之前写的，算线性方程组的

m.cpp

#include <iostream>
#include <fstream>
#include <string>
#include <sstream>
using namespace std;
int main()
{
    ifstream in("in.txt");
    string s;
    getline(in, s);
    // read for length
    int length;
    {
        istringstream sin(s);
        sin >> length;
    }
    // dim array
    double** arr = new double*[length];
    for (int i = 0; i < length + 1; ++i)
    arr[i] = new double[length + 1];
    for (int i = 0; i < length && getline(in, s); ++i)
    {
        istringstream sin(s);
        for (int j = 0; j < length + 1; ++j)
            sin >> arr[i][j];
    }
    double tmp;
    // before
    cout << "before:" << endl;
    for (int i=0; i<length; ++i)
    {
        for (int j=0; j<length+1; ++j)
            cout << arr[i][j] << " ";
        cout << endl;
    }
    // 1st step
    for (int i=1; i<length; ++i)
    {
        for (int j=i; j<length; ++j)
        {
            tmp = 1.0 * arr[j][i-1] / arr[i-1][i-1];
            for (int k=i; k<length+1; ++k)
                arr[j][k] -= 1.0 * tmp * arr[i-1][k];
        }
    }
    cout << "after:" << endl;
    for (int i=0; i<length; ++i)
    {
        for (int j=0; j<length+1; ++j)
            cout << arr[i][j] << " ";
        cout << endl;
    }
    // 2nd step
    for (int i=length-1; i>=0; --i)
    {
        tmp = 0.0;
        for (int j=length-1; j>i; --j)
            tmp += arr[i][j] * arr[j][length];
        arr[i][length] = 1.0 * (arr[i][length] - tmp) / arr[i][i];
    }
    cout << "result:" << endl;
    for (int i=0; i<length; ++i)
        cout << arr[i][length] << endl;
    return 0;
}

in.txt

3
12 2 -6 7
2 4 -6 -1
-6 -6 24 6

output

before:
12 2 -6 7 
2 4 -6 -1 
-6 -6 24 6 
after:
12 2 -6 7 
2 3.66667 -5 -2.16667 
-6 -5 14.1818 6.54545 
result:
0.807692
0.0384615
0.461538