Example for my son

lesson2.cpp

#include <iostream>
#include <fstream>
#include <valarray>
/*
This code read matrix like this: 
4 5
1 2 3 4 5
6 12 8 9 10
11 12 12 14 15
16 17 18 12 20
from file, calc summ and write output to file and stdio 
*/
int main() {

    size_t rows,cols;
    std::ifstream fin("first.txt");
    std::ofstream fout("end.txt");

    fin  >>rows >>cols;
     //  std::cout << "\t Debug: Size: ROW=" <<rows << " COL="<<cols << "\n";
    // using  dynamic numeric array;
    std::valarray<int>va(rows*cols);
    size_t i=0;
    while (!fin.eof())
            fin >>  va[i++];
    /*  more fit for lesson %-)
    for(size_t _r=0; _r<rows;_r++) {
        for (size_t _c = 0; _c <cols; _c++) {
            fin >>  va[_r*cols+_c];
        }
    }
    */
    std::cout << "Summ: " << va.sum() <<"\n";  //  min or  max  also works
    fout << va.sum();  
    fin.close();
    fout.close();
    return 0;
}