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{{Built-ins|Nest|⊆|⊃}}, also called '''Conditional Enclose''' or '''Enclose If Simple''' (commonly abbreviated as '''eis'''), is a [[monadic]] [[primitive function]] that applies [[Enclose]] to the given [[argument]], but only if it is [[simple]]. Nest first appeared as an extension to [[SHARP APL]]<ref>[https://www.jsoftware.com/papers/satn45.htm "Language Extensions of May 1983"]. SATN-45, 1983-05-02.</ref> using the [[glyph]] < | {{Built-ins|Nest|⊆|⊃}}, also called '''Conditional Enclose''' or '''Enclose If Simple''' (commonly abbreviated as '''eis'''), is a [[monadic]] [[primitive function]] that applies [[Enclose]] to the given [[argument]], but only if it is [[simple]]. Nest first appeared as an extension to [[SHARP APL]]<ref>[https://www.jsoftware.com/papers/satn45.htm "Language Extensions of May 1983"]. SATN-45, 1983-05-02.</ref> using the [[glyph]] <syntaxhighlight lang=apl inline>⊃</syntaxhighlight>, and was added to [[Dyalog APL]] 16.0 using the glyph <syntaxhighlight lang=apl inline>⊆</syntaxhighlight>. | ||
== Examples == | == Examples == | ||
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Nest is useful when a [[nested array]] is expected but the user may supply a simple array instead. For example, consider a function which expects one or more English words in uppercase and counts the words that include the letter E. | Nest is useful when a [[nested array]] is expected but the user may supply a simple array instead. For example, consider a function which expects one or more English words in uppercase and counts the words that include the letter E. | ||
< | <syntaxhighlight lang=apl> | ||
EWords←{+/'E'∊¨⍵} | EWords←{+/'E'∊¨⍵} | ||
</syntaxhighlight>{{Works in|[[Dyalog APL]]}} | </syntaxhighlight>{{Works in|[[Dyalog APL]]}} | ||
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If the user gives multiple words in the usual notation, it works correctly: | If the user gives multiple words in the usual notation, it works correctly: | ||
< | <syntaxhighlight lang=apl> | ||
EWords 'I' 'ATE' 'DINNER' 'AND' 'WENT' 'TO' 'SLEEP' ⍝ ATE, DINNER, WENT, SLEEP | EWords 'I' 'ATE' 'DINNER' 'AND' 'WENT' 'TO' 'SLEEP' ⍝ ATE, DINNER, WENT, SLEEP | ||
4 | 4 | ||
</syntaxhighlight>{{Works in|[[Dyalog APL]]}} | </syntaxhighlight>{{Works in|[[Dyalog APL]]}} | ||
But if the user gives only one word, < | But if the user gives only one word, <syntaxhighlight lang=apl inline>EWords</syntaxhighlight> will count E's in each ''letter'' instead, giving the wrong answer: | ||
< | <syntaxhighlight lang=apl> | ||
EWords 'SLEEP' | EWords 'SLEEP' | ||
2 | 2 | ||
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In this case, the programmer can apply Nest to the argument so that the array has a consistent structure. | In this case, the programmer can apply Nest to the argument so that the array has a consistent structure. | ||
< | <syntaxhighlight lang=apl> | ||
EWords2←{+/'E'∊¨⊆⍵} | EWords2←{+/'E'∊¨⊆⍵} | ||
EWords2 'I' 'ATE' 'DINNER' 'AND' 'WENT' 'TO' 'SLEEP' | EWords2 'I' 'ATE' 'DINNER' 'AND' 'WENT' 'TO' 'SLEEP' |