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| 22 | <title>Reordering of Operands (GNU C Language Manual)</title>
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| 57 | <body lang="en">
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| 58 | <span id="Reordering-of-Operands"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Associativity-and-Ordering.html" accesskey="n" rel="next">Associativity and Ordering</a>, Up: <a href="Order-of-Execution.html" accesskey="u" rel="up">Order of Execution</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="Reordering-of-Operands-1"></span><h3 class="section">10.1 Reordering of Operands</h3>
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| 64 | <span id="index-ordering-of-operands"></span>
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| 65 | <span id="index-reordering-of-operands"></span>
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| 66 | <span id="index-operand-execution-ordering"></span>
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| 67 |
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| 68 | <p>The C language does not necessarily carry out operations within an
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| 69 | expression in the order they appear in the code. For instance, in
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| 70 | this expression,
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| 71 | </p>
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| 72 | <div class="example">
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| 73 | <pre class="example">foo () + bar ()
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| 74 | </pre></div>
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| 75 |
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| 76 | <p><code>foo</code> might be called first or <code>bar</code> might be called first.
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| 77 | If <code>foo</code> updates a datum and <code>bar</code> uses that datum, the
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| 78 | results can be unpredictable.
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| 79 | </p>
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| 80 | <p>The unpredictable order of computation of subexpressions also makes a
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| 81 | difference when one of them contains an assignment. We already saw
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| 82 | this example of bad code,
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| 83 | </p>
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| 84 | <div class="example">
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| 85 | <pre class="example">x = 20;
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| 86 | printf ("%d %d\n", x, x = 4);
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| 87 | </pre></div>
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| 88 |
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| 89 | <p>in which the second argument, <code>x</code>, has a different value
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| 90 | depending on whether it is computed before or after the assignment in
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| 91 | the third argument.
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| 92 | </p>
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| 93 |
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| 94 |
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| 95 |
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| 96 | </body>
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| 97 | </html>
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