//编写一个程序输入一个n*n的矩阵，求出两条对角线元素值之和
#include <iostream>
using namespace std;
int main() {
    int num;
    cout << "请输入对角矩阵的大小" << endl;
    cin >> num;
    int **p = new int*[num];//数组大小动态，二维
    for (int i = 0; i <num; i++)
        p[i] = new int[num];
    int sum = 0;
    int number;
    for (int i = 0; i < num; i++) {
        cout << "请输入矩阵元素";
        cout << "第" << (i + 1) << "行" << endl;
        for (int j = 0; j < num; j++){
            cout << "第" << (j + 1) << "列" << endl;
            cin >> number;
            p[i][j] = number;
        }           
    }
    for (int i = 0; i < num; i++) {
        sum += p[i][i] + p[i][num-i-1];//两个对角线元素相加
    }
    cout << "sum=" << sum << endl;
    return 0;
}

另外还有一篇文章，类似，但是只求出主对角线值 查看