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group_with_key(+Key, +Structs, -Grouped)

Partition a list into sublists by key
Key argument position (non-negative integer, or list of those)
List of structures
Variable, or list of KeyVal-SubList structures


The list Structs is partitioned into maximal sublists of consecutive elements with identical keys, such that concatenating all these sublists would 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.

The output argument Grouped is bound to a list of KeyVal-SubList pairs, where KeyVal is the value of the Key'th argument that all the structures in SubList have in common.

Modes and Determinism


(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


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

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

See Also

group_by_key / 3, same_key_prefix / 5, sort / 4, merge / 5