Normally, if I want to find whether the key exists in map STL(C++), the first thing come up to my mind is if(map1[key1]). However, this is wrong, because if there is no key equals to key1, map1[key1] will return 0 and if there is a key1 but its value is 0, then it will also return 0; So this method is not feasible.

There are generally two ways to determine whether the key exists in map.

## Find function

find function returns an iterator.