A C program meant to find the product of matrices isn't working

Question

The lines of codes that come after the input of the elements of the matrix 'b' are not being executed. Also the nothing happens on pressing the enter button. I wanted t know if this is a logic error or a semantic error.

#include<stdio.h>
main()
{
    int a[10][10],b[10][10],c[10][10],i,j,k,m1,m2,n1,n2;
    printf("Enter the order of the matrix a: ");
    scanf("%d%d",&m1,&n1);
    printf("Enter the order of the matrix b: ");
    scanf("%d%d",&m2,&n2);
    //test for eligibility of matrices for multiplication
    if(n1==m2)
    {
        //input of the elements of matrix a
        printf("Enter the elements of the matrix a: ");
        for(i=0;i<m1;i++)
        {
            for(j=0;j<n1;j++)
                scanf("%d",&a[i][j]);
        }
        //Input of the elements of the matrix b
        printf("Enter the elements of the matrix b: ");
        for(i=0;i<m2;i++)
        {
            for(j=0;j<n2;j++)
                scanf("%d",&b[i][j]);
        }
        //calculating the result of multiplication of matrices 'a' and 'b'
        printf("The result of multiplication of 'a' and 'b' is: ");
        for(i=0;i<n1;i++)
        {
            for(j=0;j<m2;j++)
            {
                for(k=0;k<n2;k++)
                {
                    c[i][j]=c[i][j]+a[i][k]*b[k][j];
                    scanf("%d",&c[i][j]);
                }
            }
        }
        //print the resultant matrix
        for(i=0;i<m1;i++)
        {
            for(j=0;j<n2;j++)
            {
                printf("%4d",c[i][j]);
            }
        }
    }
    else
        printf("The matrices are not eligible for multiplication");
}

`c[i][j]=c[i][j]+a[i][k]*b[k][j];` -- what's the value of `c[i][j]` at the first iteration *before* you assign something to it? `scanf("%4d",&c[i][j];` what does this do exactly? — n. m. could be an AI, Aug 15 '16 at 10:04
I am very sorry for the mistake due to oversight. But even after the correction I am getting the same exact result as before. — Chetan Pujar, Aug 15 '16 at 10:08
What correction? Edit your question so that it contains up-to-date code. — n. m. could be an AI, Aug 15 '16 at 10:10
You mean I need to initialize the elements of the matrix 'c' to zeroes. — Chetan Pujar, Aug 15 '16 at 10:12
Boy, there are _syntax errors_ in the code posted. (#7, #35), it's not even compiling... — Koshinae, Aug 15 '16 at 10:12
You are using wrong variables in your loop. It should be m1 instead of n1 and n2 instead of m2 and n1 or m2 instead of n2! — kiner_shah, Aug 15 '16 at 10:13
Yes you do have to initialize everything you use. If you did, please edit the code in your question so that it's up to date. What's about the `scanf`? — n. m. could be an AI, Aug 15 '16 at 10:22
@n.m. There were two missing `"`-s, but in the 2nd edit they were introduced. — Koshinae, Aug 15 '16 at 10:26
I have edited the code as per the suggestions of @n.m. and Koshinae. Now it works perfectly. Thank you for helping me out at the cost of your time. — Chetan Pujar, Aug 15 '16 at 10:41

score 0 · Accepted Answer · answered Aug 15 '16 at 10:31

Okay, I took the time to fix it up:

#include <stdio.h>
#include <string.h> // New, for memset()

int main(void) // "Proper" signature
{
    int a[10][10],b[10][10],c[10][10],i,j,k,m1,m2,n1,n2;
    memset(c, 0, 100); // New, Ug Lee way of zeroing of the array
    printf("Enter the order of the matrix a: ");
    scanf("%d%d",&m1,&n1);
    printf("Enter the order of the matrix b: ");
    scanf("%d%d",&m2,&n2);
    //test for eligibility of matrices for multiplication
    if(n1==m2)
    {
        //input of the elements of matrix a
        printf("Enter the elements of the matrix a: ");
        for(i=0;i<m1;i++)
        {
            for(j=0;j<n1;j++)
                scanf("%d",&a[i][j]);
        }
        //Input of the elements of the matrix b
        printf("Enter the elements of the matrix b: ");
        for(i=0;i<m2;i++)
        {
            for(j=0;j<n2;j++)
                scanf("%d",&b[i][j]);
        }
        //calculating the result of multiplication of matrices 'a' and 'b'
        printf("The result of multiplication of 'a' and 'b' is: \n");
        for(i=0;i<n1;i++)
        {
            for(j=0;j<m2;j++)
            {
                for(k=0;k<n2;k++)
                {
                    c[i][j]=c[i][j]+a[i][k]*b[k][j];
                    // scanf removed
                }
            }
        }
        //print the resultant matrix
        for(i=0;i<m1;i++)
        {
            for(j=0;j<n2;j++)
            {
                printf("%4d ",c[i][j]);
            }
            printf("\n"); // New, add newline
        }
    }
    else
        printf("The matrices are not eligible for multiplication");
}

The algorithm itself is fundamentally good. Your next step is to improve the code to use calloc for array allocation for the exact size you need for the matrices.

Thank you for letting me know of the alternate way of initializing the array to zero. — Chetan Pujar, Aug 15 '16 at 10:43
@ChetanPujar Remember, that when you use `calloc`, it specifically does the zeroing out, but the other `*alloc`-s do not. — Koshinae, Aug 15 '16 at 10:51
Note: `calloc` zeroes all **bits**. For integers this also means to set the values to zero (they are defined that way), but that is not necessarily true for other types like floating point or pointers (_null pointer_). Oh, and: code-only answers are not really helpful to OP, less other readers. — too honest for this site, Aug 15 '16 at 12:28
What do you mean with "tried without it"? That's why I clearly wrote that it is fine for integers as used here. Btw., `memset` has the same issue. — too honest for this site, Aug 15 '16 at 14:14
And for the `memset` issue: http://stackoverflow.com/a/4630036/357403 by this, I assume we can do it. — Koshinae, Aug 15 '16 at 14:33

A C program meant to find the product of matrices isn't working

1 Answers1