Arrays

We use arrays for vectors. So

   double c[5];

allocates space for five consecutive double precision values called the elements of the array, or components of the vector. Subscripts are indicated with brackets. By convention the first element has subscript zero c[0], so the fifth element is c[4]. This standard is sometimes called zero-base subscripting. It can lead to confusion if you aren't expecting it.

We can also initialize an array in the declaration statement. The initializer is a comma-separated list, enclosed in braces:

   double c[5] = {1.2, -3.7, 5.4, 99.5, -45};

To declare h to be a matrix with 3 rows and 2 columns we write

   double h[3][2];

In Fig.  we show the resulting storage allocation.

Notice that the storage order for the matrix has the last subscript varying fastest.

There are more convenient and elegant ways to handle matrices, but this method works.

A matrix can also be initialized in the declaration. The order of assignment follows the storage order shown in Fig. .

The identifiers c[2] and h[1][2] refer to double precision values. But it is a convention that the unsubscripted identifiers c and h are pointers to the first element. So we could assign the address of c to the pointer variable ap:

   ap = c;

This assignment is equivalent to

   ap = &c[0];

In the former case we should not use an ampersand, because c is already a pointer by convention. In the latter case c[0] is a number, so we need the ampersand to specify its address.

Following this assignment, we could then change the value of the third element of c to 7.5 this way:

   ap[2] = 7.5;

Any pointer can be turned into an implied array this way.