Multidimensional Array

Multidimensional Array

The declaration of an array of arrays looks like this:

                    int a2[5][7];

You have to read complicated declarations like these ``inside out.'' What this one says is that a2 is an array of 5 something’s, and that each of the something’s is an array of 7 ints. More briefly, ``a2 is an array of 5 arrays of 7 ints,'' or, ``a2 is an array of array of int.'' In the declaration of a2, the brackets closest to the identifier a2 tell you what a2 first and foremost is. That's how you know it's an array of 5 arrays of size 7, not the other way around. You can think of a2 as having 5 ``rows'' and 7 ``columns,'' although this interpretation is not mandatory. (You could also treat the ``first'' or inner subscript as ``x'' and the second as ``y.'' Unless you're doing something fancy, all you have to worry about is that the subscripts when you access the array match those that you used when you declared it, as in the examples below.)

To illustrate the use of multidimensional arrays, we might fill in the elements of the above array a2 using this piece of code:

                    int i, j;

               for(i = 0; i < 5; i = i + 1)

                               {

                               for(j = 0; j < 7; j = j + 1)

                a2[i][j] = 10 * i + j;

                               }

This pair of nested loops sets a[1][2] to 12, a[4][1] to 41, etc. Since the first dimension of a2 is 5, the first subscripting index variable, i, runs from 0 to 4. Similarly, the second subscript varies from 0 to 6.

We could print a2 out (in a two-dimensional way, suggesting its structure) with a similar pair of nested loops:

                    for (i = 0; i < 5; i = i + 1)

                               {

                               for (j = 0; j < 7; j = j + 1)

                                              printf ("%d\t", a2[i][j]);

                               printf ("\n");

                               }

(The character \t in the printf string is the tab character.)

Just to see more clearly what's going on, we could make the ``row'' and ``column'' subscripts explicit by printing them, too:

                    for(j = 0; j < 7; j = j + 1)

                               printf("\t%d:", j);

               printf ("\n");

               for(i = 0; i < 5; i = i + 1)

                               {

                               printf("%d:", i);

                               for(j = 0; j < 7; j = j + 1)

                                              printf("\t%d", a2[i][j]);

                               printf("\n");

                               }

This last fragment would print

        0:      1:      2:      3:      4:      5:      6:

0:      0       1       2       3       4       5       6

1:      10      11      12      13      14      15      16

2:      20      21      22      23      24      25      26

3:      30      31      32      33      34      35      36

4:      40      41      42      43      44      45      46