Matrix Multiplication - C - c

Question

I created a program that does matrix addition,subtraction, and multiplication. I have handled the addition and subtraction portions, but when I reached the multiplication I am having trouble outputting the correct values. I have only placed the code with the multiplication function below.
#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
typedef struct {
int *elements;
int rows;
int columns;
} matrix;
void main() {
matrix a, b, c;
void read_matrix(matrix *);
void deallocate(matrix *);
void print(matrix);
matrix add(matrix, matrix);
matrix subtract(matrix, matrix);
matrix multiply(matrix, matrix);
read_matrix(&a);
read_matrix(&b);
/*
c = add(a, b);
printf("The answer of Matrix (a + b) is \n\n");
print(a);
printf("\n +\n\n");
print(b);
printf("\n =\n\n");
print(c);
printf("\n");
deallocate(&c);
c = subtract(a, b);
printf("The answer of Matrix (a - b) is \n\n");
print(a);
printf("\n -\n\n");
print(b);
printf("\n =\n\n");
print(c);
printf("\n");
deallocate(&c);
*/
c = multiply(a, b);
printf("The answer of Matrix (a * b) is \n\n");
print(a);
printf("\n *\n\n");
print(b);
printf("\n =\n\n");
print(c);
printf("\n");
}
void read_matrix(matrix *z) {
int d1, d2, allc, i, x, y, j, val;
int res;
printf("\nWhat is the first dimension of the array? ");
res = scanf("%d", &d1);
if (res != 1) {
fprintf(stderr, "Something went wrong with your first dimension!");
return;
}
printf("What is the second dimension of the array? ");
res = scanf("%d", &d2);
if (res != 1) {
fprintf(stderr, "Something went wrong with your second dimension!");
return;
}
printf("Matrix Dimension is %dx%d\n", d1, d2);
allc = d1*d2;
(*z).elements = (int *)calloc(allc, sizeof(int));
(*z).rows = d1;
(*z).columns = d2;
x = 0;
j = 0;
printf("\n");
for (i = 0; i < d1; i++) {
x++;
for (y = 0; y < d2; y++) {
printf("Enter the value for row %d column %d: ", x, y + 1);
res = scanf("%d", &val);
if (res != 1) {
fprintf(stderr, "Something went wrong while reading value %d\n", x);
return;
}
(*z).elements[j++] = val;
}
}
}
void deallocate(matrix *c) {
free((*c).elements);
(*c).elements = NULL;
(*c).rows = 0;
(*c).columns = 0;
}
void print(matrix z) {
int i, j, x;
x = 0;
for (i = 0; i < z.rows; i++) {
printf("[ ");
for (j = 0; j < z.columns; j++) {
printf("%-4d", z.elements[x++]);
}
printf("]\n");
}
}
matrix multiply(matrix a, matrix b) {
matrix c;
int a1, a2, b1, b2, allc, i, j, x, y, z, alc, addval, val;
i = 0;
j = 0;
alc = 0;
val = 0;
addval = 0;
a1 = a.rows;
a2 = a.columns;
b1 = b.rows;
b2 = b.columns;
allc = (a1 * b2);
c.elements = (int *)calloc(allc, sizeof(int));
c.columns = a1;
c.rows = b2;
if (a2 != b1) {
printf("\n\nThe inner dimensions of your matrices do not match! Multiplication cannot be done!\n\n");
exit(1);
}
for (x = 0; x < c.rows; x++) {
for (y = 0; y < c.rows; y++) {
for (z = 0; z < c.rows; z++) {
i = (i * c.rows);
addval = (a.elements[j]) * (b.elements[i]);
val += addval;
j++;
i++;
}
c.elements[alc] = val;
printf("VAL IS: %d\n\n", val);
val = 0;
i = 0;
alc++;
}
}
printf("\n\n");
return c;
}
In the multiplication function, the triple nested for loop is supposed to go through enough times to print out the correct number of items for the dimension of the new array. I am aware how to do matrix multiplication, but I am not sure if I have represented it correctly here.
The output for an example is:
What is the first dimension of the array? 3
What is the second dimension of the array? 3
Matrix Dimension is 3x3
Enter the value for row 1 column 1: 1
Enter the value for row 1 column 2: 2
Enter the value for row 1 column 3: 3
Enter the value for row 2 column 1: 4
Enter the value for row 2 column 2: 5
Enter the value for row 2 column 3: 6
Enter the value for row 3 column 1: 7
Enter the value for row 3 column 2: 8
Enter the value for row 3 column 3: 9
What is the first dimension of the array? 3
What is the second dimension of the array? 3
Matrix Dimension is 3x3
Enter the value for row 1 column 1: 1
Enter the value for row 1 column 2: 2
Enter the value for row 1 column 3: 3
Enter the value for row 2 column 1: 4
Enter the value for row 2 column 2: 5
Enter the value for row 2 column 3: 6
Enter the value for row 3 column 1: 7
Enter the value for row 3 column 2: 8
Enter the value for row 3 column 3: 9
The answer of Matrix (a * b) is
[ 1 2 3 ]
[ 4 5 6 ]
[ 7 8 9 ]
*
[ 1 2 3 ]
[ 4 5 6 ]
[ 7 8 9 ]
=
[ 216847534336951265054271]
[ 1641572693-138635672036124672]
[ 1352368309-50514195286739134]
Press any key to continue . . .

Answer 1

Below is my rework of your code that does matrix multiplication. The problem was with your multiply() function. You can see that it is doomed with it walking the a matrix by just incrementing the variable j - in a working solution every element of matrix a participates in the multiplication multiple times.
I also changed your routines to all pass pointers to matrix structures rather than your mix of passing pointers and passing/returning matrix structures directly by value. I think this is more consistent and easier to follow but it means you'll need to change your other matrix math routines to work the same, if you keep it this way.
#include <stdio.h>
#include <stdlib.h>
typedef struct {
int *elements;
int rows;
int columns;
} matrix;
void read_matrix(matrix *z) {
int d1, d2, val;
printf("\nWhat is the first dimension of the array? ");
int result = scanf("%d", &d1);
if (result != 1) {
fprintf(stderr, "Something went wrong with your first dimension!\n");
exit(1);
}
printf("What is the second dimension of the array? ");
result = scanf("%d", &d2);
if (result != 1) {
fprintf(stderr, "Something went wrong with your second dimension!\n");
exit(1);
}
printf("Matrix Dimension is %dx%d\n", d1, d2);
z->elements = calloc(d1 * d2, sizeof(int));
z->rows = d1;
z->columns = d2;
int x = 0;
int j = 0;
printf("\n");
for (int i = 0; i < d1; i++) {
x++;
for (int y = 0; y < d2; y++) {
printf("Enter the value for row %d column %d: ", x, y + 1);
result = scanf("%d", &val);
if (result != 1) {
fprintf(stderr, "Something went wrong while reading value (%d, %d)\n", x, y + 1);
exit(1);
}
z->elements[j++] = val;
}
}
}
void deallocate(matrix *c) {
free(c->elements);
c->elements = NULL;
c->rows = 0;
c->columns = 0;
}
void print(matrix *z) {
int x = 0;
for (int i = 0; i < z->rows; i++) {
printf("[ ");
for (int j = 0; j < z->columns; j++) {
printf("%-4d", z->elements[x++]);
}
printf("]\n");
}
}
void multiply(matrix *a, matrix *b, matrix *c) {
if (a->columns != b->rows) {
fprintf(stderr, "The inner dimensions of your matrices do not match! Multiplication cannot be done!\n");
exit(1);
}
c->elements = calloc(a->rows * b->columns, sizeof(int));
c->columns = b->columns;
c->rows = a->rows;
int alc = 0;
for (int x = 0; x < a->rows; x++) {
for (int y = 0; y < b->columns; y++) {
int value = 0;
for (int z = 0; z < b->rows; z++) {
value += a->elements[z + x * a->columns] * b->elements[y + z * b->columns];
}
c->elements[alc++] = value;
}
}
}
int main() {
matrix a, b, c;
read_matrix(&a);
read_matrix(&b);
multiply(&a, &b, &c);
printf("The answer of Matrix (a * b) is \n\n");
print(&a);
printf("\n *\n\n");
print(&b);
printf("\n =\n\n");
print(&c);
printf("\n");
deallocate(&a);
deallocate(&b);
deallocate(&c);
return 0;
}