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| 21 | <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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| 22 | <title>Modifying Assignment (GNU C Language Manual)</title>
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| 23 |
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| 24 | <meta name="description" content="Modifying Assignment (GNU C Language Manual)">
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| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
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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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| 33 | <link href="Increment_002fDecrement.html" rel="next" title="Increment/Decrement">
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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="Modifying-Assignment"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Increment_002fDecrement.html" accesskey="n" rel="next">Increment/Decrement</a>, Previous: <a href="Lvalues.html" accesskey="p" rel="prev">Lvalues</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="Modifying-Assignment-1"></span><h3 class="section">7.3 Modifying Assignment</h3>
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| 64 | <span id="index-modifying-assignment"></span>
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| 65 | <span id="index-assignment_002c-modifying"></span>
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| 66 |
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| 67 | <p>You can abbreviate the common construct
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| 68 | </p>
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| 69 | <div class="example">
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| 70 | <pre class="example"><var>lvalue</var> = <var>lvalue</var> + <var>expression</var>
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| 71 | </pre></div>
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| 72 |
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| 73 | <p>as
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| 74 | </p>
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| 75 | <div class="example">
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| 76 | <pre class="example"><var>lvalue</var> += <var>expression</var>
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| 77 | </pre></div>
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| 78 |
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| 79 | <p>This is known as a <em>modifying assignment</em>. For instance,
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| 80 | </p>
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| 81 | <div class="example">
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| 82 | <pre class="example">i = i + 5;
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| 83 | i += 5;
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| 84 | </pre></div>
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| 85 |
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| 86 | <p>shows two statements that are equivalent. The first uses
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| 87 | simple assignment; the second uses modifying assignment.
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| 88 | </p>
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| 89 | <p>Modifying assignment works with any binary arithmetic operator. For
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| 90 | instance, you can subtract something from an lvalue like this,
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| 91 | </p>
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| 92 | <div class="example">
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| 93 | <pre class="example"><var>lvalue</var> -= <var>expression</var>
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| 94 | </pre></div>
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| 95 |
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| 96 | <p>or multiply it by a certain amount like this,
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| 97 | </p>
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| 98 | <div class="example">
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| 99 | <pre class="example"><var>lvalue</var> *= <var>expression</var>
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| 100 | </pre></div>
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| 101 |
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| 102 | <p>or shift it by a certain amount like this.
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| 103 | </p>
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| 104 | <div class="example">
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| 105 | <pre class="example"><var>lvalue</var> <<= <var>expression</var>
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| 106 | <var>lvalue</var> >>= <var>expression</var>
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| 107 | </pre></div>
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| 108 |
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| 109 | <p>In most cases, this feature adds no power to the language, but it
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| 110 | provides substantial convenience. Also, when <var>lvalue</var> contains
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| 111 | code that has side effects, the simple assignment performs those side
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| 112 | effects twice, while the modifying assignment performs them once. For
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| 113 | instance,
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| 114 | </p>
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| 115 | <div class="example">
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| 116 | <pre class="example">x[foo ()] = x[foo ()] + 5;
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| 117 | </pre></div>
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| 118 |
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| 119 | <p>calls <code>foo</code> twice, and it could return different values each
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| 120 | time. If <code>foo ()</code> returns 1 the first time and 3 the second
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| 121 | time, then the effect could be to add <code>x[3]</code> and 5 and store the
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| 122 | result in <code>x[1]</code>, or to add <code>x[1]</code> and 5 and store the
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| 123 | result in <code>x[3]</code>. We don’t know which of the two it will do,
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| 124 | because C does not specify which call to <code>foo</code> is computed first.
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| 125 | </p>
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| 126 | <p>Such a statement is not well defined, and shouldn’t be used.
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| 127 | </p>
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| 128 | <p>By contrast,
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| 129 | </p>
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| 130 | <div class="example">
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| 131 | <pre class="example">x[foo ()] += 5;
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| 132 | </pre></div>
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| 133 |
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| 134 | <p>is well defined: it calls <code>foo</code> only once to determine which
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| 135 | element of <code>x</code> to adjust, and it adjusts that element by adding 5
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| 136 | to it.
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| 137 | </p>
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| 138 | <hr>
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| 139 | <div class="header">
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| 140 | <p>
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| 141 | Next: <a href="Increment_002fDecrement.html" accesskey="n" rel="next">Increment/Decrement</a>, Previous: <a href="Lvalues.html" accesskey="p" rel="prev">Lvalues</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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| 142 | </div>
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