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Multidimensional Arrays (C)

A subscript expression can also have multiple subscripts, as follows:

expression1 [expression2] [expression3]...

Subscript expressions associate from left to right. The leftmost subscript expression, expression1[expression2], is evaluated first. The address that results from adding expression1 and expression2 forms a pointer expression; then expression3 is added to this pointer expression to form a new pointer expression, and so on until the last subscript expression has been added. The indirection operator (*) is applied after the last subscripted expression is evaluated, unless the final pointer value addresses an array type (see examples below).

Expressions with multiple subscripts refer to elements of "multidimensional arrays." A multidimensional array is an array whose elements are arrays. For example, the first element of a three-dimensional array is an array with two dimensions.

For the following examples, an array named prop is declared with three elements, each of which is a 4-by-6 array of int values.

int prop[3][4][6];
int i, *ip, (*ipp)[6];

A reference to the prop array looks like this:

i = prop[0][0][1];

The example above shows how to refer to the second individual int element of prop. Arrays are stored by row, so the last subscript varies most quickly; the expression prop[0][0][2] refers to the next (third) element of the array, and so on.

i = prop[2][1][3];

This statement is a more complex reference to an individual element of prop. The expression is evaluated as follows:

  1. The first subscript, 2, is multiplied by the size of a 4-by-6 int array and added to the pointer value prop. The result points to the third 4-by-6 array of prop.

  2. The second subscript, 1, is multiplied by the size of the 6-element int array and added to the address represented by prop[2].

  3. Each element of the 6-element array is an int value, so the final subscript, 3, is multiplied by the size of an int before it is added to prop[2][1]. The resulting pointer addresses the fourth element of the 6-element array.

  4. The indirection operator is applied to the pointer value. The result is the int element at that address.

These next two examples show cases where the indirection operator is not applied.

ip = prop[2][1];

ipp = prop[2];

In the first of these statements, the expression prop[2][1] is a valid reference to the three-dimensional array prop; it refers to a 6-element array (declared above). Since the pointer value addresses an array, the indirection operator is not applied.

Similarly, the result of the expression prop[2] in the second statement ipp = prop[2]; is a pointer value addressing a two-dimensional array.

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