Programs often need to work with groups of values rather than one value at a time. Collections organize several values according to rules that make particular operations convenient.

Sequences

A list or array stores values in an order. Elements can be visited one after another and are commonly accessed by position.

temperatures = [18, 21, 19, 23]

A sequence is appropriate when order and repeated values matter.

Maps

A map or dictionary associates keys with values:

population["Bilbao"] = 346000

The important operation is lookup by key rather than by numerical position.

Sets

A set stores distinct values and supports questions such as whether an element is present, or which elements belong to the union or intersection of two collections.

Choosing a structure

Different structures make different operations natural or efficient. A sequence is useful for ordered iteration, a map for keyed lookup and a set for membership and uniqueness.

Mutation

Some collections can be changed after creation by adding, removing or replacing elements; others are immutable. Mutation changes program state, so references to the same mutable collection can observe one another's changes.

A data structure is therefore not just a container: it organizes data around the operations the program needs to perform.