c++ - Why does size of map changes on comparison?

Question

This question already has answers here: question from:https://stackoverflow.com/questions/65650927/why-does-size-of-map-changes-on-comparison

Answer 1

if(mp[ch])

This will call the map::operator[] with key ch.

If key matches to an element in map it will return the value of that element. If key does not match to any element in the map, the function inserts a new element with that key and returns a reference to its mapped value.

You can use map::find to check if the key is present or not in the map.