Nested for loop not looping through second array

Question

How do I get the second array to be properly looped through? It is a simple matrix multiplication operation (using Visual Studio 2019) and arr is being looped through, but arr2 isn't being looped through completely. Only the first 2 elements are being multiplied by all elements of arr.

#include <iostream>

using namespace std;

int main() {
    int r1 = 2, c1 = 3, r2 = 3, c2 = 3;

    int** arr = new int* [r1];
    for (int i = 0; i < c1; i++) {
        arr[i] = new int[c1];
    }
    for (int i = 0; i < r1; i++) {
        for (int j = 0; j < c1; j++) {
            arr[i][j] = j;
        }
    }

    int** arr2 = new int*[r2];
    for (int i = 0; i < c2; i++) {
        arr2[i] = new int[c2];
    }
    for (int i = 0; i < c2; i++) {
        for (int j = 0; j < r2; j++) {
            arr2[i][j] = j;
        }
    }

    int** arr3 = new int*[r1];
    for (int i = 0; i < c2; i++) {
        arr3[i] = new int[c2];
    }
    for (int i = 0; i < r1; i++) {
        for (int j = 0; j < c2; j++) {
            arr3[i][j] = 0;
        }
    }


    for (int i = 0; i < r1; i++) {
        for (int j = 0; j < c2; j++) {
            for (int g = 0; g < c1; g++) {
                arr3[i][j] += arr[i][g] * arr2[g][j];

                cout << "[" << i << "][" << g << "] " << arr[i][g] << "\tX\t" << "[" << g << "][" << j << "] " << arr2[g][j] << " => " << arr3[i][j] << endl;
            }
        }
    }

    cout << "\n" << endl;

    for (int i = 0; i < r1; i++) {
        for (int j = 0; j < c2; j++) {
            cout << "New Array Element: [" << i << "][" << j << "] => " << arr3[i][j] << endl;
        }
    }

    return 0;
}

Answer 1

You make the same basic mistake in all of your loops -- that mistake being that you are not iterating over the rows correctly.

You are using the column count when you should be using the row count:

 int** arr = new int* [r1];
    for (int i = 0; i < c1; i++) {  // <-- This should be i < r1, not i < c1;
        arr[i] = new int[c1];
    }

Second, you could have written a function to lessen the chance of error, and also have a better chance of finding bugs like this:

 int **createArray(int rows, int cols)
 { 
    int** arr = new int* [rows];
      for (int i = 0; i < rows; i++) { 
        arr[i] = new int[cols];
    }
    
    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < cols; j++) {
            arr[i][j] = j;
        }
    }
    return arr;
}

int main()
{
   int r1 = 2, c1 = 3, r2 = 3, c2 = 3;

   int** arr = createArray(r1, c1);
   int** arr2 = createArray(r2,c2);
   int** arr3 = createArray(r1,c2);
   //...
}

Now the creation code is not repeated over and over again, plus the issue of using the wrong variables becomes minimized, if not eliminated.

---

Having said this, this way of creating 2D arrays is one of the worst ways of doing this. Right now, your code has memory leaks (it still is considered a leak, even though the program will terminate -- memory tools such as valgrind and others would indicate the errors as soon as your program exits).

Ideally, you would use a container class such as std::vector<int> to handle the dynamic memory management automatically.

But even if this is an exercise where you must use int**, it is still a very bad way of creating two-dimensional arrays under those constraints. Here is an answer that goes in detail about this issue, why it's bad, and how it can be alleviated.