From

(jokingly referred to as Sane Indexing) is a primitive function that allows selecting multiple major cells using indices of the cells. It is equivalent to  and , only providing a neater notation for such a fundamental concept. It was introduced in Extended Dyalog APL, and then adopted into dzaima/APL and KAP.

Common usage
Select is commonly used to reorder the major cells of an array. For example, the following shuffles any array into random order: Try it online! Without Select, one would have to write.

In a case where the left argument is a permutation vector for the right argument, the functionality can rightfully be called Permute.

Select especially cleans up expressions for reordering. An ascending sort can be represented as  and "sort by" can be written as  :

Try it online!

Potential extension
Select can be defined as  thus allowing both the above usage and "scatter point indexing":