Pointers

Multi Dimensional Arrays & Pointer

We should think of multidimensional arrays in a different way in C:

A 2D array is really a 1D array, each of whose elements is itself an array

Hence

a[n][m] notation.

Array elements are stored row by row.

When we pass a 2D array to a function we must specify the number of columns and the number of rows is irrelevant.

The reason for this is pointers again. C needs to know how many columns in order that it can jump from row to row in memory.

Considerint a[5][35] to be passed in a function:

We can do:

f(int a[][35]) {.....}

or even:

f(int (*a)[35]) {.....}

We need parenthesis (*a) since [] have a higher precedence than *

So:

int (*a)[35]; /*declares a pointer to an array of 35 int */

int *a[35]; /*declares an array of 35 pointers to int */

Now lets look at the (subtle) difference between pointers and arrays. Strings are a common application of this.

Consider:

char *name[10];

char Aname[10][20];

We can legally do name[3][4] and Aname[3][4] in C.