Question

I haven’t used C++ in quite a while and I seem to be making what I’m sure is a very stupid mistake. Could someone tell me why

#include <cstdio>
#include <cstdlib>
#include <iostream>

using namespace std;

int main() {
        double* atoms;

        atoms = (double*)malloc(10 * 3*sizeof(double));
        for (int i = 0; i < 10; i++) {
                for (int j = 0; j < 3; j++) {
                        atoms[i*10 + j] = 2.0;
                }
        }

        for (int i = 0; i < 10; i++) {
                for (int j = 0; j < 3; j++) {
                        cout << atoms[i*10 + j] << endl;
                }
                cout << endl;
        }


        free(atoms);

        return 0;
}

is printing

2
2
2

2
2
2

2
2
2

6.94528e-310
6.94528e-310
0

0
4.24399e-314
4.24399e-314

2
2
2

2
2
2

2
2
2

2
2
2

2
2
2

instead of all 2’s? Thanks

Answer 1

malloc(10 * 3*sizeof(double)) allocates enough memory for 30 doubles.

The loop:

    for (int i = 0; i < 10; i++) {
            for (int j = 0; j < 3; j++) {
                    atoms[i*10 + j] = 2.0;
            }
    }

accesses well past the last allocated element (which would be atoms[29]). For example, when i == 3 and j == 0 you’re accessing atoms[30]. Any accesses when i >= 3 are out of bounds.