Course of Raku / Advanced / More about built-in types / Sets, bags, and mixes

Bags and mixes

A set remembers only whether a value is present. Sometimes you also want to know how many times it occurs. That is what a Bag is for: it keeps a count for each value.

You create a bag with the bag routine. Repeated values are not collapsed — they are counted:

my $b = bag(<a b a c a>);
say $b<a>; # 3
say $b<b>; # 1

Asking the bag for a value with the < > subscript returns how many times that value was put in. The total method gives the sum of all the counts:

say bag(<a b a c a>).total; # 5

A Mix is the same idea as a bag, except that the counts (called weights) may be fractional rather than whole numbers. To assign explicit weights, pair each value with its weight and coerce the list with .Mix:

my $m = (flour => 2.5, sugar => 0.75).Mix;
say $m<flour>; # 2.5
say $m.total;  # 3.25

Looking up a weight with < > and summing them with total work exactly as for a bag — only the numbers need not be whole. Here total adds the weights 2.5 and 0.75, giving 3.25 — it is the sum of the weights, not a count of the values. Bags and mixes are handy whenever you need to track quantities — for example, counting how often each word appears in a text, or assigning weights to options.

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