Question

I have a string in c++ and it represents an upper triangular matrix, What I want to do is meake a complete matrix from this string

    std::string input = "1,2,1,3,6,1,4,7,9,1";
    //this represents


 //1  2  3 4
 //2  1  6 7
 //3  6  1 9
 //4  7  9 1

std::replace(input.begin(), input.end(), ',', ' ');
std::vector<double> Matrix;
std::istringstream inputStream(input);
double value;
int rowNum = 0;
int colNum = 0; 

while (inputStream >> value){
    for (colNum = 0; colNum < 2; colNum++){
        if (colNum >= rowNum){
            Matrix.push_back( value ); 
        }
        else{
            Matrix.push_back( Matrix[colNum * 2 + rowNum]); 
        }
    }

    rowNum++;
}
inputStream >> std::ws;

Instead of getting

 1  2  3 4
 2  1  6 7
 3  6  1 9
 4  7  9 1

But I am getting

   1.0000   1.0000   1.0000   2.0000
   1.0000   1.0000   2.0000   1.0000
   1.0000   2.0000   1.0000   1.0000
   2.0000   1.0000   1.0000   2.0000

What is it my error? I can not see it…

Answer 1

You should show the indexing scheme used for printing the output (i.e. how do you expect the indexes works): your choice of using a vector instead of a matrix make hard to correct the code. For sure, I see the following points that have no clear connection with the input pattern:

1) each number you read you increment the rowNum index. The row should be incremented instead at ‘steps’ 1, 1+2, 1+2+3,…

2) colNum should range from 0 to current rowNum, instead assumes just 0,1

3) there is no chance to fill a row (say the first) before you read (say the last). You could do if the input would be 1 2 3 4 1 6 7 1 9 1

all these points are related, and origin from the wrong data representation, that make difficult a trivial task.

In C++, a very effective way to tackle these problems is data hiding: consider how easily we can write a class that gives the correct logical representation and doesn’t waste space:

#include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <algorithm>

template <class T = double>
class upper_triangular_matrix
{
    std::vector<T> Matrix;

public:

    upper_triangular_matrix(std::string input)
    {
        // trade time for space: store the values, compute indexing
        std::replace(input.begin(), input.end(), ',', ' ');
        std::istringstream inputStream(input);
        T value;
        while (inputStream >> value)
            Matrix.push_back(value);

        // validate size: ok 1,1+2,1+2+3 etc
    }

    T operator()(int r, int c) const
    {
        // compute indexing accounting for miss duplicated
        if (c > r)
            std::swap(c, r);
        int p = 0, n = 1;
        while (r > 0)
        {
            p += n++;
            r--;
        }
        return Matrix[p + c];
    }
};

int main()
{
    upper_triangular_matrix<> m("1,2,1,3,6,1,4,7,9,1");
    for (int r = 0; r < 4; ++r)
    {
        for (int c = 0; c < 4; ++c)
            std::cout << m(r, c) << ' ';
        std::cout << std::endl;
    }
}

when run, this prints

1 2 3 4 
2 1 6 7 
3 6 1 9 
4 7 9 1