How to multiply matrix using pointers - C - c

Question

I have to write a program which multiply matrix using pointers.
My program does it correctly, but there are two problems:
1) I can do only one multiplication - when I want next one there is error
2) when I input data of first and second matrix i have to put one more
char to make my program work.
What should I change in my code to solve those problems?
#include <stdio.h>
#include <stdlib.h>
while (z--) { // the number of sets
scanf ("%d %d", &n1, &m1); // first matrix n1 x m1
int **A = malloc(n1 * sizeof(int*));
for (i = 0; i < n1; i++) {
*(A+i) = malloc(m1 * sizeof(int));
}
for (i = 0; i < n1; i++) {
for (j = 0; j < m1; j++) {
scanf("%d ", (A+i*m1+j));
}
}
scanf("%d %d", &n2, &m2); // second matrix n2 x m2
int **B = malloc(n2 * sizeof(int*));
for (i = 0; i < n2; i++) {
*(B+i) = malloc(m2 * sizeof(int));
}
for (i = 0; i < n2; i++) {
for (j = 0; j < m2; j++) {
scanf("%d ", (B+i*m2+j));
}
}
if (m1 != n2) {
printf ("ERROR\n");
} else {
int **C = malloc(n1 * sizeof(int*)); //matrix with product
for (i = 0; i < n1; i++) {
*(C+i) = malloc(m2 * sizeof(int));
}
for (i=0; i<n1; i++) {
for (j=0; j<m2; j++) {
*(C+i*m2+j) = 0;
}
}
for (k = 0; k < n2; k++) {
for (i = 0; i < n1; i++) {
for (j = 0; j < m2; j++) {
C1 = 0;
A1 = *(A+i*m1+k);
B1 = *(B+k*m2+j);
C1 = (C1+A1*B1)%1000;
temp = *(C+i*m2+j);
*(C+i*m2+j) = temp+C1;
}
}
}
for (i = 0; i < n1; i++) {
for (j = 0; j < m2; j++) {
printf("%d ", *(C+i*m2 +j));
}
printf("\n");
}
for (i = 0; i < n1; i++) { // free third matrix
free(*(C+i));
}
free(C);
}
for (i = 0; i < n1; i++) { // free first matrix
free(*(A+i));
}
free(A);
for (i = 0; i < n2; i++) { // free second matrix
free(*(B+i));
}
free(B);
}
return 0;

Answer 1

You write to memory beyond the allocated size. This invokes undefined behavior, a good explanation for the erratic behavior you observe.
The offending code is this:
for (k=0; k<n2; k++){
for (i=0; i<n1; i++) {
for (j=0; j<m2; j++) {
C1=0;
A1=*(A+i*m1+k);
B1=*(B+k*m2+j);
C1=(C1+A1*B1)%1000;
temp=*(C+i*m2+j);
*(C+i*m2+j)=temp+C1;
}
}
}
You assume that the matrices are allocated as a single packed 2D array.
Since you allocate them as arrays of pointers to arrays of int, you cannot use this code. Use the much simpler version below:
for (k = 0; k < n2; k++){
for (i = 0; i < n1; i++) {
for (j = 0; j < m2; j++) {
C[i][j] += A[i][k] * B[k][j] % 1000;
}
}
}
It is not clear why you compute modulo 1000. If you really mean that, you should use this:
C[i][j] = (C[i][j] + A[i][k] * B[k][j]) % 1000;
If you are not supposed to use the [] syntax, replace it with the equivalent for pointer dereferencing, not 2D array addressing. I understand what they are trying to teach you, but a diagram would be more effective than this.
*(*(C+i)+j) = (*(*(C+i)+j) + *(*(A+i)+k) * *(*(B+k)+j)) % 1000;
As you may have noticed, the [] syntax is the same for 2D arrays and arrays of pointers to arrays, but the code generated for pointer arithmetic and dereferencing is quite different.
There are other places where you need to change the indexing method, here is the corrected code:
while (z--) { // the number of sets
scanf("%d %d", &n1, &m1); // first matrix n1 x m1
int **A = malloc(n1 * sizeof(int*));
for (i = 0; i < n1; i++) {
*(A+i) = malloc(m1 * sizeof(int));
for (j = 0; j < m1; j++) {
scanf("%d ", *(A+i)+j);
}
}
scanf("%d %d", &n2, &m2); // second matrix n2 x m2
int **B = malloc(n2 * sizeof(int*));
for (i = 0; i < n2; i++) {
*(B+i) = malloc(m2 * sizeof(int));
for (j = 0; j < m2; j++) {
scanf("%d ", *(B+i)+j);
}
}
if (m1 != n2) {
printf ("ERROR\n");
} else {
int **C = malloc(n1 * sizeof(int*)); //matrix with product
for (i = 0; i < n1; i++) {
// using calloc so matrix is initialized to 0
*(C+i) = calloc(m2, sizeof(int));
}
for (k = 0; k < n2; k++) {
for (i = 0; i < n1; i++) {
for (j = 0; j < m2; j++) {
*(*(C+i)+j) = (*(*(C+i)+j) + *(*(A+i)+k) * *(*(B+k)+j)) % 1000;
}
}
}
for (i = 0; i < n1; i++) {
for (j = 0; j < m2; j++) {
printf("%d ", *(*(C+i)+j));
}
printf("\n");
}
for (i = 0; i < n1; i++) { // free third matrix
free(*(C+i));
}
free(C);
}
for (i = 0; i < n1; i++) { // free first matrix
free(*(A+i));
}
free(A);
for (i = 0; i < n2; i++) { // free second matrix
free(*(B+i));
}
free(B);
}
return 0;