How to filter a vector in C++

This post will discuss how to filter a vector in C++.

1. Using `std::copy_if`

Starting with C++11, we can filter a vector using the std::copy_if algorithm. It copies all the elements in the specified range for which the predicate returns true. Following is a simple example demonstrating the usage of this function, which filters odd elements from the vector.

#include <iostream>

#include <vector>

#include <algorithm>

int main()

{

std::vector<int> v = {2, 1, 3, 6, 7, 9, 8};

std::vector<int> odd;

std::copy_if(v.begin(), v.end(), std::back_inserter(odd), [](int i) {

return i % 2 == 1;

});

for (int &i: odd) {

std::cout << i << ' ';

}

return 0;

}

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Output:

1 3 7 9

2. Using Ranges

Starting with C++20, we can use filter view from the ranges library. Here is a simple one-liner C++20 solution with std::ranges::views::filter from header <ranges>.

#include <iostream>

#include <vector>

#include <ranges>

int main()

{

std::vector<int> v = {2, 1, 3, 6, 7, 9, 8};

auto odd = v | std::ranges::views::filter([](int &i) {

return i % 2 == 1;

});

for (int &i: odd) {

std::cout << i << ' ';

}

return 0;

}

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Output:

1 3 7 9

3. Using Erase-remove idiom

We can in-place filter a vector with Erase-remove idiom, as shown below using the std::remove_if with a single call to std::vector::erase.

#include <iostream>

#include <vector>

#include <algorithm>

int main()

{

std::vector<int> v = {2, 1, 3, 6, 7, 9, 8};

v.erase(std::remove_if(v.begin(), v.end(), [&](const int& i) {

return i % 2 == 0;

}), v.end());

for (int &i: v) {

std::cout << i << ' ';

}

return 0;

}

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Output:

1 3 7 9

That’s all about filtering a vector in C++.

How to filter a vector in C++

1. Using std::copy_if

2. Using Ranges

3. Using Erase-remove idiom

1. Using `std::copy_if`