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| 22 | <title>Initializers (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="Variables.html" rel="up" title="Variables">
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| 33 | <link href="Designated-Inits.html" rel="next" title="Designated Inits">
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| 55 | </head>
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| 57 | <body lang="en">
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| 58 | <span id="Initializers"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Designated-Inits.html" accesskey="n" rel="next">Designated Inits</a>, Previous: <a href="Variable-Declarations.html" accesskey="p" rel="prev">Variable Declarations</a>, Up: <a href="Variables.html" accesskey="u" rel="up">Variables</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="Initializers-1"></span><h3 class="section">20.2 Initializers</h3>
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| 64 | <span id="index-initializers"></span>
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| 65 |
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| 66 | <p>A variable’s declaration, unless it is <code>extern</code>, should also
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| 67 | specify its initial value. For numeric and pointer-type variables,
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| 68 | the initializer is an expression for the value. If necessary, it is
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| 69 | converted to the variable’s type, just as in an assignment.
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| 70 | </p>
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| 71 | <p>You can also initialize a local structure-type (see <a href="Structures.html">Structures</a>) or
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| 72 | local union-type (see <a href="Unions.html">Unions</a>) variable this way, from an
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| 73 | expression whose value has the same type. But you can’t initialize an
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| 74 | array this way (see <a href="Arrays.html">Arrays</a>), since arrays are not first-class
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| 75 | objects in C (see <a href="Limitations-of-C-Arrays.html">Limitations of C Arrays</a>) and there is no array
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| 76 | assignment.
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| 77 | </p>
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| 78 | <p>You can initialize arrays and structures componentwise,
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| 79 | with a list of the elements or components. You can initialize
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| 80 | a union with any one of its alternatives.
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| 81 | </p>
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| 82 | <ul>
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| 83 | <li> A component-wise initializer for an array consists of element values
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| 84 | surrounded by ‘<samp>{<span class="roman">…</span>}</samp>’. If the values in the initializer
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| 85 | don’t cover all the elements in the array, the remaining elements are
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| 86 | initialized to zero.
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| 87 |
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| 88 | <p>You can omit the size of the array when you declare it, and let
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| 89 | the initializer specify the size:
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| 90 | </p>
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| 91 | <div class="example">
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| 92 | <pre class="example">int array[] = { 3, 9, 12 };
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| 93 | </pre></div>
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| 94 |
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| 95 | </li><li> A component-wise initializer for a structure consists of field values
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| 96 | surrounded by ‘<samp>{<span class="roman">…</span>}</samp>’. Write the field values in the same
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| 97 | order as the fields are declared in the structure. If the values in
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| 98 | the initializer don’t cover all the fields in the structure, the
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| 99 | remaining fields are initialized to zero.
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| 100 |
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| 101 | </li><li> The initializer for a union-type variable has the form <code>{
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| 102 | <var>value</var> }</code>, where <var>value</var> initializes the <em>first alternative</em>
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| 103 | in the union definition.
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| 104 | </li></ul>
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| 105 |
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| 106 | <p>For an array of arrays, a structure containing arrays, an array of
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| 107 | structures, etc., you can nest these constructs. For example,
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| 108 | </p>
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| 109 | <div class="example">
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| 110 | <pre class="example">struct point { double x, y; };
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| 111 |
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| 112 | struct point series[]
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| 113 | = { {0, 0}, {1.5, 2.8}, {99, 100.0004} };
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| 114 | </pre></div>
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| 115 |
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| 116 | <p>You can omit a pair of inner braces if they contain the right
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| 117 | number of elements for the sub-value they initialize, so that
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| 118 | no elements or fields need to be filled in with zeros.
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| 119 | But don’t do that very much, as it gets confusing.
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| 120 | </p>
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| 121 | <p>An array of <code>char</code> can be initialized using a string constant.
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| 122 | Recall that the string constant includes an implicit null character at
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| 123 | the end (see <a href="String-Constants.html">String Constants</a>). Using a string constant as
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| 124 | initializer means to use its contents as the initial values of the
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| 125 | array elements. Here are examples:
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| 126 | </p>
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| 127 | <div class="example">
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| 128 | <pre class="example">char text[6] = "text!"; /* <span class="roman">Includes the null.</span> */
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| 129 | char text[5] = "text!"; /* <span class="roman">Excludes the null.</span> */
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| 130 | char text[] = "text!"; /* <span class="roman">Gets length 6.</span> */
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| 131 | char text[]
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| 132 | = { 't', 'e', 'x', 't', '!', 0 }; /* <span class="roman">same as above.</span> */
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| 133 | char text[] = { "text!" }; /* <span class="roman">Braces are optional.</span> */
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| 134 | </pre></div>
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| 135 |
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| 136 | <p>and this kind of initializer can be nested inside braces to initialize
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| 137 | structures or arrays that contain a <code>char</code>-array.
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| 138 | </p>
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| 139 | <p>In like manner, you can use a wide string constant to initialize
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| 140 | an array of <code>wchar_t</code>.
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| 141 | </p>
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| 142 | <hr>
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| 143 | <div class="header">
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| 144 | <p>
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| 145 | Next: <a href="Designated-Inits.html" accesskey="n" rel="next">Designated Inits</a>, Previous: <a href="Variable-Declarations.html" accesskey="p" rel="prev">Variable Declarations</a>, Up: <a href="Variables.html" accesskey="u" rel="up">Variables</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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