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| 22 | <title>Omitted for-Expressions (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="Loop-Statements.html" rel="up" title="Loop Statements">
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| 33 | <link href="for_002dIndex-Declarations.html" rel="next" title="for-Index Declarations">
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| 34 | <link href="Example-of-for.html" rel="prev" title="Example of for">
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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="Omitted-for_002dExpressions"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="for_002dIndex-Declarations.html" accesskey="n" rel="next">for-Index Declarations</a>, Previous: <a href="Example-of-for.html" accesskey="p" rel="prev">Example of for</a>, Up: <a href="Loop-Statements.html" accesskey="u" rel="up">Loop 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="Omitted-for_002dExpressions-1"></span><h4 class="subsection">19.6.6 Omitted <code>for</code>-Expressions</h4>
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| 64 |
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| 65 | <p>A fully-fleshed <code>for</code> statement contains all these parts,
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| 66 | </p>
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| 67 | <div class="example">
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| 68 | <pre class="example">for (<var>start</var>; <var>continue-test</var>; <var>advance</var>)
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| 69 | <var>body</var>
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| 70 | </pre></div>
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| 71 |
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| 72 | <p>but you can omit any of the three expressions inside the parentheses.
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| 73 | The parentheses and the two semicolons are required syntactically, but
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| 74 | the expressions between them may be missing. A missing expression
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| 75 | means this loop doesn’t use that particular feature of the <code>for</code>
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| 76 | statement.
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| 77 | </p>
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| 78 | <p>Instead of using <var>start</var>, you can do the loop preparation
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| 79 | before the <code>for</code> statement: the effect is the same. So we
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| 80 | could have written the beginning of the previous example this way:
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| 81 | </p>
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| 82 | <div class="example">
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| 83 | <pre class="example">int i = 0;
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| 84 | for (; i < n; ++i)
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| 85 | </pre></div>
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| 86 |
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| 87 | <p>instead of this way:
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| 88 | </p>
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| 89 | <div class="example">
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| 90 | <pre class="example">int i;
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| 91 | for (i = 0; i < n; ++i)
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| 92 | </pre></div>
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| 93 |
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| 94 | <p>Omitting <var>continue-test</var> means the loop runs forever (or until
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| 95 | something else causes exit from it). Statements inside the loop can
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| 96 | test conditions for termination and use ‘<samp>break;</samp>’ to exit. This
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| 97 | is more flexible since you can put those tests anywhere in the loop,
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| 98 | not solely at the beginning.
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| 99 | </p>
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| 100 | <p>Putting an expression in <var>advance</var> is almost equivalent to writing
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| 101 | it at the end of the loop body; it does almost the same thing. The
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| 102 | only difference is for the <code>continue</code> statement (see <a href="continue-Statement.html">continue Statement</a>). So we could have written this:
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| 103 | </p>
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| 104 | <div class="example">
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| 105 | <pre class="example">for (i = 0; i < n;)
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| 106 | {
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| 107 | <span class="roman">…</span>
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| 108 | ++i;
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| 109 | }
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| 110 | </pre></div>
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| 111 |
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| 112 | <p>instead of this:
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| 113 | </p>
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| 114 | <div class="example">
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| 115 | <pre class="example">for (i = 0; i < n; ++i)
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| 116 | {
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| 117 | <span class="roman">…</span>
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| 118 | }
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| 119 | </pre></div>
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| 120 |
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| 121 | <p>The choice is mainly a matter of what is more readable for
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| 122 | programmers. However, there is also a syntactic difference:
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| 123 | <var>advance</var> is an expression, not a statement. It can’t include
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| 124 | loops, blocks, declarations, etc.
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| 125 | </p>
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| 126 | <hr>
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| 127 | <div class="header">
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| 128 | <p>
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| 129 | Next: <a href="for_002dIndex-Declarations.html" accesskey="n" rel="next">for-Index Declarations</a>, Previous: <a href="Example-of-for.html" accesskey="p" rel="prev">Example of for</a>, Up: <a href="Loop-Statements.html" accesskey="u" rel="up">Loop 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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| 130 | </div>
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