Thread: 2D Dynamically allocated pointer arrays

No. It just creates a single dimension array. That's what you were doing, and like I said, free wasn't the problem. Look at the bottom of page one, where someone asked me how to allocate one. You've been shown how already. You're just ignoring what everyone's been telling you.

You will of course have to free the memory in reverse of how you allocate it. (Free each row's data, then free the main pointer.)

Quzah.

Could you explain that example?

Code:

char **twodee;
int numofrows = 5, numofcols = 10, x;

twodee = malloc( sizeof( char * ) * numofrows );
for( x = 0; x < numofrows; x++ )
    twodee[ x ] = malloc( sizeof( char ) * numofcols );

I don't understand the use of 2 asterisks in the first line.

Also, when you use the first malloc you have "char *", and in the second malloc you have "char" (without the asterisk). Why is that?

mw

PS: Just to clarify: the "rows" is the first subscript and the "columns" is the second subscript, right?

Originally Posted by Lionmane

I don't understand the use of 2 asterisks in the first line.

It is a pointer to a pointer, used to allocate an array of pointers.

Originally Posted by Dave_Sinkula

http://www.eskimo.com/~scs/C-faq/q6.16.html

It is usually best to allocate an array of pointers, and then initialize each pointer to a dynamically-allocated ``row.'' Here is a two-dimensional example:

Code:

#include <stdlib.h>

	int **array1 = (int **)malloc(nrows * sizeof(int *));
	for(i = 0; i < nrows; i++)
		array1[i] = (int *)malloc(ncolumns * sizeof(int));

(In real code, of course, all of malloc's return values would be checked.)

Originally Posted by Lionmane

Thanks Dave. I looked at that website, but I didn't follow what it was trying to say.

From my first introduction to clc's FAQ to the present there have been and may yet be some things that I don't quite fully follow either. The more I learn, the more I find out that what is there is very succinctly put. These days I heed the advice first, and do my damndest to understand what may be confusing as I progress.