如何从具有特定值作为第一个值的对向量中删除所有对

Question

Let's say the pairs I wanted to remove would have 2 as their first value.

A vector like this:

vector<pair<int,int>> v = {{1,2}, {2,3} , {2,4} , {5,4}};

would become after the removal of pairs:

v = {{1,2}, {5,4}}

How would I do this? Is it some implementation of the erase-remove idiom? Because I don't know how it should check the first value in the pair.

Answer 1

The standard library has remove_if to do this :

#include <algorithm>
#include <iostream>
#include <vector>
#include <utility>

int main()
{
    std::vector<std::pair<int, int>> v = { {1,2}, {2,3} , {2,4} , {5,4} };

    // use std::remove_if
    // this will move al elements to erase to the end of the vector so you can erase them after
    // https://en.cppreference.com/w/cpp/algorithm/remove
    // the 3d argument is a lambda (you can also use a function)
    // https://en.cppreference.com/w/cpp/language/lambda
    auto removed_it = std::remove_if(v.begin(), v.end(), [](const std::pair<int, int>& pair)
    {
        // return true if the first value of the pair is 2.
        return pair.first == 2;
    });

    // cleanup the vector
    v.erase(removed_it, v.end());

    // show the cleaned up content
    for (const auto& pair : v)
    {
        std::cout << pair.first << ", " << pair.second << "\n";
    }

}