Function to left shift a 2D array in C

Question

I\'m writing a function within my program to left shift all the numbers in an array that was passed to the function. An example is:

1 2 3 
4 5 6

<

Answer 1

array[r][c] = array[r][c-1];

should be

array[r][c-1] = array[r][c];

Likewise for the row shift.

Answer 2

2-D arrays are contiguous in memory, so you can iterate over it as if it were a 1-D array:

void left_rotate(int array[][N])
{
    int *flat = (int *)array;
    int temp = flat[0];

    for ( size_t i = 1; i < M*N; ++i )
        flat[i-1] = flat[i];

    flat[M*N-1] = temp;

}

The for loop could also be replaced with a single block move:

memmove(&flat[0], &flat[1], (M*N-1) * sizeof *flat);

Answer 3

You can go throw the array, having a pointer following the last place you visited, and moving the elements, as in this example:

#define nrows 4
#define ncols 4

int array[nrows][ncols] = {
    { 1, 2, 3, 4, },
    { 5, 6, 7, 8, },
    { 9, 10, 11, 12, },
    { 13, 14, 15, 16, },
};

int main()
{
    int row, col;
    int saved_int;
    int *p = &saved_int;

    /* rotate elements */
    for(row = 0; row < nrows; row++)
        for (col = 0; col < ncols; col++) {
            *p = array[row][col];
            p = &array[row][col];
        } /* for */
    *p = saved_int;

    /* print matrix */
    for (row = 0; row < nrows; row++) {
        for (col = 0; col < ncols; col++)
            printf( "%s%d",
                    (col ? "\t" : ""),
                    array[row][col]);
        printf("\n");
    } /* for */
    return 0;
} /* main */