Thread: Hi, problem with pointers and matrix multiplication

Here is my code:

Code:

#include <stdio.h>
#include <stdlib.h>
int main(void)
{
        FILE *infile;
        int row, col, rows, cols;
        double **a, *b, *c, sum;
/* Define infile */
        infile = fopen("matrix5.dat", "r");
/* read rows and cols */
        fscanf(infile, "%d  %d", &rows, &cols);
        printf("Rows = %d  Cols = %d\n", rows, cols);

/* Allocate memory to **a */
        a=( double **)calloc((size_t)cols, sizeof(double *));
/* Allocate memory for a[row], row=0, 1, ..., rows-1 */
        for(row=0; row<rows; row++){
                a[row] = (double *)calloc((size_t)cols, sizeof(double));
        }
/* Allocate memory for b[rows] and c[rows] */
        b=(double *)calloc((size_t)cols, sizeof(double *));
        c=(double *)calloc((size_t)cols, sizeof(double *));
/* read all rows and save the values in a[][] */

        for(row=0; row<rows; row++){
                for(col=0; col<cols; col++)
                        fscanf(infile, "%lf", a[row]+col);
        }

/* read b[] from the last line */
        for(col=0; col<cols; col++){
                fscanf(infile, "%lf", b+col);
        }
/* print a[][] */
        for(row=0; row<rows; row++){
                for(col=0; col<cols; col++)
                        printf("%9.3f ", a[row][col]);
                printf("\n");
        }

/* print b[] */
        printf("\n");
        for(col=0; col<cols; col++){
                printf("%9.3f ", b[col]);
        }

/* compute c[] = a[][] * b[] */
        for(row=0; row<rows; row++){
                for(col=0; col<cols; col++){
                        sum += a[row][col]*b[col];
                }
                c[row]=sum;
                sum=0;
        }
/* print c[] */
        printf("\n");
        for(row=0; row<rows; row++){
                printf("%9.3f ", c[row]);
        }
        printf("\n");
/* return memory */

/* close infile */
fclose(infile);
}

When it multiplies the matrix, b[5] seems to take on the value of the previous answer, which was 44, throwing the rest of the answers off.

a[][] is a 7x7 matrix and b[] is a 1x7 matrix.
I seriously do not know what is wrong, I've been at it for hours. I'm using vi editor, compile with gcc.
Thanks!

sum is used uninitialized.


Quzah.

Code:

/* Allocate memory for b[rows] and c[rows] */
        b=(double *)calloc((size_t)cols, sizeof(double *));
        c=(double *)calloc((size_t)cols, sizeof(double *));

And these are the wrong size.
They should be sizeof(double)

Also, calloc for doubles does not guarantee that the resultant memory has a value of 0.0
Question 7.31

Also, you should not be casting the return result of malloc in a C program.
Question 7.7b


You can fix both things easily with

Code:

b = malloc( cols * sizeof *b );
c = malloc( cols * sizeof *c );

Using the "dereferenced pointer" makes the compiler work out what size is really needed.
And no, this is not dereferencing a variable at run time - this is all in the compiler at compile time.