C++ Reading txt directly into a custom vector that should be grouped accordingly to the struct by each line

Question

I'm working with a custom vector, and currently the code looks like this:

struct Edge {
    int source, dest, weight;
};

int main()
{
    // initialize edges as per the above diagram
    // (u, v, w) represent edge from vertex `u` to vertex `v` having weight `w`
    vector<Edge> edges =
    {
        {0, 1, 10}, {0, 4, 3}, {1, 2, 2}, {1, 4, 4}, {2, 3, 9},
        {3, 2, 7}, {4, 1, 1}, {4, 2, 8}, {4, 3, 2}
    };
 
    // total number of nodes in the graph (labelled from 0 to 4)
    int n = 5;
 
    // construct graph
    Graph graph(edges, n);
}

I want to change from using hard-coded values into using a .txt file that will look like this:

How can I switch into taking those numbers in the same fashion as before, but with a .txt input instead of hard-coded numbers?

I've tried things like this:

std::vector<std::string> vecOfStr;
bool result = getFileContent("my/path/to/file", vecOfStr);
    
std::vector<int> ints;
std::transform(vecOfStr.begin(), vecOfStr.end(), std::back_inserter(edges),
    [&](std::string s) {
        std::stringstream ss(s);
        int i;
        ss >> i;
        return i;
    });
 
for (auto &i: edges) {
    //std::cout << i << ' ';
}

But didn't succeed.

I have a problem because reading from a file is always as strings and I somehow need to transform each line to my custom struct.

Offtopic: BTW, it's a Dijkstra algorithm path finding program, finding path for each vertice...

An idea: Perhaps break this up into pieces. Implement `operator>>` for `std::istream` in [the canonical way](https://stackoverflow.com/a/4421719/364696), with the reading perhaps reading a single line from the provided `std::istream`, putting it in a `std::stringstream`, and then extracting the three `int`s from that (you could extract the three `int`s directly, but that has issues with handling invalid data; not fun when you leave the stream out of sync). Test that functionality alone to read a single `Edge` at a time. Once that's working, reading from a whole file is relatively easy. — ShadowRanger, Jun 08 '22 at 19:10
"*And didn't succeed*" avoid generic phrases like this, instead try to describe what you actually tried (you have a `vector` of `Edge`, not `int`) and the problem you encountered. — MatG, Jun 08 '22 at 19:14
@ShadowRanger Thank you shadow ranger, you've given me an idea on how to approach the problem and I managed to get it working. I am very thankful. — Patryk Piwowarczyk, Jun 08 '22 at 19:30

score 0 · Answer 1 · answered Jun 08 '22 at 19:29

I am sorry for how this question was stated. I agree it wasn't well paraphrased. Anyways... I managed to do the job by adding this code:

void readInstance(string filename, Instance &instance){
    instance.instanceName = filename;
    ifstream file(filename);
    
    if (file.is_open()) {
        string line;
        int i = 0;
        
        while (getline(file, line)) {
            if(i == 0){
                instance.n = stoi(line);
            }
            else{
                istringstream ss(line);
                string aux1, aux2, aux3;
                ss >> aux1;
                ss >> aux2;
                ss >> aux3;
                Edge p;
                p.source = stod(aux1);
                p.dest = stod(aux2);
                p.weight = stod(aux3);
                instance.Edges.push_back(p);
            }
            
            i++;
        }
        
        file.close();
    }
    else {
        cout << "Error opening instance file" << endl;
        exit(1);
    }
}

Using streams I was able to break it into pieces and get each thing that I needed from the file and then use it in my program. I am leaving this question for future ppl and closing the thread. — Patryk Piwowarczyk, Jun 08 '22 at 19:31

Aamir · Answer 2 · 2022-06-08T19:48:34.760

Assuming that you have input in the following format in the text file

You can use the following code to read integers directly into the vector. No need to use std::transform to convert string into integer.

#include <iostream>
#include <vector>
#include <fstream>

struct Edge {
    int source, dest, weight;
};

std::ostream& operator<<(std::ostream& os, const Edge edge) {
    os << "(" << edge.source << ", " << edge.dest << ", " << edge.weight << ")\n";
    return os;
}

int main()
{
    // initialize edges as per the above diagram
    // (u, v, w) represent edge from vertex `u` to vertex `v` having weight `w`
    std::vector<Edge> edges;

    std::ifstream fin("in.txt");

    if (!fin.is_open()) {
        std::cerr << "fail";
        return 1;
    }

    int s, d, w;
    while (fin >> s >> d >> w) {
        edges.emplace_back(Edge{s, d, w});
    }
    fin.close();

    for (auto &edge: edges) {
        std::cout << edge;
    }

    return 0;
}

C++ Reading txt directly into a custom vector that should be grouped accordingly to the struct by each line

2 Answers2