Sum of the elements of diagonal of matrix c++

Question

I want to find the sum of the diagonals of matrix that defined by user with this code , it works for main diagonal but not for secondary diagonal and i don't understand what the problem is. Any help would be appreciated.

#include <iostream>
using namespace std;

int main() {
    int r, c, j,i,k,sum1,sum2;
    cout << "Enter the size of matrix: ";
    cin >> j;
    int matrix[j][j];

    for (r = 0; r < j; r++) {
        for (c = 0; c < j; c++) {
            cout << "matrix" << "[" << r << "]" << "[" << c << "] = ";
            cin >> matrix[r][c];
        }
    }
    cout << "\n";

    for ( i = 0; i < j; i++) {
        for ( k = 0; k < j; k++) {
            cout << " "<<matrix[i][k] << " ";
        }
        cout << "\n";
    }

cout<<"\n";

cout<<"Main diagonal is: ";
for( i=0;i<j;i++){
    cout<<matrix[i][i];
}
cout<<"\n";


for( i=0;i<j;i++){
    sum1=sum1+matrix[i][i];}
    cout<<"Sum of the elements of main diagonal is: "<<sum1;
    cout<<"\n";
    cout<<"\n";

cout<<"Secondary diagonal is: ";
for(i=0,k=j-1;i<j;i++,k--){
    cout<<matrix[i][k];
}


for(i=0,k=j-1;i<j;i++,k--){
    sum2=sum2+matrix[i][k];
    }

    cout<<"Sum of the elements of secondary diagonal is: "<<sum2;
    return 0;
}

Answer 1

For starters Variable Length Arrays is not a standard C++ feature. Use instead the class template std::vector.

For example

std::vector<std::vector<int>> matrix( j, std::vector<int>( j ) );

Or at least allocate arrays dynamically as for example

int **matrix = new int *[j];
for ( i = 0; i < j; i++ ) matrix[i] = new int[j];

In this case you should free all allocated memory before exiting the program as

for ( i = 0; i < j; i++ ) delete [] matrix[i];
delete [] matrix;

Secondly neither sum1 nor sum2 were initialized.

int r, c, j,i,k,sum1,sum2;

To calculate the sum of elements of the secondary diagonal you can write

sum2 = 0;
for ( i = 0; i < j; i++ ){
    sum2 += matrix[i][j - i - 1];
}