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Select () (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 0 , only providing a neater notation for such a fundamental concept. It was introduced in Extended Dyalog APL, and then adopted into dzaima/APL.

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!

      Shuffle 'abcdef'

Without Select, one would have to write Shuffle⊢⌷⍨∘⊂?⍨∘.

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!

      'abcde' ⍨∘ 3 1 4 1 5
APL built-ins [edit]
Primitive functions
Monadic ConjugateNegateSignumNotRollType
Dyadic AddSubtractEqual to (Xnor) ∙ Not Equal to (Xor) ∙ MinimumMaximumComparison functions
Structural ShapeReshapeTallyDepthRavelReverseRazeMixCut (K)PairReplicatePartitioned Enclose
Selection TakeDropUniqueIdentitySelect
Selector Index generatorInterval IndexIndices
Computational MatchNot MatchNub SieveFormatExecute
Primitive operators Monadic EachReplicate
Dyadic Reverse Compose
Quad names
Arrays Index originMigration level
Other ZildeHigh minusFunction axis