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    Jonas Altrock ew20b126@technikum-wien.at

    To overview.

    Implementation with malloc-allocated arrays: u7_matrices.c.

    The whole source file u7_matrices_99.c.

    /* Compile with `clang -std=c99 -Wall -o u7_matrices_c99 u7_matrices_c99.c` */
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    Include C-libraries for input/output.

    #include <stdio.h>
#include <stdlib.h>
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    Exercise 7: Arrays

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    For the whole explanation of the exercise task, see u7_matrices.c.

    Here I am only going to highlight the differences to the malloc-based implementation.

    Forward declarations

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    The dimensions of array parameters can reference earlier int parameters, allowing us to specify that the matrix parameters are size by size. We could also pass width and height separately (or rows and cols).

    void read_matrix(int size, int matrix[size][size]);
void print_matrix(int size, int matrix[size][size]);
void matrix_multiply(int size, int m1[size][size], int m2[size][size], int m3[size][size]);
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    main()

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    int main() {
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    We use ‘dynamically sized arrays’ as the type for the matrix variables. This means we have to read in the size first, and then declare them.

        int size;
    scanf("%d", &size);

    int matrix1[size][size], matrix2[size][size], matrix3[size][size];
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    Read in both matrices, multiply into the third variable, and print. This looks exactly the same as in the malloc-version.

        read_matrix(size, matrix1);
    read_matrix(size, matrix2);
    matrix_multiply(size, matrix1, matrix2, matrix3);
    print_matrix(size, matrix3);
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    No need for free here!

    Return code 0 to indicate everything went OK.

        return 0;
}
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    read_matrix()

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    Because we store the matrix in a proper C99 array, we can use the row and col loop variables directly as array indices.

    void read_matrix(int size, int matrix[size][size]) {
    int row, col;

    for (row = 0; row < size; row++) {
        for (col = 0; col < size; col++) {
            scanf("%i", &matrix[row][col]);
        }
    }
}
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    print_matrix()

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    void print_matrix(int size, int matrix[size][size]) {
    int row, col;

    for (row = 0; row < size; row++) {
        for (col = 0; col < size; col++) {
            printf("% 3i", matrix[row][col]);
        }
        printf("\n");
    }
}
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    matrix_multiply

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    void matrix_multiply(int size, int m1[size][size], int m2[size][size], int m3[size][size]) {
    int row, col, i;
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    Here we can see the best readability improvement of the C99 dynamic array syntax:

        for (row = 0; row < size; row++) {
        for (col = 0; col < size; col++) {
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    the scalar product of the row of m1 and column of m2 becomes more obvious by [row][i] and [i][col] than the multiply and add combo used in the linear storage.

                int sum = 0;

            for (i = 0; i < size; i++) {
                sum += m1[row][i] * m2[i][col];
            }

            m3[row][col] = sum;
        }
    }
}