segmentation fault : matrix multiplication ??

Hi !

I tried to compile this program on red hat 9.0 but I keep getting segementation fault when I run the output. The program multiplies two matricies and is very simple one :

Code:

---

#include<stdio.h>

#include<stdlib.h>



int main()

{

        int **A, **B, **C, row1, row2, col1, col2, sum, i, j, k;

        printf("Enter the rows and columns of matrices:\n");

        printf("Matrix one row and columns\n");

        scanf("%d %d", &row1, &col1);

        printf("Matrix two row and columns\n");

        scanf("%d %d", &row2, &col2);

        if(col1 != row2)

        {

                printf("Matrices cannot be multiplied\n");

                exit(1);

        }

        A = malloc(row1*sizeof(int*));

        B = malloc(row2*sizeof(int*));

        C = malloc(row1*sizeof(int*));

        for(i=0; i<row1; i++)

                A[i] = malloc(col1*sizeof(int));

        for(i=0; i<row2; i++)

                B[i] = malloc(col2*sizeof(int));

        for(i=0; i<row1; i++)

                C[i] = malloc(col2*sizeof(int));

        printf("Enter elements for first matrix row by row\n");

        for(i=0; i<row1; i++)

                for(j=0; j<col1; j++)

                        scanf("%d", &A[i][j]);

    printf("Enter elements for second matrix row by row\n");

        for(i=0; i<row2; i++)

                for(j=0; j<col2; j++)




/*---------MULTIPLICATION------------------------- */





        for(i=0; i<row1; i++)

        {

                for(j=0; j<col2; j++)

                {

                        sum = 0;

                        for(k=0; k<col1; k++)

                                sum += A[i*k][j]*B[i][j*k];

                        C[i][j] = sum;

                }

        }



        for(i=0; i<row1; i++)

        {

                for(j=0; j<col2; j++)

                        printf("%d ", C[i][j]);

                printf("\n");

        }
        return 0;

}

---

Code:

---

printf("Enter elements for second matrix row by row\n");

        for(i=0; i<row2; i++)

                for(j=0; j<col2; j++)




/*---------MULTIPLICATION------------------------- */





        for(i=0; i<row1; i++)

---

Shouldn't you be actually scanning something in there? ;)


Quzah.

inserted the line " scanf("%d", &B[i][j]); "

Still there is a segmentation fault ??????

Repost your updated code. And how about squeezing a lot of that whitespace out so it takes up less space?

Here is the updated code:

Code:

---

#include<stdio.h>
#include<stdlib.h>

int main()
{
        int **A, **B, **C, row1, row2, col1, col2, sum;
        printf("Enter the rows and columns of matrices:\n");
        printf("Matrix one row and columns\n");
        scanf("%d %d", &row1, &col1);
        printf("Matrix two row and columns\n");
        scanf("%d %d", &row2, &col2);
        if(col1 != row2)
        {
                printf("Matrices cannot be multiplied\n");
                exit(1);
        }
        A = malloc(row1* sizeof(int*));
        B = malloc(row2* sizeof(int*));
        C = malloc(row1* sizeof(int*));
        for(int i=0; i<row1; i++)
                A[i] = malloc(col1* sizeof(int));
        for(i=0; i<row2; i++)
                B[i] = malloc(col2* sizeof(int));
        for(i=0; i<row1; i++)
                C[i] = malloc(col2* sizeof(int));
        printf("Enter elements for first matrix row by row\n");
        for(i=0; i<row1; i++)
                for(int j=0; j<col1; j++)
                        scanf("%d", &A[i][j]);
    printf("Enter elements for second matrix row by row\n");
        for(i=0; i<row2; i++)
                for(int j=0; j<col2; j++)
                        scanf("%d", &B[i][j]);
//-------------------------------- MULTIPLICATION ------------------------------------------------------


        for(i=0; i<row1; i++)
        {
                for(int j=0; j<col2; j++)
                {
                        sum = 0;
                        for(int k=0; k<col1; k++)
                                sum += A[i*k][j]*B[i][j*k];
                        C[i][j] = sum;
                }
        }

        for(i=0; i<row1; i++)
        {
                for(int j=0; j<col2; j++)
                        printf("%d ", C[i][j]);
                printf("\n");
        }
        return 0;
}

---

well, it seems to work fine for me, and i am using Dev-c++ compiler. i have made some changes as well

Code:

---

#include<stdio.h>
#include<stdlib.h>

int main()
{
        int **A, **B, **C, row1, row2, col1, col2, sum;
        int i,j,k;
        printf("Enter the rows and columns of matrices:\n");
        printf("Matrix one row and columns\n");
        scanf("%d %d", &row1, &col1);
        printf("Matrix two row and columns\n");
        scanf("%d %d", &row2, &col2);
        if(col1 != row2)
        {
                printf("Matrices cannot be multiplied\n");
                exit(1);
        }
        A = malloc(row1* sizeof(int*));
        B = malloc(row2* sizeof(int*));
        C = malloc(row1* sizeof(int*));
        for(i=0; i<row1; i++)
                A[i] = malloc(col1* sizeof(int));
        for(i=0; i<row2; i++)
                B[i] = malloc(col2* sizeof(int));
        for(i=0; i<row1; i++)
                C[i] = malloc(col2* sizeof(int));
        printf("Enter elements for first matrix row by row\n");
        for(i=0; i<row1; i++)
                for( j=0; j<col1; j++)
                        scanf("%d", &A[i][j]);
    printf("Enter elements for second matrix row by row\n");
        for(i=0; i<row2; i++)
                for(j=0; j<col2; j++)
                        scanf("%d", &B[i][j]);
//-------------------------------- MULTIPLICATION ------------------------------------------------------


        for(i=0; i<row1; i++)
        {
                for( j=0; j<col2; j++)
                {
                        sum = 0;
                        for( k=0; k<col1; k++)
                                sum += A[i*k][j]*B[i][j*k];
                        C[i][j] = sum;
                }
        }

        for(i=0; i<row1; i++)
        {
                for( j=0; j<col2; j++)
                        printf("%d ", C[i][j]);
                printf("\n");
        }
       
        fflush(stdin);
        getchar();
        return 0;
}

/*my ouput
Enter the rows and columns of matrices:
Matrix one row and columns
2
2
Matrix two row and columns
2
2
Enter elements for first matrix row by row
2
2
2
2
Enter elements for second matrix row by row
2
2
2
2
8 8
8 8
*/

---

have declared all the varibales which are used in the loop at the beginning of the main, ;)

what compiler and OS are using ??

s.s.harish

> sum += A[i*k][j]*B[i][j*k];
Isn't i*k going to result in some very large values - much larger than the values you've allocated for?

Code:

---

        for(i=0; i<row1; i++)
        {
                for( j=0; j<col2; j++)
                {
                        sum = 0;
                        for( k=0; k<col1; k++)
                                sum += A[i*k][j]*B[i][j*k];
                        C[i][j] = sum;
                }
        }

---

https://cboard.cprogramming.com/showthread.php?p=409919

Now that's spooky Dave :)

sorry for my dumbness but I still didn't understand what is going on ?

This is following an algorithm in book and it seems to work fine on dev C++ but gives me segementation fault in red hat gcc compiler

my sincere apologies for my dumbness and the stupidity of the text , I researched the algorithm and found that it works only for square matricies, hence both matricies should have equal number of rows and colums .

The program works fine it u enter a 2 X 2 matrix, but surprisingly when i enter a 3 X 3 matrix it gives a crazy result with the last row or element messed up and a 4 X 4 matrix gives a segmentation fault ?????

I ran these on gcc compiler in red hat 9.0.

> sorry for my dumbness but I still didn't understand what is going on ?
Which bit?
The "why it crashes on one machine and not another"?
Because programs are not obliged to crash at the first moment you make a mistake. Life would be a lot simpler if it did, because then you'd get rid of all those bugs.

Well think about how big your array indices get.

Code:

---

                        for( k=0; k<col1; k++) {
                                int p = i * k, q = j * k;
                                printf( "New indices = %d %d\n", p, q );
                                sum += A[p][j]*B[i][q];
                        }

---

Now do THOSE look sensible to you?