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22<title>Designated Inits (GNU C Language Manual)</title>
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58<span id="Designated-Inits"></span><div class="header">
59<p>
60Next: <a href="Auto-Type.html" accesskey="n" rel="next">Auto Type</a>, Previous: <a href="Initializers.html" accesskey="p" rel="prev">Initializers</a>, Up: <a href="Variables.html" accesskey="u" rel="up">Variables</a> &nbsp; [<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>
61</div>
62<hr>
63<span id="Designated-Initializers"></span><h3 class="section">20.3 Designated Initializers</h3>
64<span id="index-initializers-with-labeled-elements"></span>
65<span id="index-labeled-elements-in-initializers"></span>
66<span id="index-case-labels-in-initializers"></span>
67<span id="index-designated-initializers"></span>
68
69<p>In a complex structure or long array, it&rsquo;s useful to indicate
70which field or element we are initializing.
71</p>
72<p>To designate specific array elements during initialization, include
73the array index in brackets, and an assignment operator, for each
74element:
75</p>
76<div class="example">
77<pre class="example">int foo[10] = { [3] = 42, [7] = 58 };
78</pre></div>
79
80<p>This does the same thing as:
81</p>
82<div class="example">
83<pre class="example">int foo[10] = { 0, 0, 0, 42, 0, 0, 0, 58, 0, 0 };
84</pre></div>
85
86<p>The array initialization can include non-designated element values
87alongside designated indices; these follow the expected ordering
88of the array initialization, so that
89</p>
90<div class="example">
91<pre class="example">int foo[10] = { [3] = 42, 43, 44, [7] = 58 };
92</pre></div>
93
94<p>does the same thing as:
95</p>
96<div class="example">
97<pre class="example">int foo[10] = { 0, 0, 0, 42, 43, 44, 0, 58, 0, 0 };
98</pre></div>
99
100<p>Note that you can only use constant expressions as array index values,
101not variables.
102</p>
103<p>If you need to initialize a subsequence of sequential array elements to
104the same value, you can specify a range:
105</p>
106<div class="example">
107<pre class="example">int foo[100] = { [0 ... 19] = 42, [20 ... 99] = 43 };
108</pre></div>
109
110<p>Using a range this way is a GNU C extension.
111</p>
112<p>When subsequence ranges overlap, each element is initialized by the
113last specification that applies to it. Thus, this initialization is
114equivalent to the previous one.
115</p>
116<div class="example">
117<pre class="example">int foo[100] = { [0 ... 99] = 43, [0 ... 19] = 42 };
118</pre></div>
119
120<p>as the second overrides the first for elements 0 through 19.
121</p>
122<p>The value used to initialize a range of elements is evaluated only
123once, for the first element in the range. So for example, this code
124</p>
125<div class="example">
126<pre class="example">int random_values[100]
127 = { [0 ... 99] = get_random_number() };
128</pre></div>
129
130<p>would initialize all 100 elements of the array <code>random_values</code> to
131the same value&mdash;probably not what is intended.
132</p>
133<p>Similarly, you can initialize specific fields of a structure variable
134by specifying the field name prefixed with a dot:
135</p>
136<div class="example">
137<pre class="example">struct point { int x; int y; };
138
139struct point foo = { .y = 42; };
140</pre></div>
141
142<p>The same syntax works for union variables as well:
143</p>
144<div class="example">
145<pre class="example">union int_double { int i; double d; };
146
147union int_double foo = { .d = 34 };
148</pre></div>
149
150<p>This casts the integer value 34 to a double and stores it
151in the union variable <code>foo</code>.
152</p>
153<p>You can designate both array elements and structure elements in
154the same initialization; for example, here&rsquo;s an array of point
155structures:
156</p>
157<div class="example">
158<pre class="example">struct point point_array[10] = { [4].y = 32, [6].y = 39 };
159</pre></div>
160
161<p>Along with the capability to specify particular array and structure
162elements to initialize comes the possibility of initializing the same
163element more than once:
164</p>
165<div class="example">
166<pre class="example">int foo[10] = { [4] = 42, [4] = 98 };
167</pre></div>
168
169<p>In such a case, the last initialization value is retained.
170</p>
171<hr>
172<div class="header">
173<p>
174Next: <a href="Auto-Type.html" accesskey="n" rel="next">Auto Type</a>, Previous: <a href="Initializers.html" accesskey="p" rel="prev">Initializers</a>, Up: <a href="Variables.html" accesskey="u" rel="up">Variables</a> &nbsp; [<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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