Matrix Multiplication in using 2D arrays

Question

I am trying to learn 2d arrays. I have stumbled upon a problem. It says to read two matrices of order a*b and m*n using a function named readMatrix(), multiply the matrices using processMatric() and show the result using showMatrix().

I am trying to solve it using only arrays.

Here's what I tried to do.

#include <stdio.h>
#include<math.h>
void readMatrix();
void processMatrix(int p[][100],int q[][100],int a,int b,int m,int n);
void showMatrix(int c[][100],int a,int n);

int main() {
    
     readMatrix();
}
void readMatrix()
{
    int i,j,m,n,a,b;
    
    printf("For matrix 1 \n");
    
    printf("Enter the number of rows: ");
    scanf("%d",&a);
    
    printf("Enter the number of columns: ");
    scanf("%d",&b);
    
    printf("For matrix 2 \n");
    
    printf("Enter the number of rows: ");
    scanf("%d",&m);
    
    printf("Enter the number of columns: ");
    scanf("%d",&n);
    int p[a][b];
    int q[m][n];
    if (b == m)
    {
        printf("For 1st Matrix\n");
    
        for(i = 0;i<a;i++)
        {
            for(j = 0; j < b;j++)
            {
                printf("%d row %d column: ",i+1,j+1);
                scanf("%d",&p[i][j]);
            }
        }
        printf("For 2nd Matrix\n");

        for(i = 0;i < m;i++)
        {
            for(j = 0; j < n;j++)
            {
                printf("%d row %d column: ",i+1,j+1);
                scanf("%d",&q[i][j]);
            }
        }
    }
    else
    {   
        printf("Enter a valid dimension. ");
    }    
    processMatrix(p,q,a,b,m,n);
}

void processMatrix(int p[][100],int q[][100],int a,int b,int m,int n)
{
    int i,j,k;
    int c[a][n];
    for(i=0;i < a;i++)
    {   
        for(j = 0; j < n;j++)
        {
            c[i][j]=0;
            for(k = 0; k < b;k++)
            {
                c[i] [j] = c[i][j] + p[i][k] * q[k][j];
            }
        }
    }
    showMatrix(c,a,n);
}

void showMatrix(int c[][100],int a,int n)
{
    int i,j;
    for(i = 0; i < a; i++)
    {
        for(j = 0; j< n; j++)
            printf("%d ",c[i][j]);
    }
    printf("\n");
}

This program does read the elements but displays grabage value. Where am I going wrong?

Answer 1

You define the arrays with dimensions of a by b and m by n:

int p[a][b];
int q[m][n];

but you pass them to routines expecting something by 100:

void processMatrix(int p[][100],int q[][100],int a,int b,int m,int n);
void showMatrix(int c[][100],int a,int n);

That cannot work unless b and n are 100. Defining a matrix as p[a][b] says there are a rows each of which contains b int. If row 0 starts at location x in memory, then row 1 starts b elements beyond x. But, when processMatrix receives location x, it expects row 1 to start 100 elements beyond x. So it reads data from the wrong location in memory.

Since your C compiler appears to support variable length arrays, simply declare your routines to use the proper dimensions:

void processMatrix(int a, int b, int m, int n, int p[a][b], int q[m][n]);

Note the first dimension in each case, a in p and m in q, is not necessary for the compiler, as it does not need to know how big the array is, just how its elements are laid out. However, including it may help make the intent of the program clear.

(I did not check your code for additional problems beyond this one.)