Thread: 2D array pointers

I need some help with some 2D pointers and passing arrays to functions. Heres the code i got so far but i get a segmentation fault:

Code:

#include <stdio.h>

float avg(int **array, int row, int col)
{
	int i,j, count=0;
	float calculation = 0, average;
	for(i=0; i<row; i++)
	{
		for(j=0; j<col; j++)
		{
			calculation += array[i][j];
			count++;
		}
	}
	average = calculation/count;
	return average;
}

int main(void)
{
	int i, j;
	int row=3, col=3, array[row][col];
	for(i=0; i<row; i++)
	{
		for(j=0; j<col; j++)
		{
			printf("Enter value %d,%d \n", i, j);
			scanf("%d",&array[i][j]);
		}	
	}
	float average = avg(&array[0][0], row, col);
}

If array is defined as int ** (which, in your function, it is), then "array[i][j]" doesn't do what you want it to do. Namely, it will interpret your array[i] -- the number i spots away from where you started -- as a pointer, not an integer, and will then try to follow that pointer. It's unlikely that you typed in, not only a valid pointer, but the pointers where the right things are stored in memory.

In summary: int ** and int[][] are not compatible.

What would be the fix as I then tried this: (*(*(array+i)+j))
and still got the error

You have to make them match: either the function says

Code:

float avg(int array[][3], int row, int col)

or the variable says

Code:

int **array;

I posted literally in the last 24 hours how to fake a 2D array using int** in the middle of a thread with a long discussion of why and what, so search.

The function parameters should stay how i have them.
I tried this:
...
value = &array[i][j];
calculation += value;
....

Do i need to type cast value? what do i type cast to?

If the function parameters must stay, then array[row][col] CANNOT work. There is no way to take a 2D array and make it something that an int** can understand. You MUST use int** throughout.

Edit: Unless you are willing to forgo double-subscripting and calculate all the offsets yourself. (I.e., instead of saying array[2][1] you say something like array[2*row+1].) You'd have to cast your int** down to an int* first, though.

but i tried this:
calculation += (*(*(array+i)+j));

But i still get segmentation fault?

Well, yes. Because (*(*(array+i)+j)) makes no sense. Again, you're using the values you typed in via the for-loop as pointers, and then trying to follow those pointers.... there's nothing there, except the Land of Segfaults and Broken Dreams.

How do i get that bit working. I think i need to cast something but not sure

Have you read any of these replies?

Originally Posted by tabstop

Edit: Unless you are willing to forgo double-subscripting and calculate all the offsets yourself. (I.e., instead of saying array[2][1] you say something like array[2*row+1].) You'd have to cast your int** down to an int* first, though.

If for whatever reason you are unable to change the function header and the variable itself (which I still recommend, since this is apparently supposed to be dynamic in some way), then you'll have to put the pointer in an int* variable and calculate the offsets yourself.

Thats where am confused, how do i do that?

Originally Posted by taurus

Thats where am confused, how do i do that?

Do you understand how a 2-D array is laid out in memory?

Say I define array[3][3]. Then memory contains
array: [0][0] [0][1] [0][2] [1][0] [1][1] [1][2] [2][0] [2][1] [2][2]
in that order. Your job is, given [i][j], to find the offset (which item in the list it is).

How come i cant do this then
Calculation += *pointer

given Calculation is an int
?

You can. But you need to have the pointer first, and brother, (*(array+i)+j) ain't it.