如何在c++中输出二维数组的偶数列索引并对其进行计算

当我在第二次循环后用大括号将循环迭代以相反的顺序(先遍历列，然后遍历行)放在一个2D数组上时，它将输出数组作为原始数组的转置。但是在我输入任何条件或数学函数之后，它没有输出任何东西。

我试图输出只有偶数列索引的元素。我以前解决了这个问题，只打印偶数行的所有列。

//outputs the sum, average, STD of even column indices and any row indices
if ( choice == 30 )
{
    {
        int sum = 0, c = 0;
        float avg, total = 0,std;
        cout << "the even column indices elements:" << endl;
        for( j=0; j < m; j++ )
        {
            for( i = 0; i < n; i++ )
                if ( i % 2 == 0 )
                {
                    sum = sum + a[i][j];
                    c++;
                    cout << a[i][j] << endl;
                }
        }
        avg = float(sum / c);
        cout << "the sum of even columns:" << sum << endl;
        cout << "the average of even columns:" << avg << endl;
        for ( j = 0; j < m; j++ )
        {
            for( i=0; i < n; i++ )
                if ( i % 2 == 0 )
                {
                    total = total + ( a[i][j] * avg ) * ( a[i][j] - avg );
                    std = float( total / c );
                }
        }
        cout << "the STD of even columns:" << std << endl;
        // here is the output of the maximum and minimum of even columns elements
        int max = -999, min = 1000;
        for( j = 0;j < m; j++ )
        {
            for( i = 0; i < n; i++ )
                if( i % 2 == 0 )
                {
                    if ( a[i][j] > max )
                        max = a[i][j];
                    if ( a[i][j] < min )
                        min = a[i][j];
                }
        }
        cout << "and their max value:" << max << endl;
        cout << "and their min value:" << min << endl;
    }
}