CODEWITHSUNDEEP
c++sorting
Zack Lee
Zack Lee

Reputation: 3044

How to sort vector of pairs <float, string> regardless of string

I'm trying to sort std::vector<std::pair<float, std::string>> in ascending order.

Although using std::sort works, I found out the strings affect the order if the floats have the same values.

I would like to sort the vector regardless of the string so the former element always gets located first than latter ones.

My Code :

#include <iostream>
#include <vector>
#include <string>

int main()
{
    std::vector<std::pair<float, std::string>> vec;
    vec = {{1, "e"}, {1, "d"}, {1, "c"}, {1, "b"}, {1, "a"}};
    std::sort(vec.begin(), vec.end());
    for (auto i : vec)
        std::cout << i.first << ", " << i.second << '\n';
}

The Result I Get : (alphabetical order when value ties)

1, a
1, b
1, c
1, d
1, e
Program ended with exit code: 0

The Result I Want : (former element first when value ties)

1, e
1, d
1, c
1, b
1, a
Program ended with exit code: 0

Upvotes: 2

Views: 1459

Answers (3)

Yksisarvinen
Yksisarvinen

Reputation: 22229

std::pair has it's comparing operators overloaded (See the reference)

By default, operator < for std::pair compares first elements, if they are equal, then second elements.

You should provide your own predicate and use std::stable_sort to preserve the order of elements if first elements in pair are equal.

std::stable_sort(vec.begin(), vec.end(),
                 [](const auto& a, const auto& b){return a.first < b.first;});

Upvotes: 11

Ishpreet
Ishpreet

Reputation: 5880

You can provide a compare function to std::sort. Like 

std::sort(vec.begin(), vec.end(), comp);

In your case the compare function can be:

bool comp(const std::pair<float, std::string>&a, const std::pair<float, std::string>&b){
    if(a.first == b.first) return 0;
    else return a.first < b.first;
}

Live Demo

Upvotes: -1

Korni
Korni

Reputation: 464

You are probably searching for std::stable_sort. It sorts elements while preserving the order of equivalents.

Upvotes: 0

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