keysort(+List1, -List2)

List1

List of elements of the form Term-Term.

List2

List or variable.

Description

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,[1]-w,7.2-b,"k"-e,n-q],L).
             (gives L = [7.2-b,"k"-e,n-q,f(1)-a,[1]-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).

See Also