Warnings in matrix multiplication using functions

Question

I'm getting three errors:

1. Assignment makes integer from pointer without a cast
2. Passing argument 1 of read from incompatible pointer type
3. Expected int * (*)[10] but argument is of type int (*)[10][10]

Here is the code:

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

void read(int *(arr[10][10]), int row, int col) { //Third error here
    int i, j;
    for (i = 0; i < row; i++)
        for (j = 0; j < col; j++)
            scanf("%d", &arr[i][j]);
}
void multiply(int arr1[10][10], int row1, int col1,
              int arr2[10][10], int row2, int col2,
              int *prod[10][10]) { //Third error here
    int i, j, k, temp;
    for (i = 0; i < row1; i++)
        for (j = 0; j < col2; j++) {
            temp = 0;
            for (k = 0; k < col1; k++)
                temp += arr1[i][k] * arr2[k][j];
            prod[i][j] = temp; //First error here
        }
}
void display(int arr[10][10], int row, int col) {
    int i, j;
    for (i = 0; i < row; i++) {
        for (j = 0; j <col; j++)
            printf("%d\t",arr[i][j]);
        printf("\n");
    }
}
int main() {
    int a[10][10], b[10][10], c[10][10], m, n, p, q, i, j, k;
    printf("Enter the order of matrix A:");
    scanf("%d %d", &m, &n);
    printf("Enter the order of matrix B:");
    scanf("%d %d", &p, &q);
    if (n != p) {
        printf("Matrix multiplication is not possible.");
        exit(0);
    }
    printf("Enter the elements of matrix A:\n");
    read(&a, m, n); //Second error here
    printf("Enter the elements of matrix B:\n");
    read(&b, p, q); //Second error here
    multiply(a, m, n, b, p, q, &c);
    printf("Matrix A is:\n");
    display(a, m, n);
    printf("Matrix B is:\n");
    display(b, p, q);
    printf("Product matrix is:\n");
    display(c, m, q);
    return 0;
}

Answer 1

The type int *(arr[10][10]) for a function argument is not the same as int arr[10][10]. Change the prototype this way:

void read_matrix(int arr[10][10], int row, int col)

You should not name your function read as it is the name of the Posix system call used by the C library to implement the FILE* stream interface. Rename it read_matrix and invoke it this way:

printf("Enter the elements of matrix A:\n");
read_matrix(a,m,n); //Second error here
printf("Enter the elements of matrix B:\n");
read_matrix(b,p,q); //Second error here

The prototype for the matrix multiplication should be changed similarly to:

void multiply(int arr1[10][10], int row1, int col1,
              int arr2[10][10], int row2, int col2,
              int prod[10][10])

The confusion stems from the way arrays are passed to functions in C: they are passed as a pointer to their first element. We say that arrays automatically decay as pointers when used in expressions and passed as function arguments or return values. multiply should be called this way:

multiply(a, m, n, b, p, q, c);

Note that you should use braces { and } for all your for loops whose body has more than one line. Your omitting them for the outer loops in read and multiply are not strictly incorrect, but error prone and not recommended.

You should also check the return value from scanf() and validate the matrix dimensions.

Here is an improved version:

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

int matrix_read(int arr[10][10], int row, int col) {
    int i, j;

    for (i = 0; i < row; i++) {
        for (j = 0; j < col; j++) {
            if (scanf("%d", &arr[i][j]) != 1)
                return -1;
        }
    }
    return 0;
}

int matrix_multiply(int arr1[10][10], int row1, int col1,
                    int arr2[10][10], int row2, int col2,
                    int prod[10][10]) {
    int i, j, k, temp;

    if (col1 != row2)
        return -1;

    for (i = 0; i < row1; i++) {
        for (j = 0; j < col2; j++) {
            temp = 0;
            for (k = 0; k < col1; k++) {
                temp += arr1[i][k] * arr2[k][j];
            }
            prod[i][j] = temp;
        }
    }
    return 0;
}

void matrix_display(int arr[10][10], int row, int col) {
    int i, j;

    for (i = 0; i < row; i++) {
        for (j = 0; j <col; j++) {
            printf("%d\t", arr[i][j]);
        }
        printf("\n");
    }
}

int main(void) {
    int a[10][10], b[10][10], c[10][10], m = 0, n = 0, p = 0, q = 0;

    printf("Enter the order of matrix A: ");
    if (scanf("%d %d", &m, &n) != 2 || m < 1 || m > 10 || n < 1 || n > 10) {
        printf("Invalid matrix size: %d x %d.\n", m, n);
        exit(1);
    }
    printf("Enter the order of matrix B: ");
    if (scanf("%d %d", &p, &q) != 2 || p < 1 || p > 10 || q < 1 || q > 10) {
        printf("Invalid matrix size: %d x %d.\n", p, q);
        exit(1);
    }
    if (n != p) {
        printf("Matrix multiplication is not possible.");
        exit(1);
    }
    printf("Enter the elements of matrix A:\n");
    if (matrix_read(a, m, n)) {
        printf("Invalid matrix data.\n");
        exit(1);
    }
    printf("Enter the elements of matrix B:\n");
    if (matrix_read(b, p, q)) {
        printf("Invalid matrix data.\n");
        exit(1);
    }
    if (matrix_multiply(a, m, n, b, p, q, c)) {
        printf("Matrix multiplication error.\n");
        exit(1);
    }
    printf("Matrix A is:\n");
    matrix_display(a, m, n);
    printf("Matrix B is:\n");
    matrix_display(b, p, q);
    printf("Product matrix is:\n");
    matrix_display(c, m, q);
    return 0;
}