Question

I am having trouble “undoing” this method, that dumps essentially a matrix of numbers of variable size into a text file:

void vectorToFile(char *name, vector<vector<double>>* a){
    FILE* fp = fopen(name, "w");
    for(int i=0;i<a->size();i++){
        for(int j=0;j<a->at(i).size();j++){
            fprintf(fp, "%f ", a->at(i).at(j));
        }
        fprintf(fp, "\n");
    }
    fclose(fp);
}

I am having trouble implementing the reverse:

vector<vector<double>> fileToVector(char *name){ ??? }

I am guaranteed that the numbers in the file form a “rectangle”, i.e. the sizes of inner vectors are all equal, but I don’t know how to figure out the number of entries per row, and the number of columns.

Can anyone point me in the right direction? Every example I found so far implements something much easier, with hardcoded size, or sizes given in first row (which I cannot afford to do unfortunately)

Answer 1

I’m new at C++ so I’m not sure if this is a good approach or not, but I would open the file, read in the input line by line, parsing each line as I read it. Here’s some example code (untested, uncompiled):

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

std::vector<std::vector<double> > fileToVector(const char *name)
{
    std::vector<std::vector<double> > result;
    std::ifstream input (name);
    std::string lineData;

    while(getline(input, lineData))
    {
        double d;
        std::vector<double> row;
        std::stringstream lineStream(lineData);

        while (lineStream >> d)
            row.push_back(d);

        result.push_back(row);
    }

    return result;
}