How to update std::map after using the find method?

c++ map stl stdmap
jaykumarark picture jaykumarark · Dec 24, 2010 · Viewed 158.3k times · Source

How to update the value of a key in std::map after using the find method?

I have a map and iterator declaration like this:

map <char, int> m1;
map <char, int>::iterator m1_it;
typedef pair <char, int> count_pair;

I'm using the map to store the number of occurrences of a character.

I'm using Visual C++ 2010.

Answer

James McNellis picture James McNellis · Dec 24, 2010

std::map::find returns an iterator to the found element (or to the end() if the element was not found). So long as the map is not const, you can modify the element pointed to by the iterator:

std::map<char, int> m;
m.insert(std::make_pair('c', 0));  // c is for cookie

std::map<char, int>::iterator it = m.find('c'); 
if (it != m.end())
    it->second = 42;

Related questions

Checking for existence in std::map - count vs find

So there seem to be two generally acceptable methods of determining whether or not a key exists in a std::map: map.find(key) != map.end() map.count(key) > 0 Is one more efficient than the other? Specifically, the concept …

Read More
Determine if map contains a value for a key?

What is the best way to determine if a STL map contains a value for a given key? #include <map> using namespace std; struct Bar { int i; }; int main() { map<int, Bar> m; Bar b = {0}; Bar …

Read More
How to check if std::map contains a key without doing insert?

The only way I have found to check for duplicates is by inserting and checking the std::pair.second for false, but the problem is that this still inserts something if the key is unused, whereas what I want is …

Read More