Under

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⍢

Under (⍢), or Dual (¨), is a primitive dyadic operator which takes two function operands. It pre-processes its argument(s) with the monadic right operand, before applying the left operand on/between the result(s), then undoing the pre-processing.[1]

Model

In dialects that support user-defined operators and invertible functions, it can be defined as (or similar): 

       _U_ ← {⍺←{⍵ ⋄ ⍺⍺} ⋄ ⍵⍵⍣¯1⊢(⍵⍵ ⍺)⍺⍺(⍵⍵ ⍵)}

Examples

      ⍝ https://leetcode.com/problems/add-two-numbers/
      2 4 3 +_U_(10⊥⌽) 5 6 4
7 0 8

History

The Dual operator, written ¨, appeared in 1983 both in Rationalized APL and SHARP APL[2] just after. In both cases it was defined to be a close composition, so that the overall function has the rank of the right operand; this allows ¨> to serve the purpose of Each from a nested APL. Iverson changed the name to Under for his 1987 A Dictionary of APL, and J uses this name as well. It includes both a close form &. like SHARP and a non-close form &.:.

Roger Hui proposed Under as a potential addition to Dyalog APL at Dyalog '15, using the glyph ⍢. This definition was added to both Extended Dyalog APL and dzaima/APL in 2018. Structural Under was developed by Marshall Lochbaum around 2017,[3] and was included in Extended Dyalog initially and added to dzaima/APL in 2020. Later dzaima would implement ⍢× and ⍢| in the same spirit, which are featured in dzaima/APL and dzaima/BQN. The language BQN has included structural and computational Under, using the glyph ⌾, since its initial design in 2020.

External links

References

  1. ↑ BQN: Circle Jot (⌾)
  2. ↑ "Language Extensions of May 1983". SATN-45, 1983-05-02.
  3. ↑ Marshall Lochbaum. BQN's development history: Structural Under.
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)
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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