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keysort(+List1, -List2)

Succeeds if List2 is a sorted list version of List1, whose elements are of the form Key-Value. The sort is done according to the value of the key Key.
List1
List of elements of the form Term-Term.
List2
List or variable.

Description

The elements of List1 are of the form Key-Value, where Key and Value are both arbitrary terms.

List1 is sorted according to the value of the key Key and the result is unified with List2. No sorting is carried out on Value. The sort is stable, i.e. the order of elements with the same key is preserved.

The sort is done according to the standard ordering of terms. See compare/3 for this standard ordering. Note in particular that numbers are first ordered by their type (integer, float, etc) and only then by their magnitude, i.e. sorting numbers of different types may not give the expected result.

keysort(R, S) is equivalent to sort(1, =<, R, S).

Modes and Determinism

• keysort(+, -) is det

Exceptions

(4) instantiation fault
List1 is a partial list.
(5) type error
Either List1 or List2 is instantiated, but not to a list of the form Term-Term.

Examples

Success:
keysort([n-1,4-a],L).     (gives L = [4-a,n-1]]).
keysort([f(1)-a,-w,7.2-b,"k"-e,n-q],L).
(gives L = [7.2-b,"k"-e,n-q,f(1)-a,-w]).
keysort([f(1,2),g(1)],M). (gives M = [f(1,2),g(1)]).
keysort([g(1,2)-a, f(1,2)-a],M).
(gives M = [f(1,2)-a,g(1,2)-a]).
keysort([f(4,3)-a, f(3,4)-b],M).
(gives M = [f(3,4)-b,f(4,3)-a]).

Fail:
keysort([n-1,M-a],[n-1,M-a]).

Error:
keysort(L1,L2).              (Error 4).
keysort([n-1,m],L).          (Error 5).