A collection refers to a collection of concrete or abstract objects with certain attributes. Among them, these objects that make up the set are called elements of the set.

For example, the collection of all China people, its element is every China person. Generally, uppercase letters such as A, B, S, T, ... are used to represent a set, while lowercase letters such as A, B, X, Y, ... are used to represent elements in the set. If X is an element of the set S, it is said that X belongs to S and is marked as X ∈ S ... If Y is not an element of the set S, it is said that Y does not belong to S and is marked as Y? s .

Extended dataset properties:

1, certainty

Given a set, any element, whether it belongs to the set or not, must be one of them, and there is no ambiguity.

2. Interrelation

Any two elements in a collection are considered different, that is, each element can only appear once. Sometimes it is necessary to describe the situation where the same element appears many times. You can use multiset, where elements are allowed to appear multiple times.

3. Chaos

In a set, the state of each element is the same and the elements are out of order. You can define an order relation on the set. After defining the order relation, you can sort the elements according to the order relation. But as far as the characteristics of the set itself are concerned, there is no necessary order between elements.