#include <stdio.h>

#define SIZE 4

int read_matrix();
void multiply_matrix();
void output_matrix();

int main()
{
	int A[SIZE][SIZE];
	int A_rows, A_cols; // size of A
	int B[SIZE][SIZE];
	int B_rows, B_cols; // size of B
	int C[SIZE][SIZE];
	int C_rows, C_cols; // size of C
	int err;

	err = read_matrix("Input first matrix:\n", A, &A_rows, &A_cols);
	if (err < 0)
		return 0;
	err = read_matrix("Input second matrix:\n", B, &B_rows, &B_cols);
	if (err < 0)
		return 0;

	if (A_cols != B_rows) { // cannot multiply matrices
		printf("Matrix sizes do not match!\n");
		return 0;
	}
	multiply_matrix(A, A_rows, A_cols, B, B_cols, C);
	output_matrix("The product matrix is:\n", C, A_rows, B_cols);
}


/* Reads a vector */
void read_vector(int vector[], int size)
{
	for (int i = 0; i < size; i++)
		scanf("%d", &vector[i]);
}


/* Reads matrix A from input */
int read_matrix(char *statement, int A[][SIZE], int *rows, int *cols)
{
	printf(statement);
	printf("Enter number of rows: ");
	scanf("%d", rows);
	printf("Enter number of columns: ");
	scanf("%d", cols);
	if ((*rows > SIZE) || (*cols > SIZE)) { // error, exit
		printf("Matrix too large!\n");
		return -1;
	}
	printf("Enter the matrix now:\n");

	for (int i = 0; i < *rows; i++) // read i-th row
		read_vector(A[i], *cols);

	return 0; // all OK
}


/* Calculates C = A * B */
void multiply_matrix(int A[][SIZE], int A_rows, int A_cols, int B[][SIZE], int B_cols, int C[][SIZE])
{
	for (int i = 0; i < A_rows; i++)
		for (int j = 0; j < B_cols; j++) {
			C[i][j] = 0; // initialize
			for (int k = 0; k < A_cols; k++)
				C[i][j] = C[i][j] + A[i][k] * B[k][j];
		}
}


/* Outputs matrix A */
void output_matrix(char *statement, int A[][SIZE], int rows, int cols)
{
	printf(statement);
	for (int i = 0; i < rows; i++) { // output i-th row
		for (int j = 0; j < cols; j++)
			// output j-th element of i-th row
			printf("%d ", A[i][j]);
		printf("\n");
	}
}