# 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 > 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 c.

Equivalent to

    auto old_size = c.size();
    for (auto first = c.begin(), last = c.end(); first != last;)
    {
        if (pred(*first))
            first = c.erase(first);
        else
            ++first;
    }
    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 <iostream>
    #include <unordered_map>

    void println(auto rem, auto const& container)
    {
        std::cout << rem << '{';
        for (char sep[]{0, ' ', 0}; const auto& [key, value] : container)
            std::cout << sep << '{' << key << ", " << value << '}', *sep = ',';
        std::cout << "}\n";
    }

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

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

        println("Erase items with odd keys:\n", data);
        std::cout << 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

removeremove_if | removes elements satisfying specific criteria (function template)
ranges::removeranges::remove_if (C++20)(C++20) | removes elements satisfying specific criteria (algorithm function object)

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