Turning a subselect into an array

I'm sure this has been answered before, but the search seems to be down
again.

How can I convert the results of a subselect into an array? IE:

CREATE TABLE a(a int, b int, c int[]);
INSERT INTO table_a
SELECT a, b, (SELECT c FROM table_c WHERE table_c.parent = table_b.id)
FROM table_b
;
--
Jim C. Nasby, Database Consultant decibel(at)decibel(dot)org
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