Question

i’m learning C, currently pointers.

#include <cstdio>
#include <cstring>
#include <cstdlib>

using namespace std;

int f1(int **w){
    for (int i=0;i<2;i++){
        for (int j=0;j<10;j++){
            w[i][j]=10;
            printf("%d ",w[i][j]);
        }
        printf("\n");
    }
    printf("----\n");
}

int main () {


    int **w = (int **) malloc(sizeof(int*)*2);
    for (int i=0;i<2;i++)
        w[i] = (int*)malloc(sizeof(int)*10);

    for (int i=0;i<2;i++){
        for (int j=0;j<10;j++){
            w[i][j]=i*10 + j;
            printf("%d ",w[i][j]);
        }
        printf("\n");
    }
    printf("---\n");
    f1(w);
        for (int i=0;i<2;i++){
        for (int j=0;j<10;j++){
            w[i][j]=i*10 + j;
            printf("%d ",w[i][j]);
        }
        printf("\n");
    }

    return 0;
}

So, I have this code

and here is the output:

0 1 2 3 4 5 6 7 8 9 10
11 12 13 14 15 16 17 18 19
--- 
10 10 10 10 10 10 10 10 10 10 
10 10 10 10 10 10 10 10 10 10
---- 
0 1 2 3 4 5 6 7 8 9 10 
11 12 13 14 15 16 17 18 19

I’m wondering, WHY the array values (see last 2 rows) are different to 10?.
My guess, not sending the correct pointer, but, in that case, WHERE is stored the array with 10…10?, is it created magically??

thanks

Answer 1

Seems the program is doing exactly what you have programmed it to do.

The f1 function sets all values to 10

 w[i][j]=10;

the other places set it to

w[i][j]=i*10 + j;