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| 22 | <title>Assignment in Subexpressions (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="Assignment-Expressions.html" rel="up" title="Assignment Expressions">
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| 58 | <span id="Assignment-in-Subexpressions"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Write-Assignments-Separately.html" accesskey="n" rel="next">Write Assignments Separately</a>, Previous: <a href="Postincrement_002fPostdecrement.html" accesskey="p" rel="prev">Postincrement/Postdecrement</a>, Up: <a href="Assignment-Expressions.html" accesskey="u" rel="up">Assignment Expressions</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="Pitfall_003a-Assignment-in-Subexpressions"></span><h3 class="section">7.6 Pitfall: Assignment in Subexpressions</h3>
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| 64 | <span id="index-assignment-in-subexpressions"></span>
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| 65 | <span id="index-subexpressions_002c-assignment-in"></span>
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| 66 |
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| 67 | <p>In C, the order of computing parts of an expression is not fixed.
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| 68 | Aside from a few special cases, the operations can be computed in any
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| 69 | order. If one part of the expression has an assignment to <code>x</code>
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| 70 | and another part of the expression uses <code>x</code>, the result is
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| 71 | unpredictable because that use might be computed before or after the
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| 72 | assignment.
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| 73 | </p>
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| 74 | <p>Here’s an example of ambiguous code:
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| 75 | </p>
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| 76 | <div class="example">
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| 77 | <pre class="example">x = 20;
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| 78 | printf ("%d %d\n", x, x = 4);
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| 79 | </pre></div>
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| 80 |
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| 81 | <p>If the second argument, <code>x</code>, is computed before the third argument,
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| 82 | <code>x = 4</code>, the second argument’s value will be 20. If they are
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| 83 | computed in the other order, the second argument’s value will be 4.
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| 84 | </p>
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| 85 | <p>Here’s one way to make that code unambiguous:
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| 86 | </p>
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| 87 | <div class="example">
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| 88 | <pre class="example">y = 20;
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| 89 | printf ("%d %d\n", y, x = 4);
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| 90 | </pre></div>
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| 91 |
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| 92 | <p>Here’s another way, with the other meaning:
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| 93 | </p>
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| 94 | <div class="example">
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| 95 | <pre class="example">x = 4;
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| 96 | printf ("%d %d\n", x, x);
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| 97 | </pre></div>
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| 98 |
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| 99 | <p>This issue applies to all kinds of assignments, and to the increment
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| 100 | and decrement operators, which are equivalent to assignments.
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| 101 | See <a href="Order-of-Execution.html">Order of Execution</a>, for more information about this.
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| 102 | </p>
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| 103 | <p>However, it can be useful to write assignments inside an
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| 104 | <code>if</code>-condition or <code>while</code>-test along with logical operators.
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| 105 | See <a href="Logicals-and-Assignments.html">Logicals and Assignments</a>.
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| 106 | </p>
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