Pointers in bidimensional array

Question

I have this code for matrix multiplication, but its given me a compilation error. I need t have a function that receives as arguments the pointers for the 3 matrices and their dimension N.

#include <stdio.h>
#define N 100

void matrixmul(int **matA, int **matB, int **matC, int n){

    int i,j,k;

    for(i=0;i<n;i++){   
    for(j=0;j<n;j++){
        matC[i][j] = 0;
        for(k=0;k<n;k++){
        matC[i][j] += matA[i][k] * matB[k][j];
        }
    }
    }

}

int main(){

    int matA[N][N]={1};
    int matB[N][N]={3};
    int matC[N][N];

    int i,j,k;

    matrixmul(&matA, &matB, &matC, N);

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

    return 0;

}

The error is:

teste.c: In function ‘main’:
teste.c:28:5: warning: passing argument 1 of ‘matrixmul’ from incompatible pointer type [enabled by default]
teste.c:5:6: note: expected ‘int **’ but argument is of type ‘int (*)[100][100]’
teste.c:28:5: warning: passing argument 2 of ‘matrixmul’ from incompatible pointer type [enabled by default]
teste.c:5:6: note: expected ‘int **’ but argument is of type ‘int (*)[100][100]’
teste.c:28:5: warning: passing argument 3 of ‘matrixmul’ from incompatible pointer type [enabled by default]
teste.c:5:6: note: expected ‘int **’ but argument is of type ‘int (*)[100][100]’

Have you made an effort to *understand* what the error messages *say*? — n. m. could be an AI, Nov 27 '13 at 14:17
@n.m. error message... or warning message? I am going to try — Jekyll, Nov 27 '13 at 14:19
@Jekyll all warnings are errors *unless you understand precisely what they mean and why it's OK to leave them alone*. — n. m. could be an AI, Nov 27 '13 at 14:19
@n.m I tried... -1 for me... I had to do some extra work to pass the matrix and work with the pointer.... — Jekyll, Nov 27 '13 at 14:26
'int**' p can be seen as an array to pointers to integer while the col vector doesn't have any pointer but it has integers inside so the matrix will never decay to 'int**' but to int*[]. In order to use 'int**' the only trick I found is creating a new object... @n.m. I learned the lesson :) — Jekyll, Nov 27 '13 at 14:45

haccks · Accepted Answer · 2013-11-27T14:38:14.760

Pointers are not arrays.
All of the &matA, &matB and &matC are of type int (*)[100][100] (pointer to an array of 100 arrays of 100 integers) but your function matrixmul is expecting parameters of type int ** (except for N: int type).
Change your function definition to

void matrixmul(int (*matA)[N], int (*matB)[N], int (*matC)[N], int n){ ... }

and call it from main as

 matrixmul(matA, matB, matC, N);

score 2 · Answer 2 · answered Nov 27 '13 at 14:15

2

You can try these changes.

void matrixmul(int matA[][N], int matB[][N], int matC[][N], int n)

and can call this function like this

matrixmul(matA, matB, matC, N);

answered Nov 27 '13 at 14:15

Vishal R

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Ilie NEACSU · Answer 3 · 2013-11-27T14:35:44.913

The error is caused by the fact that a pointer to a pointer is different by an array. An array is a fixed-length collection of objects, which are stored sequentially in memory. An pointer to a pointer even if it can be used as as an array the compiler doesn't guarantee that the memory of the 2d matrix is continuous. One way to decalare your fonction is:

void matrixmul(int matA[][N], int matB[][N], int matC[][N], int n)

If you want to pass de matrix as a pointer to a pointer you have to alloc the memory like this:

int **array = new int *[N];

for(int i = 0; i<N; i++)

    array[i] = new int[N];`

Pointers in bidimensional array

3 Answers3