vector of vector and computes

[mickey]

Hello,
I have a matrix of int (vector of vector); the problem is that I need to compute the mean of each column (the matrix can be 500 x200); so to do this I did:

foreach (iter_column) {
   float sum = 0;
   iter_line.setFirstLine;
   foreach(iter_line) {
        sum += iter_line[iter_coulmn]. value;   
   }
   mean = sum / nOfLine;
}

foreach (iter_column) {
   float sum = 0;
   iter_line.setFirstLine;
   foreach(iter_line) {
        sum += iter_line[iter_coulmn]. value;   
   }
   mean = sum / nOfLine;
}

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The problem is that I have to do "two for" one inside other; I thought If is better to do something like to tie a vector<coulum> and do:

class Column {
public:
   vector<double> _value;
};
vector<Column> col; //to fill col properly
foreach (col) {
    int sum = std::accumulate(col.begin(), col.end(), 0);
    mean = sum / col.size();
}

class Column {
public:
   vector<double> _value;
};
vector<Column> col; //to fill col properly
foreach (col) {
    int sum = std::accumulate(col.begin(), col.end(), 0);
    mean = sum / col.size();
}

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Will be the second way better/faster? Any other ways to improve?