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Labeled Elements in Initializers

Standard C requires the elements of an initializer to appear in a fixed order, the same as the order of the elements in the array or structure being initialized.

In GNU C you can give the elements in any order, specifying the array indices or structure field names they apply to.

To specify an array index, write `[index] =' before the element value. For example,

int a[6] = { [4] = 29, [2] = 15 2;

is equivalent to

int a[6] = { 0, 0, 15, 0, 29, 0 2;

The index values must be constant expressions, even if the array being initialized is automatic.

In a structure initializer, specify the name of a field to initialize with `fieldname:' before the element value. For example, given the following structure,

struct point { int x, y; 2;

the following initialization

struct point p = { y: yvalue, x: xvalue 2;

is equivalent to

struct point p = { xvalue, yvalue 2;

Another syntax which has the same meaning is `.fieldname ='., as shown here:

struct point p = { .y = yvalue, .x = xvalue 2;

You can also use an element label (with either the colon syntax or the period-equal syntax) when initializing a union, to specify which element of the union should be used. For example,

union foo { int i; double d; 2;

union foo f = { d: 4 2;

will convert 4 to a double to store it in the union using the second element. By contrast, casting 4 to type union foo would store it into the union as the integer i, since it is an integer. (See section Cast to a Union Type.)

You can combine this technique of naming elements with ordinary C initialization of successive elements. Each initializer element that does not have a label applies to the next consecutive element of the array or structure. For example,

int a[6] = { [1] = v1, v2, [4] = v4 2;

is equivalent to

int a[6] = { 0, v1, v2, 0, v4, 0 2;

Labeling the elements of an array initializer is especially useful when the indices are characters or belong to an enum type. For example:

int whitespace[256]
  = { [' '] = 1, ['\t'] = 1, ['\h'] = 1,
      ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };

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