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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:

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      Shuffle←?⍨∘≢⊇⊢
      Shuffle 'abcdef'
fbdcea

Works in: dzaima/APL, Extended Dyalog APL

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

Works in: dzaima/APL, Extended Dyalog APL

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

APL built-ins [edit]
Primitives (Timeline)	Functions
Scalar
		Monadic	Conjugate ∙ Negate ∙ Signum ∙ Reciprocal ∙ Magnitude ∙ Exponential ∙ Natural Logarithm ∙ Floor ∙ Ceiling ∙ Factorial ∙ Not ∙ Pi Times ∙ Roll ∙ Type ∙ Imaginary ∙ Square Root ∙ Round
		Dyadic	Add ∙ Subtract ∙ Times ∙ Divide ∙ Residue ∙ Power ∙ Logarithm ∙ Minimum ∙ Maximum ∙ Binomial ∙ Comparison functions ∙ Boolean functions (And, Or, Nand, Nor) ∙ GCD ∙ LCM ∙ Circular ∙ Complex ∙ Root
Non-Scalar
		Structural	Shape ∙ Reshape ∙ Tally ∙ Depth ∙ Ravel ∙ Enlist ∙ Table ∙ Catenate ∙ Reverse ∙ Rotate ∙ Transpose ∙ Raze ∙ Mix ∙ Split ∙ Enclose ∙ Nest ∙ Cut (K) ∙ Pair ∙ Link ∙ Partitioned Enclose ∙ Partition
		Selection	First ∙ Pick ∙ Take ∙ Drop ∙ Unique ∙ Identity ∙ Stop ∙ Select ∙ Replicate ∙ Expand ∙ Set functions (Intersection ∙ Union ∙ Without) ∙ Bracket indexing ∙ Index ∙ Cartesian Product ∙ Sort
		Selector	Index generator ∙ Grade ∙ Index Of ∙ Interval Index ∙ Indices ∙ Deal ∙ Prefix and suffix vectors
		Computational	Match ∙ Not Match ∙ Membership ∙ Find ∙ Nub Sieve ∙ Encode ∙ Decode ∙ Matrix Inverse ∙ Matrix Divide ∙ Format ∙ Execute ∙ Materialise ∙ Range
Operators	Monadic	Each ∙ Commute ∙ Constant ∙ Replicate ∙ Expand ∙ Reduce ∙ Windowed Reduce ∙ Scan ∙ Outer Product ∙ Key ∙ I-Beam ∙ Spawn ∙ Function axis ∙ Identity (Null, Ident)
Operators	Dyadic	Bind ∙ Compositions (Compose, Reverse Compose, Beside, Withe, Atop, Over) ∙ Inner Product ∙ Determinant ∙ Power ∙ At ∙ Under ∙ Rank ∙ Depth ∙ Variant ∙ Stencil ∙ Cut ∙ Direct definition (operator) ∙ Identity (Lev, Dex)
Quad names	Index origin ∙ Comparison tolerance ∙ Migration level ∙ Atomic vector

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Common usage

Potential extension

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