when setting value in double array with swaping row and col, got two cells valued. Why?

Question

with the following code:

#include <iostream>
#define M 3
#define N 5
using namespace std;
int n;
int m;
int my_array[N][M];
void print_a(){
    cout << "array---------------------------------" << endl;
    for (int i = 0; i < m; i++){
        for (int j = 0; j < n; j++){
            cout << my_array[i][j] << " ";
        }
        cout << endl;
    }
}
int main() {
    n = N;
    m = M;
    int j = n - 1;
    for (int i = 0; i < m; i++){
        my_array[i][j] = i + j;
        print_a(); 
    }
    return 0;
}

array---------------------------------
0 0 0 0 4
0 4 0 0 0
0 0 0 0 0
array---------------------------------
0 0 0 0 4
0 4 0 0 5
0 5 0 0 0
array---------------------------------
0 0 0 0 4
0 4 0 0 5
0 5 0 0 6

tow cells in the double array are changed. I know that double array is also single array. so, even the col and row are exchanged. there should not be two cells are changed. It's Why?

You declare the array with N rows and M columns, but then you fill the array with a loop that goes over M rows and accesses column N-1 — M.M, Oct 19 '16 at 05:36
You have an array of arrays, and when you fill the values you go out of bounds of the nested array. Going out of bounds leads to *undefined behavior*. — Some programmer dude, Oct 19 '16 at 05:38

score 1 · Accepted Answer · edited May 23 '17 at 12:06

Swap n by m in your print function.

#include <iostream>
#define M 3
#define N 5
using namespace std;
int n;
int m;
int my_array[N][M];
void print_a(){
    cout << "array---------------------------------" << endl;
    for (int i = 0; i < n; i++){
        for (int j = 0; j < m; j++){
            cout << my_array[i][j] << " ";
        }
        cout << endl;
    }
}
int main() {
    n = N;
    m = M;
    int j = n - 1;
    for (int i = 0; i < m; i++){
        my_array[i][j] = i + j;
        print_a(); 
    }
    return 0;
}

it works.

array---------------------------------
0 0 0 
0 4 0 
0 0 0 
0 0 0 
0 0 0 
array---------------------------------
0 0 0 
0 4 0 
0 5 0 
0 0 0 
0 0 0 
array---------------------------------
0 0 0 
0 4 0 
0 5 0 
0 6 0 
0 0 0

Problem in your code: The number in last column is actually next row's value(second col). This is due to the cpp layout of 2D arrays on your machine. link

Thanks! I got it. Only one cell's value is changed, just because i print it twice! — user154317, Oct 19 '16 at 07:20

when setting value in double array with swaping row and col, got two cells valued. Why?

1 Answers1