CODEWITHSUNDEEP
c++algorithmdictionarystl
Pustovalov Dmitry
Pustovalov Dmitry

Reputation: 1047

Update map with values from another map

Let's assume that I have two maps of the same type and the set of keys of the second map is a subset of the keys of the first map. I want to update the first map values with values from the second map (only for the keys that the second map contains).

I have written a simple loop to do this, but I was wondering if there is a better way to write it using STL algorithms.

Code sample:

using PersonAgeMap = std::map<std::string, int>;

PersonAgeMap map1;
map1.insert(std::make_pair("John", 23));
map1.insert(std::make_pair("Liza", 19));
map1.insert(std::make_pair("Dad", 45));
map1.insert(std::make_pair("Granny", 77));

PersonAgeMap map2;
map2.insert(std::make_pair("John", 24));
map2.insert(std::make_pair("Liza", 20));

//simple cycle way
for (const auto& person: map2)
{
    map1[person.first] = person.second;
}

//is there some another way of doing this using STL algorithms???

for (const auto& person: map1)
{
    std::cout << person.first << " " << person.second << std::endl;
}

Output:

  Dad 45
  Granny 77
  John 24
  Liza 20

Upvotes: 5

Views: 1769

Answers (3)

Pustovalov Dmitry
Pustovalov Dmitry

Reputation: 1047

Seems like there is no more clear and better way to do it than simple cycle.

Thanks to everybody.

Upvotes: 0

Slava
Slava

Reputation: 44258

This is not much shorter but could be more efficient when value is something more complex than int:

for( const auto &p : map2 ) {
    auto r = map1.emplace( p );
    if( !r.second ) r.first->second = p.second;
}

PS in comments you said that map2 is a subset of map1, then your method probably the simplest and would not be less efficient than mine.

Upvotes: 3

Daniel La&#252;gt
Daniel La&#252;gt

Reputation: 1147

If you just want to merge them and keep elements from map2:

std::swap(map1, map2);
map1.insert(map2.begin(), map2.end());

Upvotes: 1

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