Prototype: Difference between revisions

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== Examples ==
== Examples ==


The prototype is used when an array is expanded or when empty elements are inserted in some other way. [[Take]] and [[Expand]] on a [[character]] array produce an array with spaces, because <code>' '</code> is the prototype of a character array:
The prototype is used when an array is expanded or when empty elements are inserted in some other way. [[Take]] and [[Expand]] on a [[character]] array produce an array with spaces, because <source lang=apl inline>' '</code> is the prototype of a character array:
<source lang=apl>
<source lang=apl>
       ¯12 ↑ 'whitespace'
       ¯12 ↑ 'whitespace'
Line 13: Line 13:
</source>
</source>


The [[Rank operator]] combines its left operand's result cells by extending them to a common shape like [[Mix]] does. Numeric vectors of different lengths will be expanded with <code>0</code>, the prototype for a simple numeric array.
The [[Rank operator]] combines its left operand's result cells by extending them to a common shape like [[Mix]] does. Numeric vectors of different lengths will be expanded with <source lang=apl inline>0</code>, the prototype for a simple numeric array.
<source lang=apl>
<source lang=apl>
       (⍳⍤0) 2 3 4
       (⍳⍤0) 2 3 4
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{{Works in|[[Dyalog APL]]}}
{{Works in|[[Dyalog APL]]}}


An array's prototype is based on its first element, so the array <code>a</code> below has a simple numeric prototype, as does any prefix of <code>a</code>. This property even applies to empty arrays: the expression <code>0 ↑ a</code> results in an empty array with the same prototype as <code>a</code>.  
An array's prototype is based on its first element, so the array <source lang=apl inline>a</code> below has a simple numeric prototype, as does any prefix of <source lang=apl inline>a</code>. This property even applies to empty arrays: the expression <source lang=apl inline>0 ↑ a</code> results in an empty array with the same prototype as <source lang=apl inline>a</code>.  
<source lang=apl>
<source lang=apl>
       ⊢a ← 5 'c' ¯2
       ⊢a ← 5 'c' ¯2

Revision as of 09:06, 29 October 2019

In the nested array model, an array's prototype is an array which gives information about the type and structure of its elements. It is derived from the first element of the array in ravel order, or, if the array is empty, from information stored alongside the array (usually this information is just the prototype itself). An array's prototype is used as a fill element for operations involving that array.

APLs with a flat array model do not typically store prototype information, instead using only the array's type (character, numeric, or boxed) to determine its fills.

Examples

The prototype is used when an array is expanded or when empty elements are inserted in some other way. Take and Expand on a character array produce an array with spaces, because ' '</code> is the prototype of a character array: <source lang=apl> ¯12 ↑ 'whitespace' whitespace (5 1 5/1 ¯2 1) \ 'whitespace' white space

The Rank operator combines its left operand's result cells by extending them to a common shape like Mix does. Numeric vectors of different lengths will be expanded with 0</code>, the prototype for a simple numeric array. <source lang=apl> (⍳⍤0) 2 3 4 1 2 0 0 1 2 3 0 1 2 3 4

Works in: Dyalog APL

An array's prototype is based on its first element, so the array a</code> below has a simple numeric prototype, as does any prefix of <source lang=apl inline>a</code>. This property even applies to empty arrays: the expression <source lang=apl inline>0 ↑ a</code> results in an empty array with the same prototype as <source lang=apl inline>a</code>. <source lang=apl> ⊢a ← 5 'c' ¯2 5 c ¯2 4 ↑ a 5 c ¯2 0 4 ↑ 2 ↑ a 5 c 0 0 4 ↑ 0 ↑ a 0 0 0 0

Works in: Dyalog APL

The prototype of an array may be nested. Here, we inspect the prototype of an array containing both character and numeric data. In the prototype, all characters are converted to spaces and all numbers to zeros. Thus the prototype retains type and structure information but not specific values.

      ⊢x ← ⊂'de'(3 4 5)   ⍝ A complicated scalar array
┌──────────┐
│┌──┬─────┐│
││de│3 4 5││
│└──┴─────┘│
└──────────┘
      0 1 \ x             ⍝ Its fill element is nested
┌──────────┬──────────┐
│┌──┬─────┐│┌──┬─────┐│
││  │0 0 0│││de│3 4 5││
│└──┴─────┘│└──┴─────┘│
└──────────┴──────────┘
      e ← 0 ↑ x           ⍝ An empty array based on x
      ⊃e                  ⍝ Disclosing gets the prototype
┌──┬─────┐
│  │0 0 0│
└──┴─────┘
Works in: Dyalog APL