1. map and unordered map #

Created Tuesday 18 February 2020

1. map ordered - BST implementation - O(logn) - ops.
2. unordered_map - Hash table. Unordered - O(1) - all ops.

map has an order because you can use inOrder. unordered is can be in any order. Order is random.

• Header: #include<unordered_map>
• It is a template.
• Remember “key then value”

The functions: mapname.f()

1. insert(pair<>). Does not overwite the value if we try to {insert key, different value}. No errors though if a key exists.
2. at(key). Acess the value for ‘key’. Throws an error if key is absent. Can be used as lval.
3. mapname[key] -Used for acess and assignment. **Note: Creates **a (key, value) pair if key was absent. Returns no errors. T.C same as insert(). But this is better. Should’nt be used if we are working on old data. i.e don’t create your own.
4. count(key) - returns the count. In a map, it is either 0 or 1. Used for checking existence.
5. size() - returns number of key value pairs.
6. erase(key), deletes a key if present. Else does nothing. No exceptions.

Notes:

1. Never use a hashmap to store coordinates, uncategorized tuples. As multiple tules could have same values for some.
2. unordered_map cannot store.