矩阵 c++ 的对角线元素之和

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];
}

Answer 2

I included some comments in the code.

#include <iostream>

/* try to avoid "using namespace ...", as it pollutes the global namespace */
using std::cout;
using std::cin;

int main() {
    /* I moved r and c into the block they belong to */
    int j,i,k,sum1,sum2;
    //cout << "Enter the size of matrix: ";
    //cin >> j;
    /* let's use a fixed-size array for now, otherwise you'll have to
     * dynamically allocate your array using "new" or better use std::vector */
    j = 4;
    int matrix[j][j];

    for (int r {0}; r < j; ++r) {
        for (int c {0}; c < j; ++c) {
            matrix[r][c] = r-c; /* this way, diagonals show very clearly */
            //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";

/* initialise your sums */
sum1 = 0;
sum2 = 0;

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] << " ";
}
cout<<"\n";
cout<<"\n";


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;
}

Run it and you'll find that it works as expected. You were very close to the solution, the main problems were the lack of initialisations of your sum variables and that you need to use new or better yet something like std::vector for dynamically-sized arrays. Also, please don't use post-increment in loops ( i++ ). What you mean is pre-increment ( ++i ).