# std::erase_if (std::unordered_map)

From cppreference.com
< cpp‎ | container‎ | unordered map

Defined in header `<unordered_map>` |  |
template< class Key, class T, class Hash, class KeyEqual, class Alloc, class Pred > typename std::unordered_map<Key, T, Hash, KeyEqual, Alloc>::size_type erase_if( std::unordered_map<Key, T, Hash, KeyEqual, Alloc>& c, Pred pred ); |  | (since C++20)

Erases all elements that satisfy the predicate `pred` from the container. Equivalent to

    auto old_size = c.size();
    for (auto i = c.begin(), last = c.end(); i != last; ) {
      if (pred(*i)) {
        i = c.erase(i);
      } else {
        ++i;
      }
    }
    return old_size - c.size();

### Parameters

c | - | container from which to erase
pred | - | predicate that returns true if the element should be erased

### Return value

The number of erased elements.

### Complexity

Linear.

### Example

Run this code

    #include <unordered_map>
    #include <iostream>

    template<typename Os, typename Container>
    inline Os& operator<<(Os& os, Container const& cont)
    {
        os << "{";
        for (const auto& item : cont) {
            os << "{" << item.first << ", " << item.second << "}";
        }
        return os << "}";
    }

    int main()
    {
        std::unordered_map<int, char> data {{1, 'a'},{2, 'b'},{3, 'c'},{4, 'd'},
                                            {5, 'e'},{4, 'f'},{5, 'g'},{5, 'g'}};
        std::cout << "Original:\n" << data << '\n';

        const auto count = std::erase_if(data, [](const auto& item) {
            auto const& [key, value] = item;
            return (key & 1) == 1;
        });

        std::cout << "Erase items with odd keys:\n" << data << '\n'
                  << count << " items removed.\n";
    }

Possible output:

    Original:
    {{5, e}{4, d}{3, c}{2, b}{1, a}}
    Erase items with odd keys:
    {{4, d}{2, b}}
    3 items removed.

### See also

remove remove_if | removes elements satisfying specific criteria (function template)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/container/unordered_map/erase_if&oldid=135906"