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{{Built-in|Constant|⍨}} is a [[monadic operator]] which takes an [[array]] as its [[operand]] and becomes a [[function]] which returns the operand array regardless of its [[argument|arguments]]. It was first introduced in [[Extended Dyalog APL]], sharing its [[glyph]] with [[Commute]], and was adopted in [[Dyalog APL 18.0]] as an alternative to the constant [[dfn]] such as < | {{Built-in|Constant|⍨}} is a [[monadic operator]] which takes an [[array]] as its [[operand]] and becomes a [[function]] which returns the operand array regardless of its [[argument|arguments]]. It was first introduced in [[Extended Dyalog APL]], sharing its [[glyph]] with [[Commute]], and was adopted in [[Dyalog APL 18.0]] as an alternative to the constant [[dfn]] such as <syntaxhighlight lang=apl inline>{0}</syntaxhighlight>. | ||
== Examples == | == 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 < | 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 <syntaxhighlight lang=apl inline>A⍨</syntaxhighlight> over a constant dfn <syntaxhighlight lang=apl inline>{A}</syntaxhighlight> is that the array <syntaxhighlight lang=apl inline>A</syntaxhighlight> is evaluated only once at definition time, rather than every time the function is called. | ||
=== Trains === | === Trains === | ||
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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. | 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. | ||
< | <syntaxhighlight lang=apl> | ||
f0←{(⍺+⍵)*3} ⍝ Converting this function to a train was a mess: | 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) | fx←+*3 ⍝ This does not work; it evaluates to a number (conjugate of exponential of 3) | ||
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=== Other uses === | === Other uses === | ||
Sometimes, one needs a constant function that returns one of the arguments of the outer dfn. Simply writing < | Sometimes, one needs a constant function that returns one of the arguments of the outer dfn. Simply writing <syntaxhighlight lang=apl inline>{⍺}</syntaxhighlight> does not work; <syntaxhighlight lang=apl inline>⍺⍨</syntaxhighlight> does. | ||
< | <syntaxhighlight lang=apl> | ||
1 0 0 1{'⎕'@{⍺}⍵}'AbcD' | 1 0 0 1{'⎕'@{⍺}⍵}'AbcD' | ||
VALUE ERROR | VALUE ERROR | ||
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Using Constant is also cleaner when doing a constant fill. | Using Constant is also cleaner when doing a constant fill. | ||
< | <syntaxhighlight lang=apl> | ||
{1}¨2 3⍴⎕A ⍝ Without Constant | {1}¨2 3⍴⎕A ⍝ Without Constant | ||
1 1 1 | 1 1 1 | ||
Latest revision as of 22:15, 10 September 2022
⍨
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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, sharing its glyph with Commute, 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 workaround using Identity
f5←+*3⍨ ⍝ A solution using ConstantWorks 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 1Works in: Extended Dyalog APL, Dyalog APL 18.0
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