Name Rascalopedia/Relation

Synopsis An unordered set of tuples.

Description In mathematics, given sets D1, D2, ... Dn, a n-ary relation R is characterized by R ⊆ D1 × D2 × ... × Dn. In other words, R consists of a set of tuples <V1, ..., Vn> where each Vi is an element of the set Di. When n = 2, we call the relation a binary relation.

In database theory, a relation is a table with a heading and an unordered set of tuples:

D1 Name1	D2 Name2	...	Dn Namen
V11	V12	...	V1n
V21	V22	...	V2n
V31	V32	...	V3n
...	...	...

In Rascal, a relation is a set of tuples and is characterized by the type: rel[D1 Name1, D2 Name2, ..., Dn Namen] See Rascal:Values/Relation and for a description of relations and their operators (since relations are sets all set operators also apply to them, see Rascal:Values/Set) and Rascal:Prelude/Relation for functions on relations (and here again, since relations are sets all set operators also apply to them, see Rascal:Prelude/Set).

Examples

Relations in daily life

• The parent-of or friend-of relation between people.
• A character relation map, describing the relations between the characters in a play or soap series.
• A listing of the top 2000 songs of all times including the position, artist name, song title, the year the song was published.

credit credit

Relations in computer science

• A relational data base.
• Login information including user name, password, home directory, etc.

Relations in Rascal

• A parent child relation:

  rel[str parent, str child] = {
  <"Paul", "Eva">,
  <"Paul", "Thomas">,
  <"Jurgen", "Simon">,
  <"Jurgen", "David">,
  <"Tijs", "Mats">
  };
  

• A fragment of the top 2000 relation:

  rel[int position, str artist, str title, int year] Top2000 = {
  <1, "Eagles", "Hotel California",1977>,
  <2, "Queen", "Bohemian rhapsody", 1975>,
  <3, "Boudewijn de Groot", "Avond", 1997>,
  ...
  };
  

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