Select

From APL Wiki
Jump to navigation Jump to search

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?⍨∘
      Shuffle 'abcdef'
fbdcea

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
bdace

Potential extension

Select can be defined as ⍨∘⍨⍤0 thus allowing both the above usage and "scatter point indexing":

      1 4 3  4 4⎕A
ABCD
MNOP
IJKL
      (1 1)(4 4)(3 4)  4 4⎕A
APL

See also

APL built-ins [edit]
Primitive functions
Scalar
Monadic ConjugateNegateSignumReciprocalMagnitudeExponentialNatural LogarithmFloorCeilingFactorialNotPi TimesRollTypeImaginarySquare Root
Dyadic AddSubtractTimesDivideResiduePowerLogarithmMinimumMaximumBinomialComparison functionsBoolean functions (And, Or, Nand, Nor) ∙ GCDLCMCircularComplexRoot
Non-Scalar
Structural ShapeReshapeTallyDepthRavelEnlistTableCatenateReverseRotateTransposeRazeMixSplitEncloseNestCut (K)PairLinkPartitioned EnclosePartition
Selection FirstPickTakeDropUniqueIdentitySelectReplicateExpandSet functions (IntersectionUnionWithout) ∙ Bracket indexingIndex
Selector Index generatorGradeIndex OfInterval IndexIndicesDeal
Computational MatchNot MatchMembershipFindNub SieveEncodeDecodeMatrix InverseMatrix DivideFormatExecuteMaterialiseRange
Primitive operators Monadic EachCommuteConstantReplicateExpandReduceWindowed ReduceScanOuter ProductKeyI-beamSpawnFunction axis
Dyadic BindCompositions (Compose, Reverse Compose, Beside, Atop, Over) ∙ Inner ProductPowerAtUnderRankDepthVariantStencilCut (J)
Quad names
Arrays Index originMigration levelAtomic vector
Functions Case convertUnicode convert
Operators SearchReplace