C++ How to remove items from std::map by value?

Question

My map:

std::map<std::array<byte, 16>, int> possible; // key is a byte array, value is a frequency of occurrence

I already filled the map. Now I need to remove the arrays that occurred once. It is necessary to remove from the map such pairs, the value of which is equal to one. How to do it?

score 2 · Answer 1 · answered Jul 02 '22 at 20:40

2

Use erase_if:

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

answered Jul 02 '22 at 20:40

Goswin von Brederlow

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1

Prior to C++20, you will have to use [`map::erase()`](https://en.cppreference.com/w/cpp/container/map/erase) in a loop instead. – Remy Lebeau Jul 02 '22 at 20:44

score 1 · Answer 2 · answered Jul 02 '22 at 20:49

std::map isn't designed to find elements by value, only by key. So, to do what you are asking for, you will have to manually loop through the map calling map::erase() on the desired items, eg:

auto iter = possible.begin();
while (iter != possible.end()) {
  if (iter->second == 1) {
    iter = possible.erase(iter);
  } else {
    ++iter;
  }
}

In C++20 and later, you can use the std::map overload of std::erase_if() instead, eg:

std::erase_if(possible, [](const auto& item) {
    return item.second == 1;
});

C++ How to remove items from std::map by value?

2 Answers2