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<h2 class="chapter-name chapter-name-short">Control Flow</h2>
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For Loops
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<a href="#TP40014097-CH9-XID_158">
While Loops
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Conditional Statements
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Control Transfer Statements
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<p class="para">
Swift provides all the familiar control flow constructs of C-like languages. These include <code class="code-voice">for</code> and <code class="code-voice">while</code> loops to perform a task multiple times; <code class="code-voice">if</code> and <code class="code-voice">switch</code> statements to execute different branches of code based on certain conditions; and statements such as <code class="code-voice">break</code> and <code class="code-voice">continue</code> to transfer the flow of execution to another point in your code.
</p>
<p class="para">
In addition to the traditional <code class="code-voice">for</code>-<code class="code-voice">condition</code>-<code class="code-voice">increment</code> loop found in C, Swift adds a <code class="code-voice">for</code>-<code class="code-voice">in</code> loop that makes it easy to iterate over arrays, dictionaries, ranges, strings, and other sequences.
</p>
<p class="para">
Swift’s <code class="code-voice">switch</code> statement is also considerably more powerful than its counterpart in C. The cases of a <code class="code-voice">switch</code> statement do not “fall through” to the next case in Swift, avoiding common C errors caused by missing <code class="code-voice">break</code> statements. Cases can match many different types of pattern, including range matches, tuples, and casts to a specific type. Matched values in a <code class="code-voice">switch</code> case can be bound to temporary constants or variables for use within the case’s body, and complex matching conditions can be expressed with a <code class="code-voice">where</code> clause for each case.
</p>
</section>
<section class="section">
<a name="TP40014097-CH9-XID_154"></a>
<h3 class="section-name" tabindex="0">For Loops</h3>
<p class="para">
A <code class="code-voice">for</code> loop performs a set of statements a certain number of times. Swift provides two kinds of <code class="code-voice">for</code> loop:
</p><ul class="list-bullet">
<li class="item"><p class="para">
<code class="code-voice">for</code>-<code class="code-voice">in</code> performs a set of statements for each item in a range, sequence, collection, or progression.
</p>
</li><li class="item"><p class="para">
<code class="code-voice">for</code>-<code class="code-voice">condition</code>-<code class="code-voice">increment</code> performs a set of statements until a specific condition is met, typically by incrementing a counter each time the loop ends.
</p>
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<section class="section">
<a name="TP40014097-CH9-XID_155"></a>
<h3 class="section-name" tabindex="0">For-In</h3>
<p class="para">
You use the <code class="code-voice">for</code>-<code class="code-voice">in</code> loop to iterate over collections of items, such as ranges of numbers, items in an array, or characters in a string.
</p><p class="para">
This example prints the first few entries in the five-times-table:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">for</span> <span class="vc">index</span> <span class="kt">in</span> <span class="m">1</span>...<span class="m">5</span> {</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"</span>\(<span class="vc">index</span>)<span class="s"> times 5 is </span>\(<span class="vc">index</span> * <span class="m">5</span>)<span class="s">"</span>)</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="c">// 1 times 5 is 5</span></code></li>
<li><code class="code-voice"><span class="c">// 2 times 5 is 10</span></code></li>
<li><code class="code-voice"><span class="c">// 3 times 5 is 15</span></code></li>
<li><code class="code-voice"><span class="c">// 4 times 5 is 20</span></code></li>
<li><code class="code-voice"><span class="c">// 5 times 5 is 25</span></code></li>
</ul>
</div>
</section><p class="para">
The collection of items being iterated is a closed range of numbers from <code class="code-voice">1</code> to <code class="code-voice">5</code> inclusive, as indicated by the use of the closed range operator (<code class="code-voice">...</code>). The value of <code class="code-voice">index</code> is set to the first number in the range (<code class="code-voice">1</code>), and the statements inside the loop are executed. In this case, the loop contains only one statement, which prints an entry from the five-times-table for the current value of <code class="code-voice">index</code>. After the statement is executed, the value of <code class="code-voice">index</code> is updated to contain the second value in the range (<code class="code-voice">2</code>), and the <code class="code-voice">println</code> function is called again. This process continues until the end of the range is reached.
</p><p class="para">
In the example above, <code class="code-voice">index</code> is a constant whose value is automatically set at the start of each iteration of the loop. As such, it does not have to be declared before it is used. It is implicitly declared simply by its inclusion in the loop declaration, without the need for a <code class="code-voice">let</code> declaration keyword.
</p><div class="note">
<a name="TP40014097-CH9-XID_156"></a>
<aside class="aside">
<p class="aside-title">Note
</p>
<p class="para">The <code class="code-voice">index</code> constant exists only within the scope of the loop. If you want to check the value of <code class="code-voice">index</code> after the loop completes, or if you want to work with its value as a variable rather than a constant, you must declare it yourself before its use in the loop.
</p>
</aside>
</div><p class="para">
If you don’t need each value from the range, you can ignore the values by using an underscore in place of a variable name:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">let</span> <span class="vc">base</span> = <span class="m">3</span></code></li>
<li><code class="code-voice"><span class="kt">let</span> <span class="vc">power</span> = <span class="m">10</span></code></li>
<li><code class="code-voice"><span class="kt">var</span> <span class="vc">answer</span> = <span class="m">1</span></code></li>
<li><code class="code-voice"><span class="kt">for</span> <span class="kt">_</span> <span class="kt">in</span> <span class="m">1</span>...<span class="vc">power</span> {</code></li>
<li><code class="code-voice"> <span class="vc">answer</span> *= <span class="vc">base</span></code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="vc">println</span>(<span class="s">"</span>\(<span class="vc">base</span>)<span class="s"> to the power of </span>\(<span class="vc">power</span>)<span class="s"> is </span>\(<span class="vc">answer</span>)<span class="s">"</span>)</code></li>
<li><code class="code-voice"><span class="c">// prints "3 to the power of 10 is 59049"</span></code></li>
</ul>
</div>
</section><p class="para">
This example calculates the value of one number to the power of another (in this case, <code class="code-voice">3</code> to the power of <code class="code-voice">10</code>). It multiplies a starting value of <code class="code-voice">1</code> (that is, <code class="code-voice">3</code> to the power of <code class="code-voice">0</code>) by <code class="code-voice">3</code>, ten times, using a half-closed loop that starts with <code class="code-voice">0</code> and ends with <code class="code-voice">9</code>. This calculation doesn’t need to know the individual counter values each time through the loop—it simply needs to execute the loop the correct number of times. The underscore character <code class="code-voice">_</code> (used in place of a loop variable) causes the individual values to be ignored and does not provide access to the current value during each iteration of the loop.
</p><p class="para">
Use the <code class="code-voice">for</code>-<code class="code-voice">in</code> loop with an array to iterate over its items:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">let</span> <span class="vc">names</span> = [<span class="s">"Anna"</span>, <span class="s">"Alex"</span>, <span class="s">"Brian"</span>, <span class="s">"Jack"</span>]</code></li>
<li><code class="code-voice"><span class="kt">for</span> <span class="vc">name</span> <span class="kt">in</span> <span class="vc">names</span> {</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"Hello, </span>\(<span class="vc">name</span>)<span class="s">!"</span>)</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="c">// Hello, Anna!</span></code></li>
<li><code class="code-voice"><span class="c">// Hello, Alex!</span></code></li>
<li><code class="code-voice"><span class="c">// Hello, Brian!</span></code></li>
<li><code class="code-voice"><span class="c">// Hello, Jack!</span></code></li>
</ul>
</div>
</section><p class="para">
You can also iterate over a dictionary to access its key-value pairs. Each item in the dictionary is returned as a <code class="code-voice">(key, value)</code> tuple when the dictionary is iterated, and you can decompose the <code class="code-voice">(key, value)</code> tuple’s members as explicitly named constants for use within in the body of the <code class="code-voice">for</code>-<code class="code-voice">in</code> loop. Here, the dictionary’s keys are decomposed into a constant called <code class="code-voice">animalName</code>, and the dictionary’s values are decomposed into a constant called <code class="code-voice">legCount</code>:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">let</span> <span class="vc">numberOfLegs</span> = [<span class="s">"spider"</span>: <span class="m">8</span>, <span class="s">"ant"</span>: <span class="m">6</span>, <span class="s">"cat"</span>: <span class="m">4</span>]</code></li>
<li><code class="code-voice"><span class="kt">for</span> (<span class="vc">animalName</span>, <span class="vc">legCount</span>) <span class="kt">in</span> <span class="vc">numberOfLegs</span> {</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"</span>\(<span class="vc">animalName</span>)<span class="s">s have </span>\(<span class="vc">legCount</span>)<span class="s"> legs"</span>)</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="c">// spiders have 8 legs</span></code></li>
<li><code class="code-voice"><span class="c">// ants have 6 legs</span></code></li>
<li><code class="code-voice"><span class="c">// cats have 4 legs</span></code></li>
</ul>
</div>
</section><p class="para">
Items in a <code class="code-voice">Dictionary</code> may not necessarily be iterated in the same order as they were inserted. The contents of a <code class="code-voice">Dictionary</code> are inherently unordered, and iterating over them does not guarantee the order in which they will be retrieved. For more on arrays and dictionaries, see <span class="x-name"><a href="Collection Types.html#TP40014097-CH8-XID_133" data-id="TP40014097-CH8-XID_133">Collection Types</a></span>.)
</p><p class="para">
In addition to arrays and dictionaries, you can also use the <code class="code-voice">for</code>-<code class="code-voice">in</code> loop to iterate over the <code class="code-voice">Character</code> values in a string:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">for</span> <span class="vc">character</span> <span class="kt">in</span> <span class="s">"Hello"</span> {</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="vc">character</span>)</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="c">// H</span></code></li>
<li><code class="code-voice"><span class="c">// e</span></code></li>
<li><code class="code-voice"><span class="c">// l</span></code></li>
<li><code class="code-voice"><span class="c">// l</span></code></li>
<li><code class="code-voice"><span class="c">// o</span></code></li>
</ul>
</div>
</section>
</section>
<section class="section">
<a name="TP40014097-CH9-XID_157"></a>
<h3 class="section-name" tabindex="0">For-Condition-Increment</h3>
<p class="para">
In addition to <code class="code-voice">for</code>-<code class="code-voice">in</code> loops, Swift supports traditional C-style <code class="code-voice">for</code> loops with a condition and an incrementer:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">for</span> <span class="kt">var</span> <span class="vc">index</span> = <span class="m">0</span>; <span class="vc">index</span> < <span class="m">3</span>; ++<span class="vc">index</span> {</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"index is </span>\(<span class="vc">index</span>)<span class="s">"</span>)</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="c">// index is 0</span></code></li>
<li><code class="code-voice"><span class="c">// index is 1</span></code></li>
<li><code class="code-voice"><span class="c">// index is 2</span></code></li>
</ul>
</div>
</section><p class="para">
Here’s the general form of this loop format:
</p><span class="caption"></span>
<div class="code-outline">
<ul class="code-outline-lines">
<li><pre class="code-voice"><span class="kt">for</span> <em class="variable-text">initialization</em>; <em class="variable-text">condition</em>; <em class="variable-text">increment</em> {</pre></li><li><pre class="code-voice"> <em class="variable-text">statements</em></pre></li><li><pre class="code-voice">}</pre></li>
</ul>
</div><p class="para">
Semicolons separate the three parts of the loop’s definition, as in C. However, unlike C, Swift doesn’t need parentheses around the entire “initialization; condition; increment” block.
</p><p class="para">
The loop is executed as follows:
</p><ol class="list-number">
<li class="item"><p class="para">
When the loop is first entered, the <em>initialization expression</em> is evaluated once, to set up any constants or variables that are needed for the loop.
</p>
</li><li class="item"><p class="para">
The <em>condition expression</em> is evaluated. If it evaluates to <code class="code-voice">false</code>, the loop ends, and code execution continues after the <code class="code-voice">for</code> loop’s closing brace (<code class="code-voice">}</code>). If the expression evaluates to <code class="code-voice">true</code>, code execution continues by executing the statements inside the braces.
</p>
</li><li class="item"><p class="para">
After all statements are executed, the <em>increment expression</em> is evaluated. It might increase or decrease the value of a counter, or set one of the initialized variables to a new value based on the outcome of the statements. After the increment expression has been evaluated, execution returns to step 2, and the condition expression is evaluated again.
</p>
</li>
</ol><p class="para">
The loop format and execution process described above is shorthand for (and equivalent to) the outline below:
</p><span class="caption"></span>
<div class="code-outline">
<ul class="code-outline-lines">
<li><pre class="code-voice"><em class="variable-text">initialization</em></pre></li><li><pre class="code-voice"><span class="kt">while</span> <em class="variable-text">condition</em> {</pre></li><li><pre class="code-voice"> <em class="variable-text">statements</em></pre></li><li><pre class="code-voice"> <em class="variable-text">increment</em></pre></li><li><pre class="code-voice">}</pre></li>
</ul>
</div><p class="para">
Constants and variables declared within the initialization expression (such as <code class="code-voice">var index = 0</code>) are only valid within the scope of the <code class="code-voice">for</code> loop itself. To retrieve the final value of <code class="code-voice">index</code> after the loop ends, you must declare <code class="code-voice">index</code> before the loop’s scope begins:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">var</span> <span class="vc">index</span>: <span class="n"></span></code></li>
<li><code class="code-voice"><span class="kt">for</span> <span class="vc">index</span> = <span class="m">0</span>; <span class="vc">index</span> < <span class="m">3</span>; ++<span class="vc">index</span> {</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"index is </span>\(<span class="vc">index</span>)<span class="s">"</span>)</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="c">// index is 0</span></code></li>
<li><code class="code-voice"><span class="c">// index is 1</span></code></li>
<li><code class="code-voice"><span class="c">// index is 2</span></code></li>
<li><code class="code-voice"><span class="vc">println</span>(<span class="s">"The loop statements were executed </span>\(<span class="vc">index</span>)<span class="s"> times"</span>)</code></li>
<li><code class="code-voice"><span class="c">// prints "The loop statements were executed 3 times"</span></code></li>
</ul>
</div>
</section><p class="para">
Note that the final value of <code class="code-voice">index</code> after this loop is completed is <code class="code-voice">3</code>, not <code class="code-voice">2</code>. The last time the increment statement <code class="code-voice">++index</code> is called, it sets <code class="code-voice">index</code> to <code class="code-voice">3</code>, which causes <code class="code-voice">index < 3</code> to equate to <code class="code-voice">false</code>, ending the loop.
</p>
</section>
</section>
<section class="section">
<a name="TP40014097-CH9-XID_158"></a>
<h3 class="section-name" tabindex="0">While Loops</h3>
<p class="para">
A <code class="code-voice">while</code> loop performs a set of statements until a condition becomes <code class="code-voice">false</code>. These kinds of loops are best used when the number of iterations is not known before the first iteration begins. Swift provides two kinds of <code class="code-voice">while</code> loop:
</p><ul class="list-bullet">
<li class="item"><p class="para">
<code class="code-voice">while</code> evaluates its condition at the start of each pass through the loop.
</p>
</li><li class="item"><p class="para">
<code class="code-voice">do</code>-<code class="code-voice">while</code> evaluates its condition at the end of each pass through the loop.
</p>
</li>
</ul>
<section class="section">
<a name="TP40014097-CH9-XID_159"></a>
<h3 class="section-name" tabindex="0">While</h3>
<p class="para">
A <code class="code-voice">while</code> loop starts by evaluating a single condition. If the condition is <code class="code-voice">true</code>, a set of statements is repeated until the condition becomes <code class="code-voice">false</code>.
</p><p class="para">
Here’s the general form of a <code class="code-voice">while</code> loop:
</p><span class="caption"></span>
<div class="code-outline">
<ul class="code-outline-lines">
<li><pre class="code-voice"><span class="kt">while</span> <em class="variable-text">condition</em> {</pre></li><li><pre class="code-voice"> <em class="variable-text">statements</em></pre></li><li><pre class="code-voice">}</pre></li>
</ul>
</div><p class="para">
This example plays a simple game of <em>Snakes and Ladders</em> (also known as <em>Chutes and Ladders</em>):
</p><figure class="figure">
<span class="caption"></span>
<img src="./resource/snakesAndLadders_2x.png" alt="image: ../Art/snakesAndLadders_2x.png" width="623" height="299">
</figure><p class="para">
The rules of the game are as follows:
</p><ul class="list-bullet">
<li class="item"><p class="para">
The board has 25 squares, and the aim is to land on or beyond square 25.
</p>
</li><li class="item"><p class="para">
Each turn, you roll a six-sided dice and move by that number of squares, following the horizontal path indicated by the dotted arrow above.
</p>
</li><li class="item"><p class="para">
If your turn ends at the bottom of a ladder, you move up that ladder.
</p>
</li><li class="item"><p class="para">
If your turn ends at the head of a snake, you move down that snake.
</p>
</li>
</ul><p class="para">
The game board is represented by an array of <code class="code-voice">Int</code> values. Its size is based on a constant called <code class="code-voice">finalSquare</code>, which is used to initialize the array and also to check for a win condition later in the example. The board is initialized with 26 zero <code class="code-voice">Int</code> values, not 25 (one each at indices <code class="code-voice">0</code> through <code class="code-voice">25</code> inclusive):
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">let</span> <span class="vc">finalSquare</span> = <span class="m">25</span></code></li>
<li><code class="code-voice"><span class="kt">var</span> <span class="vc">board</span> = <span class="vc">Int</span>[](<span class="vc">count</span>: <span class="vc">finalSquare</span> + <span class="m">1</span>, <span class="vc">repeatedValue</span>: <span class="m">0</span>)</code></li>
</ul>
</div>
</section><p class="para">
Some squares are then set to have more specific values for the snakes and ladders. Squares with a ladder base have a positive number to move you up the board, whereas squares with a snake head have a negative number to move you back down the board:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="vc">board</span>[<span class="m">03</span>] = +<span class="m">08</span>; <span class="vc">board</span>[<span class="m">06</span>] = +<span class="m">11</span>; <span class="vc">board</span>[<span class="m">09</span>] = +<span class="m">09</span>; <span class="vc">board</span>[<span class="m">10</span>] = +<span class="m">02</span></code></li>
<li><code class="code-voice"><span class="vc">board</span>[<span class="m">14</span>] = -<span class="m">10</span>; <span class="vc">board</span>[<span class="m">19</span>] = -<span class="m">11</span>; <span class="vc">board</span>[<span class="m">22</span>] = -<span class="m">02</span>; <span class="vc">board</span>[<span class="m">24</span>] = -<span class="m">08</span></code></li>
</ul>
</div>
</section><p class="para">
Square 3 contains the bottom of a ladder that moves you up to square 11. To represent this, <code class="code-voice">board[03]</code> is equal to <code class="code-voice">+08</code>, which is equivalent to an integer value of <code class="code-voice">8</code> (the difference between <code class="code-voice">3</code> and <code class="code-voice">11</code>). The unary plus operator (<code class="code-voice">+i</code>) balances with the unary minus operator (<code class="code-voice">-i</code>), and numbers lower than <code class="code-voice">10</code> are padded with zeros so that all board definitions align. (Neither stylistic tweak is strictly necessary, but they lead to neater code.)
</p><p class="para">
The player’s starting square is “square zero”, which is just off the bottom left corner of the board. The first dice roll always moves the player on to the board:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">var</span> <span class="vc">square</span> = <span class="m">0</span></code></li>
<li><code class="code-voice"><span class="kt">var</span> <span class="vc">diceRoll</span> = <span class="m">0</span></code></li>
<li><code class="code-voice"><span class="kt">while</span> <span class="vc">square</span> < <span class="vc">finalSquare</span> {</code></li>
<li><code class="code-voice"> <span class="c">// roll the dice</span></code></li>
<li><code class="code-voice"> <span class="kt">if</span> ++<span class="vc">diceRoll</span> == <span class="m">7</span> { <span class="vc">diceRoll</span> = <span class="m">1</span> }</code></li>
<li><code class="code-voice"> <span class="c">// move by the rolled amount</span></code></li>
<li><code class="code-voice"> <span class="vc">square</span> += <span class="vc">diceRoll</span></code></li>
<li><code class="code-voice"> <span class="kt">if</span> <span class="vc">square</span> < <span class="vc">board</span>.<span class="vc">count</span> {</code></li>
<li><code class="code-voice"> <span class="c">// if we're still on the board, move up or down for a snake or a ladder</span></code></li>
<li><code class="code-voice"> <span class="vc">square</span> += <span class="vc">board</span>[<span class="vc">square</span>]</code></li>
<li><code class="code-voice"> }</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="vc">println</span>(<span class="s">"Game over!"</span>)</code></li>
</ul>
</div>
</section><p class="para">
This example uses a very simple approach to dice rolling. Instead of a random number generator, it starts with a <code class="code-voice">diceRoll</code> value of <code class="code-voice">0</code>. Each time through the <code class="code-voice">while</code> loop, <code class="code-voice">diceRoll</code> is incremented with the prefix increment operator (<code class="code-voice">++i</code>), and is then checked to see if it has become too large. The return value of <code class="code-voice">++diceRoll</code> is equal to the value of <code class="code-voice">diceRoll</code> <em>after</em> it is incremented. Whenever this return value equals <code class="code-voice">7</code>, the dice roll has become too large, and is reset to a value of <code class="code-voice">1</code>. This gives a sequence of <code class="code-voice">diceRoll</code> values that is always <code class="code-voice">1</code>, <code class="code-voice">2</code>, <code class="code-voice">3</code>, <code class="code-voice">4</code>, <code class="code-voice">5</code>, <code class="code-voice">6</code>, <code class="code-voice">1</code>, <code class="code-voice">2</code> and so on.
</p><p class="para">
After rolling the dice, the player moves forward by <code class="code-voice">diceRoll</code> squares. It’s possible that the dice roll may have moved the player beyond square 25, in which case the game is over. To cope with this scenario, the code checks that <code class="code-voice">square</code> is less than the <code class="code-voice">board</code> array’s <code class="code-voice">count</code> property before adding the value stored in <code class="code-voice">board[square]</code> onto the current <code class="code-voice">square</code> value to move the player up or down any ladders or snakes.
</p><p class="para">
Had this check not been performed, <code class="code-voice">board[square]</code> might try to access a value outside the bounds of the <code class="code-voice">board</code> array, which would trigger an error. If <code class="code-voice">square</code> is now equal to <code class="code-voice">26</code>, the code would try to check the value of <code class="code-voice">board[26]</code>, which is larger than the size of the array.
</p><p class="para">
The current <code class="code-voice">while</code> loop execution then ends, and the loop’s condition is checked to see if the loop should be executed again. If the player has moved on or beyond square number <code class="code-voice">25</code>, the loop’s condition evaluates to <code class="code-voice">false</code>, and the game ends.
</p><p class="para">
A <code class="code-voice">while</code> loop is appropriate in this case because the length of the game is not clear at the start of the <code class="code-voice">while</code> loop. Instead, the loop is executed until a particular condition is satisfied.
</p>
</section>
<section class="section">
<a name="TP40014097-CH9-XID_160"></a>
<h3 class="section-name" tabindex="0">Do-While</h3>
<p class="para">
The other variation of the <code class="code-voice">while</code> loop, known as the <code class="code-voice">do</code>-<code class="code-voice">while</code> loop, performs a single pass through the loop block first, <em>before</em> considering the loop’s condition. It then continues to repeat the loop until the condition is <code class="code-voice">false</code>.
</p><p class="para">
Here’s the general form of a <code class="code-voice">do</code>-<code class="code-voice">while</code> loop:
</p><span class="caption"></span>
<div class="code-outline">
<ul class="code-outline-lines">
<li><pre class="code-voice"><span class="kt">do</span> {</pre></li><li><pre class="code-voice"> <em class="variable-text">statements</em></pre></li><li><pre class="code-voice">} <span class="kt">while</span> <em class="variable-text">condition</em></pre></li>
</ul>
</div><p class="para">
Here’s the <em>Snakes and Ladders</em> example again, written as a <code class="code-voice">do</code>-<code class="code-voice">while</code> loop rather than a <code class="code-voice">while</code> loop. The values of <code class="code-voice">finalSquare</code>, <code class="code-voice">board</code>, <code class="code-voice">square</code>, and <code class="code-voice">diceRoll</code> are initialized in exactly the same way as with a <code class="code-voice">while</code> loop:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">let</span> <span class="vc">finalSquare</span> = <span class="m">25</span></code></li>
<li><code class="code-voice"><span class="kt">var</span> <span class="vc">board</span> = <span class="vc">Int</span>[](<span class="vc">count</span>: <span class="vc">finalSquare</span> + <span class="m">1</span>, <span class="vc">repeatedValue</span>: <span class="m">0</span>)</code></li>
<li><code class="code-voice"><span class="vc">board</span>[<span class="m">03</span>] = +<span class="m">08</span>; <span class="vc">board</span>[<span class="m">06</span>] = +<span class="m">11</span>; <span class="vc">board</span>[<span class="m">09</span>] = +<span class="m">09</span>; <span class="vc">board</span>[<span class="m">10</span>] = +<span class="m">02</span></code></li>
<li><code class="code-voice"><span class="vc">board</span>[<span class="m">14</span>] = -<span class="m">10</span>; <span class="vc">board</span>[<span class="m">19</span>] = -<span class="m">11</span>; <span class="vc">board</span>[<span class="m">22</span>] = -<span class="m">02</span>; <span class="vc">board</span>[<span class="m">24</span>] = -<span class="m">08</span></code></li>
<li><code class="code-voice"><span class="kt">var</span> <span class="vc">square</span> = <span class="m">0</span></code></li>
<li><code class="code-voice"><span class="kt">var</span> <span class="vc">diceRoll</span> = <span class="m">0</span></code></li>
</ul>
</div>
</section><p class="para">
In this version of the game, the <em>first</em> action in the loop is to check for a ladder or a snake. No ladder on the board takes the player straight to square 25, and so it is not possible to win the game by moving up a ladder. Therefore, it is safe to check for a snake or a ladder as the first action in the loop.
</p><p class="para">
At the start of the game, the player is on “square zero”. <code class="code-voice">board[0]</code> always equals <code class="code-voice">0</code>, and has no effect:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">do</span> {</code></li>
<li><code class="code-voice"> <span class="c">// move up or down for a snake or ladder</span></code></li>
<li><code class="code-voice"> <span class="vc">square</span> += <span class="vc">board</span>[<span class="vc">square</span>]</code></li>
<li><code class="code-voice"> <span class="c">// roll the dice</span></code></li>
<li><code class="code-voice"> <span class="kt">if</span> ++<span class="vc">diceRoll</span> == <span class="m">7</span> { <span class="vc">diceRoll</span> = <span class="m">1</span> }</code></li>
<li><code class="code-voice"> <span class="c">// move by the rolled amount</span></code></li>
<li><code class="code-voice"> <span class="vc">square</span> += <span class="vc">diceRoll</span></code></li>
<li><code class="code-voice">} <span class="kt">while</span> <span class="vc">square</span> < <span class="vc">finalSquare</span></code></li>
<li><code class="code-voice"><span class="vc">println</span>(<span class="s">"Game over!"</span>)</code></li>
</ul>
</div>
</section><p class="para">
After the code checks for snakes and ladders, the dice is rolled, and the player is moved forward by <code class="code-voice">diceRoll</code> squares. The current loop execution then ends.
</p><p class="para">
The loop’s condition (<code class="code-voice">while square < finalSquare</code>) is the same as before, but this time it is not evaluated until the <em>end</em> of the first run through the loop. The structure of the <code class="code-voice">do</code>-<code class="code-voice">while</code> loop is better suited to this game than the <code class="code-voice">while</code> loop in the previous example. In the <code class="code-voice">do</code>-<code class="code-voice">while</code> loop above, <code class="code-voice">square += board[square]</code> is always executed <em>immediately after</em> the loop’s <code class="code-voice">while</code> condition confirms that <code class="code-voice">square</code> is still on the board. This behavior removes the need for the array bounds check seen in the earlier version of the game.
</p>
</section>
</section>
<section class="section">
<a name="TP40014097-CH9-XID_161"></a>
<h3 class="section-name" tabindex="0">Conditional Statements</h3>
<p class="para">
It is often useful to execute different pieces of code based on certain conditions. You might want to run an extra piece of code when an error occurs, or to display a message when a value becomes too high or too low. To do this, you make parts of your code <em>conditional</em>.
</p><p class="para">
Swift provides two ways to add conditional branches to your code, known as the <code class="code-voice">if</code> statement and the <code class="code-voice">switch</code> statement. Typically, you use the <code class="code-voice">if</code> statement to evaluate simple conditions with only a few possible outcomes. The <code class="code-voice">switch</code> statement is better suited to more complex conditions with multiple possible permutations, and is useful in situations where pattern-matching can help select an appropriate code branch to execute.
</p>
<section class="section">
<a name="TP40014097-CH9-XID_162"></a>
<h3 class="section-name" tabindex="0">If</h3>
<p class="para">
In its simplest form, the <code class="code-voice">if</code> statement has a single <code class="code-voice">if</code> condition. It executes a set of statements only if that condition is <code class="code-voice">true</code>:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">var</span> <span class="vc">temperatureInFahrenheit</span> = <span class="m">30</span></code></li>
<li><code class="code-voice"><span class="kt">if</span> <span class="vc">temperatureInFahrenheit</span> <= <span class="m">32</span> {</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"It's very cold. Consider wearing a scarf."</span>)</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="c">// prints "It's very cold. Consider wearing a scarf."</span></code></li>
</ul>
</div>
</section><p class="para">
The preceding example checks whether the temperature is less than or equal to 32 degrees Fahrenheit (the freezing point of water). If it is, a message is printed. Otherwise, no message is printed, and code execution continues after the <code class="code-voice">if</code> statement’s closing brace.
</p><p class="para">
The <code class="code-voice">if</code> statement can provide an alternative set of statements, known as an <em>else clause</em>, for when the <code class="code-voice">if</code> condition is <code class="code-voice">false</code>. These statements are indicated by the <code class="code-voice">else</code> keyword:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="vc">temperatureInFahrenheit</span> = <span class="m">40</span></code></li>
<li><code class="code-voice"><span class="kt">if</span> <span class="vc">temperatureInFahrenheit</span> <= <span class="m">32</span> {</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"It's very cold. Consider wearing a scarf."</span>)</code></li>
<li><code class="code-voice">} <span class="kt">else</span> {</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"It's not that cold. Wear a t-shirt."</span>)</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="c">// prints "It's not that cold. Wear a t-shirt."</span></code></li>
</ul>
</div>
</section><p class="para">
One of these two branches is always executed. Because the temperature has increased to <code class="code-voice">40</code> degrees Fahrenheit, it is no longer cold enough to advise wearing a scarf, and so the <code class="code-voice">else</code> branch is triggered instead.
</p><p class="para">
You can chain multiple <code class="code-voice">if</code> statements together, to consider additional clauses:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="vc">temperatureInFahrenheit</span> = <span class="m">90</span></code></li>
<li><code class="code-voice"><span class="kt">if</span> <span class="vc">temperatureInFahrenheit</span> <= <span class="m">32</span> {</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"It's very cold. Consider wearing a scarf."</span>)</code></li>
<li><code class="code-voice">} <span class="kt">else</span> <span class="kt">if</span> <span class="vc">temperatureInFahrenheit</span> >= <span class="m">86</span> {</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"It's really warm. Don't forget to wear sunscreen."</span>)</code></li>
<li><code class="code-voice">} <span class="kt">else</span> {</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"It's not that cold. Wear a t-shirt."</span>)</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="c">// prints "It's really warm. Don't forget to wear sunscreen."</span></code></li>
</ul>
</div>
</section><p class="para">
Here, an additional <code class="code-voice">if</code> statement is added to respond to particularly warm temperatures. The final <code class="code-voice">else</code> clause remains, and prints a response for any temperatures that are neither too warm nor too cold.
</p><p class="para">
The final <code class="code-voice">else</code> clause is optional, however, and can be excluded if the set of conditions does not need to be complete:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="vc">temperatureInFahrenheit</span> = <span class="m">72</span></code></li>
<li><code class="code-voice"><span class="kt">if</span> <span class="vc">temperatureInFahrenheit</span> <= <span class="m">32</span> {</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"It's very cold. Consider wearing a scarf."</span>)</code></li>
<li><code class="code-voice">} <span class="kt">else</span> <span class="kt">if</span> <span class="vc">temperatureInFahrenheit</span> >= <span class="m">86</span> {</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"It's really warm. Don't forget to wear sunscreen."</span>)</code></li>
<li><code class="code-voice">}</code></li>
</ul>
</div>
</section><p class="para">
In this example, the temperature is neither too cold nor too warm to trigger the <code class="code-voice">if</code> or <code class="code-voice">else if</code> conditions, and so no message is printed.
</p>
</section>
<section class="section">
<a name="TP40014097-CH9-XID_163"></a>
<h3 class="section-name" tabindex="0">Switch</h3>
<p class="para">
A <code class="code-voice">switch</code> statement considers a value and compares it against several possible matching patterns. It then executes an appropriate block of code, based on the first pattern that matches successfully. A <code class="code-voice">switch</code> statement provides an alternative to the <code class="code-voice">if</code> statement for responding to multiple potential states.
</p><p class="para">
In its simplest form, a <code class="code-voice">switch</code> statement compares a value against one or more values of the same type:
</p><span class="caption"></span>
<div class="code-outline">
<ul class="code-outline-lines">
<li><pre class="code-voice"><span class="kt">switch</span> <em class="variable-text">some value to consider</em> {</pre></li><li><pre class="code-voice"><span class="kt">case</span> <em class="variable-text">value 1</em>:</pre></li><li><pre class="code-voice"> <em class="variable-text">respond to value 1</em></pre></li><li><pre class="code-voice"><span class="kt">case</span> <em class="variable-text">value 2</em>,</pre></li><li><pre class="code-voice"><em class="variable-text">value 3</em>:</pre></li><li><pre class="code-voice"> <em class="variable-text">respond to value 2 or 3</em></pre></li><li><pre class="code-voice"><span class="kt">default</span>:</pre></li><li><pre class="code-voice"> <em class="variable-text">otherwise, do something else</em></pre></li><li><pre class="code-voice">}</pre></li>
</ul>
</div><p class="para">
Every <code class="code-voice">switch</code> statement consists of multiple possible <em>cases</em>, each of which begins with the <code class="code-voice">case</code> keyword. In addition to comparing against specific values, Swift provides several ways for each case to specify more complex matching patterns. These options are described later in this section.
</p><p class="para">
The body of each <code class="code-voice">switch</code> case is a separate branch of code execution, in a similar manner to the branches of an <code class="code-voice">if</code> statement. The <code class="code-voice">switch</code> statement determines which branch should be selected. This is known as <em>switching</em> on the value that is being considered.
</p><p class="para">
Every <code class="code-voice">switch</code> statement must be <em>exhaustive</em>. That is, every possible value of the type being considered must be matched by one of the <code class="code-voice">switch</code> cases. If it is not appropriate to provide a <code class="code-voice">switch</code> case for every possible value, you can define a default catch-all case to cover any values that are not addressed explicitly. This catch-all case is indicated by the keyword <code class="code-voice">default</code>, and must always appear last.
</p><p class="para">
This example uses a <code class="code-voice">switch</code> statement to consider a single lowercase character called <code class="code-voice">someCharacter</code>:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">let</span> <span class="vc">someCharacter</span>: <span class="n"></span> = <span class="s">"e"</span></code></li>
<li><code class="code-voice"><span class="kt">switch</span> <span class="vc">someCharacter</span> {</code></li>
<li><code class="code-voice"><span class="kt">case</span> <span class="s">"a"</span>, <span class="s">"e"</span>, <span class="s">"i"</span>, <span class="s">"o"</span>, <span class="s">"u"</span>:</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"</span>\(<span class="vc">someCharacter</span>)<span class="s"> is a vowel"</span>)</code></li>
<li><code class="code-voice"><span class="kt">case</span> <span class="s">"b"</span>, <span class="s">"c"</span>, <span class="s">"d"</span>, <span class="s">"f"</span>, <span class="s">"g"</span>, <span class="s">"h"</span>, <span class="s">"j"</span>, <span class="s">"k"</span>, <span class="s">"l"</span>, <span class="s">"m"</span>,</code></li>
<li><code class="code-voice"><span class="s">"n"</span>, <span class="s">"p"</span>, <span class="s">"q"</span>, <span class="s">"r"</span>, <span class="s">"s"</span>, <span class="s">"t"</span>, <span class="s">"v"</span>, <span class="s">"w"</span>, <span class="s">"x"</span>, <span class="s">"y"</span>, <span class="s">"z"</span>:</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"</span>\(<span class="vc">someCharacter</span>)<span class="s"> is a consonant"</span>)</code></li>
<li><code class="code-voice"><span class="kt">default</span>:</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"</span>\(<span class="vc">someCharacter</span>)<span class="s"> is not a vowel or a consonant"</span>)</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="c">// prints "e is a vowel"</span></code></li>
</ul>
</div>
</section><p class="para">
The <code class="code-voice">switch</code> statement’s first case matches all five lowercase vowels in the English language. Similarly, its second case matches all lowercase English consonants.
</p><p class="para">
It is not practical to write all other possible characters as part of a <code class="code-voice">switch</code> case, and so this <code class="code-voice">switch</code> statement provides a <code class="code-voice">default</code> case to match all other characters that are not vowels or consonants. This provision ensures that the <code class="code-voice">switch</code> statement is exhaustive.
</p>
<section class="section">
<a name="TP40014097-CH9-XID_164"></a>
<h3 class="section-name" tabindex="0">No Implicit Fallthrough</h3>
<p class="para">
In contrast with <code class="code-voice">switch</code> statements in C and Objective-C, <code class="code-voice">switch</code> statements in Swift do not fall through the bottom of each case and into the next one by default. Instead, the entire <code class="code-voice">switch</code> statement finishes its execution as soon as the first matching <code class="code-voice">switch</code> case is completed, without requiring an explicit <code class="code-voice">break</code> statement. This makes the <code class="code-voice">switch</code> statement safer and easier to use than in C, and avoids executing more than one <code class="code-voice">switch</code> case by mistake.
</p><div class="note">
<a name="TP40014097-CH9-XID_165"></a>
<aside class="aside">
<p class="aside-title">Note
</p>
<p class="para">You can still break out of a matched <code class="code-voice">switch</code> case before that case has completed its execution if you need to. See <span class="x-name"><a href="#TP40014097-CH9-XID_176" data-id="TP40014097-CH9-XID_176">Break in a Switch Statement</a></span> for details.
</p>
</aside>
</div><p class="para">
The body of each case <em>must</em> contain at least one executable statement. It is not valid to write the following code, because the first case is empty:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">let</span> <span class="vc">anotherCharacter</span>: <span class="n"></span> = <span class="s">"a"</span></code></li>
<li><code class="code-voice"><span class="kt">switch</span> <span class="vc">anotherCharacter</span> {</code></li>
<li><code class="code-voice"><span class="kt">case</span> <span class="s">"a"</span>:</code></li>
<li><code class="code-voice"><span class="kt">case</span> <span class="s">"A"</span>:</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"The letter A"</span>)</code></li>
<li><code class="code-voice"><span class="kt">default</span>:</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"Not the letter A"</span>)</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="c">// this will report a compile-time error</span></code></li>
</ul>
</div>
</section><p class="para">
Unlike a <code class="code-voice">switch</code> statement in C, this <code class="code-voice">switch</code> statement does not match both <code class="code-voice">"a"</code> and <code class="code-voice">"A"</code>. Rather, it reports a compile-time error that <code class="code-voice">case "a":</code> does not contain any executable statements. This approach avoids accidental fallthrough from one case to another, and makes for safer code that is clearer in its intent.
</p><p class="para">
Multiple matches for a single <code class="code-voice">switch</code> case can be separated by commas, and can be written over multiple lines if the list is long:
</p><span class="caption"></span>
<div class="code-outline">
<ul class="code-outline-lines">
<li><pre class="code-voice"><span class="kt">switch</span> <em class="variable-text">some value to consider</em> {</pre></li><li><pre class="code-voice"><span class="kt">case</span> <em class="variable-text">value 1</em>,</pre></li><li><pre class="code-voice"><em class="variable-text">value 2</em>:</pre></li><li><pre class="code-voice"> <em class="variable-text">statements</em></pre></li><li><pre class="code-voice">}</pre></li>
</ul>
</div><div class="note">
<a name="TP40014097-CH9-XID_166"></a>
<aside class="aside">
<p class="aside-title">Note
</p>
<p class="para">To opt in to fallthrough behavior for a particular <code class="code-voice">switch</code> case, use the <code class="code-voice">fallthrough</code> keyword, as described in <span class="x-name"><a href="#TP40014097-CH9-XID_178" data-id="TP40014097-CH9-XID_178">Fallthrough</a></span>.
</p>
</aside>
</div>
</section>
<section class="section">
<a name="TP40014097-CH9-XID_167"></a>
<h3 class="section-name" tabindex="0">Range Matching</h3>
<p class="para">
Values in <code class="code-voice">switch</code> cases can be checked for their inclusion in a range. This example uses number ranges to provide a natural-language count for numbers of any size:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">let</span> <span class="vc">count</span> = <span class="m">3_000_000_000_000</span></code></li>
<li><code class="code-voice"><span class="kt">let</span> <span class="vc">countedThings</span> = <span class="s">"stars in the Milky Way"</span></code></li>
<li><code class="code-voice"><span class="kt">var</span> <span class="vc">naturalCount</span>: <span class="n"></span></code></li>
<li><code class="code-voice"><span class="kt">switch</span> <span class="vc">count</span> {</code></li>
<li><code class="code-voice"><span class="kt">case</span> <span class="m">0</span>:</code></li>
<li><code class="code-voice"> <span class="vc">naturalCount</span> = <span class="s">"no"</span></code></li>
<li><code class="code-voice"><span class="kt">case</span> <span class="m">1</span>...<span class="m">3</span>:</code></li>
<li><code class="code-voice"> <span class="vc">naturalCount</span> = <span class="s">"a few"</span></code></li>
<li><code class="code-voice"><span class="kt">case</span> <span class="m">4</span>...<span class="m">9</span>:</code></li>
<li><code class="code-voice"> <span class="vc">naturalCount</span> = <span class="s">"several"</span></code></li>
<li><code class="code-voice"><span class="kt">case</span> <span class="m">10</span>...<span class="m">99</span>:</code></li>
<li><code class="code-voice"> <span class="vc">naturalCount</span> = <span class="s">"tens of"</span></code></li>
<li><code class="code-voice"><span class="kt">case</span> <span class="m">100</span>...<span class="m">999</span>:</code></li>
<li><code class="code-voice"> <span class="vc">naturalCount</span> = <span class="s">"hundreds of"</span></code></li>
<li><code class="code-voice"><span class="kt">case</span> <span class="m">1000</span>...<span class="m">999_999</span>:</code></li>
<li><code class="code-voice"> <span class="vc">naturalCount</span> = <span class="s">"thousands of"</span></code></li>
<li><code class="code-voice"><span class="kt">default</span>:</code></li>
<li><code class="code-voice"> <span class="vc">naturalCount</span> = <span class="s">"millions and millions of"</span></code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="vc">println</span>(<span class="s">"There are </span>\(<span class="vc">naturalCount</span>)<span class="s"> </span>\(<span class="vc">countedThings</span>)<span class="s">."</span>)</code></li>
<li><code class="code-voice"><span class="c">// prints "There are millions and millions of stars in the Milky Way."</span></code></li>
</ul>
</div>
</section>
</section>
<section class="section">
<a name="TP40014097-CH9-XID_168"></a>
<h3 class="section-name" tabindex="0">Tuples</h3>
<p class="para">
You can use tuples to test multiple values in the same <code class="code-voice">switch</code> statement. Each element of the tuple can be tested against a different value or range of values. Alternatively, use the underscore (<code class="code-voice">_</code>) identifier to match any possible value.
</p><p class="para">
The example below takes an (x, y) point, expressed as a simple tuple of type <code class="code-voice">(Int, Int)</code>, and categorizes it on the graph that follows the example:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">let</span> <span class="vc">somePoint</span> = (<span class="m">1</span>, <span class="m">1</span>)</code></li>
<li><code class="code-voice"><span class="kt">switch</span> <span class="vc">somePoint</span> {</code></li>
<li><code class="code-voice"><span class="kt">case</span> (<span class="m">0</span>, <span class="m">0</span>):</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"(0, 0) is at the origin"</span>)</code></li>
<li><code class="code-voice"><span class="kt">case</span> (<span class="kt">_</span>, <span class="m">0</span>):</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"(</span>\(<span class="vc">somePoint</span>.<span class="m">0</span>)<span class="s">, 0) is on the x-axis"</span>)</code></li>
<li><code class="code-voice"><span class="kt">case</span> (<span class="m">0</span>, <span class="kt">_</span>):</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"(0, </span>\(<span class="vc">somePoint</span>.<span class="m">1</span>)<span class="s">) is on the y-axis"</span>)</code></li>
<li><code class="code-voice"><span class="kt">case</span> (-<span class="m">2</span>...<span class="m">2</span>, -<span class="m">2</span>...<span class="m">2</span>):</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"(</span>\(<span class="vc">somePoint</span>.<span class="m">0</span>)<span class="s">, </span>\(<span class="vc">somePoint</span>.<span class="m">1</span>)<span class="s">) is inside the box"</span>)</code></li>
<li><code class="code-voice"><span class="kt">default</span>:</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"(</span>\(<span class="vc">somePoint</span>.<span class="m">0</span>)<span class="s">, </span>\(<span class="vc">somePoint</span>.<span class="m">1</span>)<span class="s">) is outside of the box"</span>)</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="c">// prints "(1, 1) is inside the box"</span></code></li>
</ul>
</div>
</section><figure class="figure">
<span class="caption"></span>
<img src="./resource/coordinateGraphSimple_2x.png" alt="image: ../Art/coordinateGraphSimple_2x.png" width="288" height="288">
</figure><p class="para">
The <code class="code-voice">switch</code> statement determines if the point is at the origin (0, 0); on the red x-axis; on the orange y-axis; inside the blue 4-by-4 box centered on the origin; or outside of the box.
</p><p class="para">
Unlike C, Swift allows multiple <code class="code-voice">switch</code> cases to consider the same value or values. In fact, the point (0, 0) could match all <em>four</em> of the cases in this example. However, if multiple matches are possible, the first matching case is always used. The point (0, 0) would match <code class="code-voice">case (0, 0)</code> first, and so all other matching cases would be ignored.
</p>
</section>
<section class="section">
<a name="TP40014097-CH9-XID_169"></a>
<h3 class="section-name" tabindex="0">Value Bindings</h3>
<p class="para">
A <code class="code-voice">switch</code> case can bind the value or values it matches to temporary constants or variables, for use in the body of the case. This is known as <em>value binding</em>, because the values are “bound” to temporary constants or variables within the case’s body.
</p><p class="para">
The example below takes an (x, y) point, expressed as a tuple of type <code class="code-voice">(Int, Int)</code> and categorizes it on the graph that follows:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">let</span> <span class="vc">anotherPoint</span> = (<span class="m">2</span>, <span class="m">0</span>)</code></li>
<li><code class="code-voice"><span class="kt">switch</span> <span class="vc">anotherPoint</span> {</code></li>
<li><code class="code-voice"><span class="kt">case</span> (<span class="kt">let</span> <span class="vc">x</span>, <span class="m">0</span>):</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"on the x-axis with an x value of </span>\(<span class="vc">x</span>)<span class="s">"</span>)</code></li>
<li><code class="code-voice"><span class="kt">case</span> (<span class="m">0</span>, <span class="kt">let</span> <span class="vc">y</span>):</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"on the y-axis with a y value of </span>\(<span class="vc">y</span>)<span class="s">"</span>)</code></li>
<li><code class="code-voice"><span class="kt">case</span> <span class="kt">let</span> (<span class="vc">x</span>, <span class="vc">y</span>):</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"somewhere else at (</span>\(<span class="vc">x</span>)<span class="s">, </span>\(<span class="vc">y</span>)<span class="s">)"</span>)</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="c">// prints "on the x-axis with an x value of 2"</span></code></li>
</ul>
</div>
</section><figure class="figure">
<span class="caption"></span>
<img src="./resource/coordinateGraphMedium_2x.png" alt="image: ../Art/coordinateGraphMedium_2x.png" width="288" height="288">
</figure><p class="para">
The <code class="code-voice">switch</code> statement determines if the point is on the red x-axis, on the orange y-axis, or elsewhere, on neither axis.
</p><p class="para">
The three <code class="code-voice">switch</code> cases declare placeholder constants <code class="code-voice">x</code> and <code class="code-voice">y</code>, which temporarily take on one or both tuple values from <code class="code-voice">anotherPoint</code>. The first case, <code class="code-voice">case (let x, 0)</code>, matches any point with a <code class="code-voice">y</code> value of <code class="code-voice">0</code> and assigns the point’s <code class="code-voice">x</code> value to the temporary constant <code class="code-voice">x</code>. Similarly, the second case, <code class="code-voice">case (0, let y)</code>, matches any point with an <code class="code-voice">x</code> value of <code class="code-voice">0</code> and assigns the point’s <code class="code-voice">y</code> value to the temporary constant <code class="code-voice">y</code>.
</p><p class="para">
Once the temporary constants are declared, they can be used within the case’s code block. Here, they are used as shorthand for printing the values with the <code class="code-voice">println</code> function.
</p><p class="para">
Note that this <code class="code-voice">switch</code> statement does not have a <code class="code-voice">default</code> case. The final case, <code class="code-voice">case let (x, y)</code>, declares a tuple of two placeholder constants that can match any value. As a result, it matches all possible remaining values, and a <code class="code-voice">default</code> case is not needed to make the <code class="code-voice">switch</code> statement exhaustive.
</p><p class="para">
In the example above, <code class="code-voice">x</code> and <code class="code-voice">y</code> are declared as constants with the <code class="code-voice">let</code> keyword, because there is no need to modify their values within the body of the case. However, they could have been declared as variables instead, with the <code class="code-voice">var</code> keyword. If this had been done, a temporary variable would have been created and initialized with the appropriate value. Any changes to that variable would only have an effect within the body of the case.
</p>
</section>
<section class="section">
<a name="TP40014097-CH9-XID_170"></a>
<h3 class="section-name" tabindex="0">Where</h3>
<p class="para">
A <code class="code-voice">switch</code> case can use a <code class="code-voice">where</code> clause to check for additional conditions.
</p><p class="para">
The example below categorizes an (x, y) point on the following graph:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">let</span> <span class="vc">yetAnotherPoint</span> = (<span class="m">1</span>, -<span class="m">1</span>)</code></li>
<li><code class="code-voice"><span class="kt">switch</span> <span class="vc">yetAnotherPoint</span> {</code></li>
<li><code class="code-voice"><span class="kt">case</span> <span class="kt">let</span> (<span class="vc">x</span>, <span class="vc">y</span>) <span class="kt">where</span> <span class="vc">x</span> == <span class="vc">y</span>:</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"(</span>\(<span class="vc">x</span>)<span class="s">, </span>\(<span class="vc">y</span>)<span class="s">) is on the line x == y"</span>)</code></li>
<li><code class="code-voice"><span class="kt">case</span> <span class="kt">let</span> (<span class="vc">x</span>, <span class="vc">y</span>) <span class="kt">where</span> <span class="vc">x</span> == -<span class="vc">y</span>:</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"(</span>\(<span class="vc">x</span>)<span class="s">, </span>\(<span class="vc">y</span>)<span class="s">) is on the line x == -y"</span>)</code></li>
<li><code class="code-voice"><span class="kt">case</span> <span class="kt">let</span> (<span class="vc">x</span>, <span class="vc">y</span>):</code></li>
<li><code class="code-voice"> <span class="vc">println</span>(<span class="s">"(</span>\(<span class="vc">x</span>)<span class="s">, </span>\(<span class="vc">y</span>)<span class="s">) is just some arbitrary point"</span>)</code></li>
<li><code class="code-voice">}</code></li>
<li><code class="code-voice"><span class="c">// prints "(1, -1) is on the line x == -y"</span></code></li>
</ul>
</div>
</section><figure class="figure">
<span class="caption"></span>
<img src="./resource/coordinateGraphComplex_2x.png" alt="image: ../Art/coordinateGraphComplex_2x.png" width="289" height="290">
</figure><p class="para">
The <code class="code-voice">switch</code> statement determines if the point is on the green diagonal line where <code class="code-voice">x == y</code>, on the purple diagonal line where <code class="code-voice">x == -y</code>, or neither.
</p><p class="para">
The three <code class="code-voice">switch</code> cases declare placeholder constants <code class="code-voice">x</code> and <code class="code-voice">y</code>, which temporarily take on the two tuple values from <code class="code-voice">point</code>. These constants are used as part of a <code class="code-voice">where</code> clause, to create a dynamic filter. The <code class="code-voice">switch</code> case matches the current value of <code class="code-voice">point</code> only if the <code class="code-voice">where</code> clause’s condition evaluates to <code class="code-voice">true</code> for that value.
</p><p class="para">
As in the previous example, the final case matches all possible remaining values, and so a <code class="code-voice">default</code> case is not needed to make the <code class="code-voice">switch</code> statement exhaustive.
</p>
</section>
</section>
</section>
<section class="section">
<a name="TP40014097-CH9-XID_171"></a>
<h3 class="section-name" tabindex="0">Control Transfer Statements</h3>
<p class="para">
<em>Control transfer statements</em> change the order in which your code is executed, by transferring control from one piece of code to another. Swift has four control transfer statements:
</p><ul class="list-bullet">
<li class="item"><p class="para">
<code class="code-voice">continue</code>
</p>
</li><li class="item"><p class="para">
<code class="code-voice">break</code>