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| 22 | <title>Logical Operators (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="Execution-Control-Expressions.html" rel="up" title="Execution Control Expressions">
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| 33 | <link href="Logicals-and-Comparison.html" rel="next" title="Logicals and Comparison">
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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="Logical-Operators"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Logicals-and-Comparison.html" accesskey="n" rel="next">Logicals and Comparison</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-1"></span><h3 class="section">8.1 Logical Operators</h3>
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| 64 | <span id="index-logical-operators"></span>
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| 65 | <span id="index-operators_002c-logical"></span>
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| 66 | <span id="index-conjunction-operator"></span>
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| 67 | <span id="index-disjunction-operator"></span>
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| 68 | <span id="index-negation-operator_002c-logical"></span>
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| 69 |
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| 70 | <p>The <em>logical operators</em> combine truth values, which are normally
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| 71 | represented in C as numbers. Any expression with a numeric value is a
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| 72 | valid truth value: zero means false, and any other value means true.
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| 73 | A pointer type is also meaningful as a truth value; a null pointer
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| 74 | (which is zero) means false, and a non-null pointer means true
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| 75 | (see <a href="Pointer-Types.html">Pointer Types</a>). The value of a logical operator is always 1
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| 76 | or 0 and has type <code>int</code> (see <a href="Integer-Types.html">Integer Types</a>).
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| 77 | </p>
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| 78 | <p>The logical operators are used mainly in the condition of an <code>if</code>
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| 79 | statement, or in the end test in a <code>for</code> statement or
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| 80 | <code>while</code> statement (see <a href="Statements.html">Statements</a>). However, they are valid
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| 81 | in any context where an integer-valued expression is allowed.
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| 82 | </p>
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| 83 | <dl compact="compact">
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| 84 | <dt>‘<samp>! <var>exp</var></samp>’</dt>
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| 85 | <dd><p>Unary operator for logical “not.” The value is 1 (true) if
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| 86 | <var>exp</var> is 0 (false), and 0 (false) if <var>exp</var> is nonzero (true).
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| 87 | </p>
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| 88 | <p><strong>Warning:</strong> if <code>exp</code> is anything but an lvalue or a
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| 89 | function call, you should write parentheses around it.
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| 90 | </p>
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| 91 | </dd>
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| 92 | <dt>‘<samp><var>left</var> && <var>right</var></samp>’</dt>
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| 93 | <dd><p>The logical “and” binary operator computes <var>left</var> and, if necessary,
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| 94 | <var>right</var>. If both of the operands are true, the ‘<samp>&&</samp>’ expression
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| 95 | gives the value 1 (which is true). Otherwise, the ‘<samp>&&</samp>’ expression
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| 96 | gives the value 0 (false). If <var>left</var> yields a false value,
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| 97 | that determines the overall result, so <var>right</var> is not computed.
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| 98 | </p>
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| 99 | </dd>
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| 100 | <dt>‘<samp><var>left</var> || <var>right</var></samp>’</dt>
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| 101 | <dd><p>The logical “or” binary operator computes <var>left</var> and, if necessary,
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| 102 | <var>right</var>. If at least one of the operands is true, the ‘<samp>||</samp>’ expression
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| 103 | gives the value 1 (which is true). Otherwise, the ‘<samp>||</samp>’ expression
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| 104 | gives the value 0 (false). If <var>left</var> yields a true value,
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| 105 | that determines the overall result, so <var>right</var> is not computed.
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| 106 | </p></dd>
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| 107 | </dl>
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| 108 |
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| 109 | <p><strong>Warning:</strong> never rely on the relative precedence of ‘<samp>&&</samp>’
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| 110 | and ‘<samp>||</samp>’. When you use them together, always use parentheses to
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| 111 | specify explicitly how they nest, as shown here:
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| 112 | </p>
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| 113 | <div class="example">
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| 114 | <pre class="example">if ((r != 0 && x % r == 0)
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| 115 | ||
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| 116 | (s != 0 && x % s == 0))
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| 117 | </pre></div>
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| 118 |
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| 119 | <hr>
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| 120 | <div class="header">
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| 121 | <p>
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| 122 | Next: <a href="Logicals-and-Comparison.html" accesskey="n" rel="next">Logicals and Comparison</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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