Flattening a 2D array in C without malloc etc

Question

So I have been struggling with this for quit some time and I'm just not able to figure out how I'm supposed to flatten this randomly generated 5*15 2D array into a 1D and print it without using malloc but instead just for loops. And I'm slowly getting unhinged

This is the code for the printing random array.

#include <stdio.h>
#include <stdlib.h>

int main() {
    unsigned short i, j;
    int ar[5][15];
    puts("The array: ");
    for(i = 0; i < 5;i++) {
        for(j = 0; j < 15;j++) {
            printf("%d ", rand()%10);
        }
    printf("\n");
    }
    return 0;
}

Answer 1

If you just want to print them as if it's 1D-array just comment printf("\n"); in the outer for loop.

But, if you want the data from 2D-array in an 1D-array, you've to copy them individually.

#include <stdio.h>
#include <stdlib.h>

#define ROWS    5
#define COLS    15

int main() {
    unsigned short i, j;
    int ar[ROWS][COLS];
    int flat_array [ROWS * COLS];
    puts ("2D array: ");
    for (i = 0; i < ROWS; i++) {
        for (j = 0; j < COLS; j++) {
            ar[i][j] = rand() % 10;
            flat_array[COLS * i + j] = ar[i][j];
            printf ("%d ", ar[i][j]);
        }
        printf ("\n");
    }
    printf("\nFlattened Array: \n");
    for (int fi = 0; fi < ROWS * COLS; )
        printf ("%d ", flat_array[fi++]);

    return 0;
}

i.e. If you've a 2D array of size [M][N] and want to flatten it to an 1D array of size [M*N]

int Arr_2D[M][N];  // with data
int Arr_1D[M*N];    

for (int ri = 0; ri < M; ++ri)
    for (int ci = 0; ci < N; ++ci)
        Arr_1D[ri * N + ci] = Arr_2D [ri][ci];

Also, Data in true-2D-array is stored in memory contiguously, as if a true-1D-array. So :

#include <stdio.h>
#include <stdlib.h>

#define ROWS    5
#define COLS    15

int main() {

    int Arr_2D[ROWS][COLS];

    printf ("Original 2D Array: \n");
    for (int ri = 0; ri < ROWS; ++ri) {
        for (int ci = 0; ci < COLS; ++ci) {
            Arr_2D[ri][ci] = rand() % 10;
            printf ("%d ", Arr_2D[ri][ci]);
        }
        printf ("\n");
    }

    printf ("\nAccessing a 2D array as 1D array: \n");
    int *AP_1D = (int *) Arr_2D;
    for (int ni =0; ni < ROWS * COLS; )
        printf ("%d ", AP_1D[ni++]);

    printf ("\n");
    return 0;
}

There is also, Row & Column major memory storage, we digress.