Constant

Constant (⍨) is a monadic operator which takes an array as its operand and becomes a function which returns the operand array regardless of its arguments. It was first introduced in Extended Dyalog APL, and was adopted in Dyalog APL 18.0 as an alternative to the constant dfn such as {0}.

Examples

The need for Constant arises in various contexts, such as at the rightmost branch in a train and mapping over an array to create a constant-filled one. The major advantage of Constant A⍨ over a constant dfn {A} is that the array A is evaluated only once at definition time, rather than every time the function is called.

Trains

If the rightmost branch of a train is an array, it is not recognised as a train at all. This problem can be worked around in many ways, but none is visually appealing. The Constant operator gives a natural solution to this problem.

f0←{(⍺+⍵)*3}  ⍝ Converting this function to a train was a mess:
fx←+*3        ⍝ This does not work; it evaluates to a number (conjugate of exponential of 3)
f1←3*⍨+       ⍝ A workaround using Commute; it changes the order of visual tokens
f2←*∘3+       ⍝ A workaround using Bind; ditto
f3←+*{3}      ⍝ A workaround using a constant dfn

f4←+*3⍨       ⍝ A solution using Constant

Works in: Extended Dyalog APL, Dyalog APL 18.0

Other uses

Sometimes, one needs a constant function that returns one of the arguments of the outer dfn. Simply writing {⍺} does not work; ⍺⍨ does.

      1 0 0 1{'⎕'@{⍺}⍵}'AbcD'
VALUE ERROR             
 1 0 0 1{'⎕'@{⍺}⍵}'AbcD'
             ∧     
      1 0 0 1{'⎕'@(⍺⍨)⍵}'AbcD'
⎕bc⎕

Works in: Extended Dyalog APL, Dyalog APL 18.0

Using Constant is also cleaner when doing a constant fill.

      {1}¨2 3⍴⎕A  ⍝ Without Constant
1 1 1
1 1 1
      1⍨¨2 3⍴⎕A   ⍝ With Constant
1 1 1
1 1 1

Works in: Extended Dyalog APL, Dyalog APL 18.0

External links

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

Constant

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Examples

Trains

Other uses

External links

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Constant

Examples

Trains

Other uses

External links

Webinars

Lessons

Documentation

Navigation menu

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