categorize — multi-key grouping

Group elements under every key a block returns, so one element can land in several groups.

categorize is classify's multi-key cousin: its block returns a list of keys per element, and the element is filed under each of them. One value can therefore appear in several groups at once.

Filing into multiple groups #

Here each number is categorised by parity and by size, so it lands in two lists.

my %g = (1..6).categorize({ ($_ %% 2 ?? "even" !! "odd", $_ > 3 ?? "big" !! "small") });
say %g<even>.sort;
say %g<small>.sort;

(2 4 6)
(1 2 3)

4 is in both even and big; 2 is in both even and small.

Notes #

• classify (List methods) is the single-key form — its block returns one key, so each element lands in exactly one group.
• The result is a hash of arrays; iterate it with .sort for reproducible output (hash order is undefined).
• It's the natural tool for tag-style grouping, where items carry several independent labels.