group_by_key(+Key, +Structs, -Grouped)

Key

Key argument position (non-negative integer, or list of those)

Structs

List of structures

Grouped

Variable, or list of list of structures

Description

The list Structs is partitioned into maximal sublists of consecutive elements with identical keys. Concatenating all these sublists will yield the original list. If the original list was ordered according to the Key'th argument, then the sublists represent a partitioning of the original elements according to their different key values.

Modes and Determinism

• group_by_key(+, +, -) is det

Exceptions

(4) instantiation fault

Arguments are insufficiently instantiated

(5) type error

Some argument or its components are of the wrong type

(6) out of range

Some structure does not have a Key'th argument

Examples

    ?- group_by_key(1, [f(a,1),f(a,2),f(b,2),f(c,2),f(c,5)], Groups).
    Groups = [[f(a,1),f(a,2)], [f(b,2)], [f(c,2),f(c,5)]]
    Yes (0.00s cpu)

    ?- group_by_key(2, [f(a,1),f(a,2),f(b,2),f(c,2),f(c,5)], Groups).
    Groups = [[f(a,1)], [f(a,2),f(b,2),f(c,2)], [f(c,5)]]
    Yes (0.00s cpu)

    ?- group_by_key(1, [a-1,a-2,b-2,c-2,c-5], Groups).
    Groups = [[a-1,a-2], [b-2], [c-2,c-5]]
    Yes (0.00s cpu)

See Also