why does GCC “expect an expression”?

Question

#define rows 2
#define cols 2
#define NUM_CORNERS 4

int main(void) {
    int i;
    int the_corners[NUM_CORNERS];
    int array[rows][cols] = {{1, 2}, {3, 4}};
    corners(array, the_corners);
    for (i = 0; i < 4; i++) printf("%d\n", the_corners[i]);
}

int corners (int array[rows][cols], int the_corners[]) {
    the_corners = {
        array[0][cols-1],
        array[0][0],
        array[rows-1][0],
        array[rows-1][cols-1]
    };
}

I get these weird errors and i have no idea why:

prog.c: In function ‘main’:
prog.c:10: warning: implicit declaration of function ‘corners’
prog.c: In function ‘corners’:
prog.c:15: error: expected expression before 

Answer 1

You're trying to use an initialiser expression as an assignment. This isn't valid, even in C99, because the type of the_corners is int* , not int[4] . In this case you would be best off assigning each element individually.

Answer 2

The the_corners = {... } syntax is an array initialization, not an assignment. I don't have a copy of the standard handy so I can't quote chapter and verse but you want to say this:

void corners (int array[rows][cols], int the_corners[]) {
    the_corners[0] = array[0][cols-1];
    the_corners[1] = array[0][0];
    the_corners[2] = array[rows-1][0];
    the_corners[3] = array[rows-1][cols-1];
}

I also took the liberty of changing int corners to void corners as you weren't returning anything. And your main also needs a return value and you forgot to #include <stdio.h> .

Answer 3

The main doesn' know about your function. Either move the function decleration above the main or prototype it before the main:

int corners (int array[rows][cols], int the_corners[NUM_CORNERS]);

Answer 4

Try this one:

#include <stdio.h>
#define NROWS 2
#define NCOLUMNS 2
#define NCORNERS 4

int corners(int (*arr)[NCOLUMNS], int* the_corners);

int main() {
    int i;
    int the_corners[NCORNERS];
    int arr[NCOLUMNS][NROWS] = {{1, 2}, {3, 4}};

    corners(arr, the_corners);

    for (i = 0; i < NCORNERS; i++)
        printf("%d\n", the_corners[i]);

    return 0;
}

int corners(int (*arr)[NCOLUMNS], int* the_corners) {

        the_corners[0] = arr[0][NCOLUMNS-1];
        the_corners[1] = arr[0][0];
        the_corners[2] = arr[NROWS-1][0];
        the_corners[3] = arr[NROWS-1][NCOLUMNS-1];

        return 0;
}

You can read here about passing a 2D array to a function.