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| 22 | <title>Logicals and Comparison (GNU C Language Manual)</title>
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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="Execution-Control-Expressions.html" rel="up" title="Execution Control Expressions">
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| 33 | <link href="Logicals-and-Assignments.html" rel="next" title="Logicals and Assignments">
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| 34 | <link href="Logical-Operators.html" rel="prev" title="Logical Operators">
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| 57 | <body lang="en">
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| 58 | <span id="Logicals-and-Comparison"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Logicals-and-Assignments.html" accesskey="n" rel="next">Logicals and Assignments</a>, Previous: <a href="Logical-Operators.html" accesskey="p" rel="prev">Logical Operators</a>, Up: <a href="Execution-Control-Expressions.html" accesskey="u" rel="up">Execution Control 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="Logical-Operators-and-Comparisons"></span><h3 class="section">8.2 Logical Operators and Comparisons</h3>
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| 64 |
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| 65 | <p>The most common thing to use inside the logical operators is a
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| 66 | comparison. Conveniently, ‘<samp>&&</samp>’ and ‘<samp>||</samp>’ have lower
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| 67 | precedence than comparison operators and arithmetic operators, so we
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| 68 | can write expressions like this without parentheses and get the
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| 69 | nesting that is natural: two comparison operations that must both be
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| 70 | true.
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| 71 | </p>
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| 72 | <div class="example">
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| 73 | <pre class="example">if (r != 0 && x % r == 0)
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| 74 | </pre></div>
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| 75 |
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| 76 | <p>This example also shows how it is useful that ‘<samp>&&</samp>’ guarantees to
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| 77 | skip the right operand if the left one turns out false. Because of
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| 78 | that, this code never tries to divide by zero.
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| 79 | </p>
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| 80 | <p>This is equivalent:
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| 81 | </p>
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| 82 | <div class="example">
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| 83 | <pre class="example">if (r && x % r == 0)
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| 84 | </pre></div>
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| 85 |
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| 86 | <p>A truth value is simply a number, so <code>r</code>
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| 87 | as a truth value tests whether it is nonzero.
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| 88 | But <code>r</code>’s meaning is not a truth value—it is a number to divide by.
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| 89 | So it is better style to write the explicit <code>!= 0</code>.
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| 90 | </p>
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| 91 | <p>Here’s another equivalent way to write it:
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| 92 | </p>
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| 93 | <div class="example">
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| 94 | <pre class="example">if (!(r == 0) && x % r == 0)
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| 95 | </pre></div>
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| 96 |
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| 97 | <p>This illustrates the unary ‘<samp>!</samp>’ operator, and the need to
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| 98 | write parentheses around its operand.
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| 99 | </p>
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| 100 |
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| 101 |
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| 102 |
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| 103 | </body>
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| 104 | </html>
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