How do you write a C program to add 2 matrices using functions?

Answer 1

We'll assume the matrix elements are doubles, but we can easily adapt the code to cater for any numeric data type.

First we need a (primitive) function that emulates the += operator for two arrays of doubles:

double* add_assign_array (double* a, double* b, size_t sz) {

for (size_t i=0; i<sz; ++i) a[i] += b[i];

return a;

}

Note that we are wholly reliant upon the caller to ensure all arguments are valid. We could test for null pointer arguments, however there's no advantage in doing so when we cannot even guarantee that a and b actually refer to at least sz elements. For efficiency it's better if the caller handles any and all necessary runtime tests and thus keep those tests to a minimum.

With this function in place we can now add two matrices, row by row:

double* add_assign_matrix (double* a, double* b, size_t rows, size_t cols) {

size_t i;

for (size_t row=0; row<rows; ++row) {

i = row * cols;

add_assign_array (a[i], b[i], cols);

}

return a;

}

Example usage:

// Utility functions: void print_array (double*, size_t);

void print_matrix (double*, size_t, size_t);

int main (void) {

const size_t rows = 3;

const size_t cols = 4;

double a[rows][cols] = {{1, 2, 3, 4}, {5, 6, 7, 8}, {9, 10, 11, 12}};

double b[rows][cols] = {{13, 14, 15, 16}, {17, 18, 19, 20}, {21, 22, 23, 24}};

printf ("Matrix a:\n");

print_matrix (a, rows, cols);

printf ("Matrix b:\n");

print_matrix (b, rows, cols);

printf ("Matrix a+=b:\n");

add_assign_matrix (a, b, rows, cols);

print_matrix (a, rows, cols);

return 0;

}

void print_array (double* a, size_t sz) {

for (size_t i=0; i<sz; ++i) printf ("%f\t") a[i];

printf ("\n");

}

void print_matrix (double* a, size_t rows, size_t cols) {

for (size_t row=0; row<rows; ++row) print_array (a[row * cols], cols);

}

Note that the add_assign function emulates a += b rather than c = a + b. However, we can easily emulate this by copying one of the matrices and then calling add_assign upon the copy:

// e.g., c = a + b;

double c[rows][cols]; // uninitialised matrix

memcpy (c, a, rows * cols * sizeof (double)); // c is a copy of a

add_assign_matrix (c, b, rows, cols); // c += b

It's far from intuitive but arrays and matrices are anything but intuitive in C Programming.