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;
		}
	}

http://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?