CODEWITHSUNDEEP
c++multi-mappingunordered-multimap
zodiac
zodiac

Reputation: 93

finding all the values with given key for multimap

I am searching for all pairs for a particular key in a multimap using the code below.

int main() {
    multimap<int,int> mp;
    mp.insert({1,2});
    mp.insert({11,22});
    mp.insert({12,42});
    mp.insert({1,2});
    mp.insert({1,2});

    for (auto itr = mp.find(1); itr != mp.end(); itr++)
        cout << itr->first<< '\t' << itr->second << '\n';
}

Upvotes: 7

Views: 6247

Answers (2)

Pavan Chandaka
Pavan Chandaka

Reputation: 12751

Like others pointed out your code looped all the elements, from the first iterator (find(1) retuned the first iterator) to last.

If you want you can use C++20 ranges (#include <ranges>)

//DATA
std::multimap<int, int> mp;    
mp.insert({ 1,2 });
mp.insert({ 11,22 });
mp.insert({ 12,42 });
mp.insert({ 1,3 });
mp.insert({ 1,4 });

//FILTER THE ELEMENTS
auto foundElements = mp | std::views::filter([](auto& v) {
    return v.first == 1;
    });

//PRINT
for (auto m : foundElements)
{
    std::cout << m.first << " " << m.second << std::endl;
}

Upvotes: 3

fabian
fabian

Reputation: 82461

You're calling find only a single time in your code. It's perfectly acceptable for this call to return the same value as mp.begin() resulting in you iterating though all the entries in the map before reaching mp.end().

You can use the equal_range member function to get iterators for the start and end of the elements with key 1:

for (auto[itr, rangeEnd] = mp.equal_range(1); itr != rangeEnd; ++itr)
{
    std::cout << itr->first<< '\t' << itr->second << '\n';
}

Upvotes: 12

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