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| 22 | <title>switch Statement (GNU C Language Manual)</title>
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Statements.html" rel="up" title="Statements">
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| 33 | <link href="switch-Example.html" rel="next" title="switch Example">
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| 55 | </head>
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| 56 |
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| 57 | <body lang="en">
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| 58 | <span id="switch-Statement"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="switch-Example.html" accesskey="n" rel="next">switch Example</a>, Previous: <a href="Loop-Statements.html" accesskey="p" rel="prev">Loop Statements</a>, Up: <a href="Statements.html" accesskey="u" rel="up">Statements</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="switch-Statement-1"></span><h3 class="section">19.7 <code>switch</code> Statement</h3>
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| 64 | <span id="index-switch-statement"></span>
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| 65 | <span id="index-statement_002c-switch"></span>
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| 66 | <span id="index-switch"></span>
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| 67 | <span id="index-case"></span>
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| 68 | <span id="index-default"></span>
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| 69 |
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| 70 | <p>The <code>switch</code> statement selects code to run according to the value
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| 71 | of an expression. The expression, in parentheses, follows the keyword
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| 72 | <code>switch</code>. After that come all the cases to select among,
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| 73 | inside braces. It looks like this:
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| 74 | </p>
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| 75 | <div class="example">
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| 76 | <pre class="example">switch (<var>selector</var>)
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| 77 | {
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| 78 | <var>cases</var><span class="roman">…</span>
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| 79 | }
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| 80 | </pre></div>
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| 81 |
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| 82 | <p>A case can look like this:
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| 83 | </p>
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| 84 | <div class="example">
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| 85 | <pre class="example">case <var>value</var>:
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| 86 | <var>statements</var>
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| 87 | break;
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| 88 | </pre></div>
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| 89 |
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| 90 | <p>which means “come here if <var>selector</var> happens to have the value
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| 91 | <var>value</var>,” or like this (a GNU C extension):
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| 92 | </p>
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| 93 | <div class="example">
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| 94 | <pre class="example">case <var>rangestart</var> ... <var>rangeend</var>:
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| 95 | <var>statements</var>
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| 96 | break;
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| 97 | </pre></div>
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| 98 |
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| 99 | <p>which means “come here if <var>selector</var> happens to have a value
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| 100 | between <var>rangestart</var> and <var>rangeend</var> (inclusive).” See <a href="Case-Ranges.html">Case Ranges</a>.
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| 101 | </p>
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| 102 | <p>The values in <code>case</code> labels must reduce to integer constants.
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| 103 | They can use arithmetic, and <code>enum</code> constants, but they cannot
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| 104 | refer to data in memory, because they have to be computed at compile
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| 105 | time. It is an error if two <code>case</code> labels specify the same
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| 106 | value, or ranges that overlap, or if one is a range and the other is a
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| 107 | value in that range.
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| 108 | </p>
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| 109 | <p>You can also define a default case to handle “any other value,” like
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| 110 | this:
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| 111 | </p>
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| 112 | <div class="example">
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| 113 | <pre class="example">default:
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| 114 | <var>statements</var>
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| 115 | break;
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| 116 | </pre></div>
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| 117 |
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| 118 | <p>If the <code>switch</code> statement has no <code>default:</code> label, then it
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| 119 | does nothing when the value matches none of the cases.
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| 120 | </p>
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| 121 | <p>The brace-group inside the <code>switch</code> statement is a block, and you
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| 122 | can declare variables with that scope just as in any other block
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| 123 | (see <a href="Blocks.html">Blocks</a>). However, initializers in these declarations won’t
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| 124 | necessarily be executed every time the <code>switch</code> statement runs,
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| 125 | so it is best to avoid giving them initializers.
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| 126 | </p>
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| 127 | <p><code>break;</code> inside a <code>switch</code> statement exits immediately from
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| 128 | the <code>switch</code> statement. See <a href="break-Statement.html">break Statement</a>.
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| 129 | </p>
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| 130 | <p>If there is no <code>break;</code> at the end of the code for a case,
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| 131 | execution continues into the code for the following case. This
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| 132 | happens more often by mistake than intentionally, but since this
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| 133 | feature is used in real code, we cannot eliminate it.
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| 134 | </p>
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| 135 | <p><strong>Warning:</strong> When one case is intended to fall through to the
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| 136 | next, write a comment like ‘<samp>falls through</samp>’ to say it’s
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| 137 | intentional. That way, other programmers won’t assume it was an error
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| 138 | and “fix” it erroneously.
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| 139 | </p>
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| 140 | <p>Consecutive <code>case</code> statements could, pedantically, be considered
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| 141 | an instance of falling through, but we don’t consider or treat them that
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| 142 | way because they won’t confuse anyone.
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| 143 | </p>
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| 144 | <hr>
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| 145 | <div class="header">
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| 146 | <p>
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| 147 | Next: <a href="switch-Example.html" accesskey="n" rel="next">switch Example</a>, Previous: <a href="Loop-Statements.html" accesskey="p" rel="prev">Loop Statements</a>, Up: <a href="Statements.html" accesskey="u" rel="up">Statements</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 148 | </div>
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