Constant: Difference between revisions
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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 <source lang=apl inline>{0}</ | {{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 <source 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 <source lang=apl inline>A⍨</ | 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 <source lang=apl inline>A⍨</syntaxhighlight> over a constant dfn <source lang=apl inline>{A}</syntaxhighlight> is that the array <source 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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f5←+*3⍨ ⍝ A solution using Constant | f5←+*3⍨ ⍝ A solution using Constant | ||
</ | </syntaxhighlight>{{Works in|[[Extended Dyalog APL]], [[Dyalog APL 18.0]]}} | ||
=== Other uses === | === Other uses === | ||
Sometimes, one needs a constant function that returns one of the arguments of the outer dfn. Simply writing <source lang=apl inline>{⍺}</ | Sometimes, one needs a constant function that returns one of the arguments of the outer dfn. Simply writing <source lang=apl inline>{⍺}</syntaxhighlight> does not work; <source lang=apl inline>⍺⍨</syntaxhighlight> does. | ||
<source lang=apl> | <source lang=apl> | ||
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1 0 0 1{'⎕'@(⍺⍨)⍵}'AbcD' | 1 0 0 1{'⎕'@(⍺⍨)⍵}'AbcD' | ||
⎕bc⎕ | ⎕bc⎕ | ||
</ | </syntaxhighlight>{{Works in|[[Extended Dyalog APL]], [[Dyalog APL 18.0]]}} | ||
Using Constant is also cleaner when doing a constant fill. | Using Constant is also cleaner when doing a constant fill. | ||
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1 1 1 | 1 1 1 | ||
1 1 1 | 1 1 1 | ||
</ | </syntaxhighlight>{{Works in|[[Extended Dyalog APL]], [[Dyalog APL 18.0]]}} | ||
== External links == | == External links == | ||
Revision as of 21:37, 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 <source lang=apl inline>{0}</syntaxhighlight>.
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 <source lang=apl inline>A⍨</syntaxhighlight> over a constant dfn <source lang=apl inline>{A}</syntaxhighlight> is that the array <source lang=apl inline>A</syntaxhighlight> 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.
<source lang=apl> 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 Constant
</syntaxhighlight>
Other uses
Sometimes, one needs a constant function that returns one of the arguments of the outer dfn. Simply writing <source lang=apl inline>{⍺}</syntaxhighlight> does not work; <source lang=apl inline>⍺⍨</syntaxhighlight> does.
<source lang=apl>
1 0 0 1{'⎕'@{⍺}⍵}'AbcD'
VALUE ERROR
1 0 0 1{'⎕'@{⍺}⍵}'AbcD'
∧
1 0 0 1{'⎕'@(⍺⍨)⍵}'AbcD'
⎕bc⎕
</syntaxhighlight>
Using Constant is also cleaner when doing a constant fill.
<source lang=apl>
{1}¨2 3⍴⎕A ⍝ Without Constant
1 1 1 1 1 1
1⍨¨2 3⍴⎕A ⍝ With Constant
1 1 1 1 1 1
</syntaxhighlight>
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