Dictionaries and sets

A dictionary (dict) is a structure of “key: value” pairs. In a list an element is found by its number (index), while in a dictionary it is found by its key: for example, a student’s grade by the student’s name. An empty dictionary is created with d = dict(), and a value is added and read with d["Ali"] = 5 and d["Ali"]. Keys are not repeated; if a new value is written under the same key, the old one is replaced.

Some operations come up often when working with a dictionary: "Ali" in d checks whether a key exists, d.get("Vali", 0) returns 0 if the key is missing, del d["Ali"] deletes, and d.items() loops over all the pairs. Searching by key in a dictionary is very fast because the place of the value is determined directly from the key. That is why dictionaries are used in counting tasks, for example to find how many times each word occurs in a text.

A set (set) is a collection of unique elements whose order does not matter. The expression set([1, 2, 2, 3, 3, 3]) holds only the elements 1, 2, 3, so a set is the shortest way to remove duplicates from a list. An element is added with s.add(4) and its presence is checked with 4 in s.

Sets support operations from mathematics: union a | b, intersection a & b, difference a - b. For example, if a = set([1, 2, 3]) and b = set([2, 3, 4]), then a & b is 2, 3 and a | b is 1, 2, 3, 4. The choice of structure depends on the task: an ordered sequence calls for a list, lookup by key for a dictionary, and removing duplicates with quick membership checks for a set.

Key takeaways

• A dictionary stores “key: value” pairs and finds a value quickly by its key.
• Dictionary keys are not repeated: a new value under the same key replaces the old one.
• A set stores unique elements and does not depend on order.
• Sets support union, intersection and difference.
• The structure is chosen to suit the task: a list, a dictionary or a set.