Accessing map value by index

Question

If I have a structure like

std::map myMap;
myMap[\"banana\"] = 1;
myMap[\"apple\"] = 1;
myMap[\"orange\"] = 1;

How can I

Answer 1

Well, actually you can't. The way you found is very unefficient, it have a computational complexity of O(n) (n operations worst case, where n is the number of elements in a map).

Accessing an item in a vector or in an array have complexity O(1) by comparison (constant computational complexity, a single operation).

Consider that map is internally implemented as a red black tree (or avl tree, it depends on the implementation) and every insert, delete and lookup operation are O(log n) worst case (it requires logarithm in base 2 operations to find an element in the tree), that is quite good.

A way you can deal with is to use a custom class that have inside both a vector and a map. Insertion at the end of the class will be averaged O(1), lookup by name will be O(log n), lookup by index will be O(1) but in this case, removal operation will be O(n).