07-27-10 - 2d arrays

Some little things I often forget and have to search for.

Say you need to change a stack two dimensional array :

int array[7][3];

into a dynamically allocated one, and you don't want to change any code. Do you know how? It's :

int (*array) [3] = (int (*) [3]) malloc(sizeof(int)*3*7);

It's a little bit cleaner if you use a typedef :

typedef int array_t [3];

int array[7][3];

array_t array[7];

array_t * array = (array_t *) malloc(7*sizeof(array_t));

those are all equivalent (*).

You can take a two dimensional array as function arg in a few reasonable ways :

void func(int array[7][3]) { }

void func(int (*array)[3]) { }

void func(int array[][3]) { }

function arg arrays are always passed by address.

2-d arrays are indexed [row][col] , which means the first index takes big steps and the second index takes small steps in memory.

(* = if your compiler is some fucking rules nazi, they are microscopically not quite identical, because array[rows][cols] doesn't actually have to be rows*cols ints all in a row (though I'm not sure how this would ever actually not be the case in practice)).