Thread: Pointers to 2D arrays

Hi, how do I declare pointers to 2D arrays, and how can I then pass these as arguments to a function? The syntax must be different to that of 1D arrays, since I keep getting compiler errors when doing either of these things. Cheers

Code:

int (*ptr)[10][10];

Perhaps?

It really depends on what you actually mean by "pointer to 2D arrays".
You may also mean a pointer to pointer to pointer, if you are dynamically allocating the content of the array.

--
Mats

I'm trying to write a function that takes two (same-size) 2D arrays of numbers and adds them element wise, returning a third 2D array.

using:

Code:

int add(int (*a1)[][],int (*a2)[][])
{...}

produced a compiler error, "array type has incompatible element type"

Code:

int add(int (*a1)[][10], int (*a2)[][10], int (*a3)[][10])

The outer dimension must be specified. Also, you would need an argument for the 3rd array to store the result (you cannot return arrays; only pointers, but that would mean you have to use malloc).

You MUST specify the all dimensions except the one to the most left when passing arrays into a function. If you don't, the compiler will not be able to calculate the positions of the elements in the array.

Edit: And most likely, what you actually want to do is to NOT pass a pointer.

--
Mats

Ah, you are right... >_<
That just goes to show why pointers to arrays tend to be confusing... Grrr.
It's also possible to just "declare" them as 2D arrays...

Code:

int add(int a1[][10], int a2[][10], int a3[][10])

Less confusion, no?

Originally Posted by itCbitC

The innermost subscript is the only one that can be left out. All the others must be specified as in.

Code:

int add(int (*a1)[][10],int (*a2)[][20])

and your return type is an int or a pointer to a 2D array??

I tried this:

Code:

#include <stdio.h>

int add(int (*a1)[][3], int (*a2)[][3], int (*a3)[][3]){
	int i,j;
	for(i=0;i<3;i++){
		for(j=0;j<3;j++){
			a3[i][j]=a1[i][j]+a2[i][j];
		}
	}
return 0;
}

int main (){

	int size1,size2;
	int i,j;	

	printf("enter the size of a square matrix:");
	scanf("%d",&size1);

	int a1[size1][size1];

	for(i=0;i<size1;i++){
		for(j=0;j<size1;j++){
			printf("enter element %d,%d: ",i+1,j+1);
			scanf("%d",&a1[i][j]);		
		}
	}

	printf("enter elements of second matrix:\n");

	int a2[size1][size1];

	for(i=0;i<size1;i++){
		for(j=0;j<size1;j++){
			printf("enter element %d,%d: ",i+1,j+1);
			scanf("%d",&a2[i][j]);		
		}
	}
	
	int a3[size1][size1];

	int *ptr1=&a1;
	int *ptr2=&a2;
	int *ptr3=&a3;

	addm(ptr1,ptr2,ptr3);

	printf("the resultant matrix is:\n\n");

	for(i=0;i<size1;i++){
		for(j=0;j<size1;j++){
			printf("%d ",a3[i][j]);
		}
		printf("\n");	
	}

return 0;
}

which gave me output:

Code:

addm.c: In function ‘add’:
addm.c:7: error: invalid use of array with unspecified bounds
addm.c:7: error: invalid use of array with unspecified bounds
addm.c:7: error: invalid use of array with unspecified bounds
addm.c:7: error: invalid operands to binary +
addm.c: In function ‘main’:
addm.c:43: warning: initialisation from incompatible pointer type
addm.c:44: warning: initialisation from incompatible pointer type
addm.c:45: warning: initialisation from incompatible pointer type

Preferably I want to be able to pass pointers to two arbitrarily but same sized arrays to the function, have it create a third array, and return a pointer to this array. How can I do that?

Thanks

P.S. I'm glad I'm not the only one who's confused ^^

Firstly,

Code:

	int *ptr1=&a1;
	int *ptr2=&a2;
	int *ptr3=&a3;

	addm(ptr1,ptr2,ptr3);

This should just be:

Code:

	addm(a1, a2, a3);

And you want to change your function to...

Code:

int add(int a1[][3], int a2[][3], int a3[][3]){

...to avoid headaches.
The problem stems from the fact that by taking the address of a 2D array, you do not get a raw pointer (ie T*), but rather T (*)[x][y].

And for the first errors... well, array size must be constant, so you'll have to look into malloc and free.

Thanks, I was under the impression I could only pass arrays to functions as pointers. Well if not, great, since it's confusing as hell.

As for the first errors, I thought I had already specified the array size to be constant, with e.g.

Code:

 int a1[][3]

However, this was just a test and what I really want is to be able to pass an arbitrary sized array. I'll have a look into malloc and free, but can you explain why the above is not a constant sized array?

itCbitC, this does not work since it says pointers to the arrays, hence the need to use & in that case.
It also causes trouble inside the function, as it's not mere arrays anymore, but pointers to arrays, so to avoid confusion, I'd go by my example.

bertazoid: It's not the passing of arrays that is the trouble, but the actual creation of them in your main function. The errors clearly point to those lines.