2.3 Pointers and arrays  (Page 5/8)

Multidimensional arrays

A multidimensional array in C is merely an array whose elements are themselves arrays. The elements of an n-dimensional array are (n-1)-dimensional arrays. For example, each element of a two-dimensional array is a one-dimensional array. The elements of a one-dimensional array, of course, do not have an array type.

A multidimensional array declaration has a pair of brackets for each dimension:

The array screen consists of the 10 elements screen[0] to screen[9]. Each of these elements is a two-dimensional array, consisting in turn of 40 one-dimensional arrays of 80 characters each. All in all, the array screen contains 32,000 elements with the type char.

Two-dimensional arrays are also called matrices. Because they are so frequently used, they merit a closer look. It is often helpful to think of the elements of a matrix as being arranged in rows and columns. Thus the matrix mat in the following definition has three rows and five columns:

The three elements mat[0], mat[1], and mat[2] are the rows of the matrix mat. Each of these rows is an array of five float elements. Thus the matrix contains a total of 3 x 5 = 15 float elements, as the following diagram illustrates:

Accessing array elements

The subscript operator [ ] provides an easy way to address the individual elements of an array by index. If myArray is the name of an one dimensional array and i is an integer, then the expression myArray[i]designates the array element with the index i. Array elements are indexed beginning with 0. Thus, if len is the number of elements in an array, the last element of the array has the index len-1.

The following code fragment defines the array myArray and assigns a value to each element.

The diagram in [link] illustrates the result of this assignment loop.

To access a char element in the three-dimensional array screen, you must specify three indices. For example, the following statement writes the character Z in a char element of the array:

Initializing arrays

If you do not explicitly initialize an array variable, the usual rules apply: if the array has automatic storage duration, then its elements have undefined values. Otherwise, all elements are initialized by default to the value 0.

• You cannot include an initialization in the definition of a variable-length array.
• If the array has static storage duration, then the array initializers must be constant expressions. If the array has automatic storage duration, then you can use variables in its initializers.
• You may omit the length of the array in its definition if you supply an initialization list. The array's length is then determined by the index of the last array element for which the list contains an initializer. For example, the definition of the array a in the previous example is equivalent to this: int a[ ] = { 1, 2, 4, 8 }; // An array with four elements.
• If the definition of an array contains both a length specification and an initialization list, then the length is that specified by the expression between the square brackets. Any elements for which there is no initializer in the list are initialized to zero (or NULL, for pointers). If the list contains more initializers than the array has elements, the superfluous initializers are simply ignored.
• A superfluous comma after the last initializer is also ignored.
• As a result of these rules, all of the following definitions are equivalent: int a[4] = {1, 2};int a[ ] = {1, 2, 0, 0};int a[ ] = {1, 2, 0, 0, };int a[4] = {1, 2, 0, 0, 5};