Nest

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 using the glyph <source lang=apl inline>⊃</syntaxhighlight>, and was added to Dyalog APL 16.0 using the glyph <source lang=apl inline>⊆</syntaxhighlight>.

Examples

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.

<source lang=apl>

     EWords←{+/'E'∊¨⍵}

</syntaxhighlight>

Works in: Dyalog APL

If the user gives multiple words in the usual notation, it works correctly:

<source lang=apl>

     EWords 'I' 'ATE' 'DINNER' 'AND' 'WENT' 'TO' 'SLEEP'  ⍝ ATE, DINNER, WENT, SLEEP

4

</syntaxhighlight>

Works in: Dyalog APL

But if the user gives only one word, <source lang=apl inline>EWords</syntaxhighlight> will count E's in each letter instead, giving the wrong answer:

<source lang=apl>

     EWords 'SLEEP'

2

</syntaxhighlight>

Works in: Dyalog APL

In this case, the programmer can apply Nest to the argument so that the array has a consistent structure.

<source lang=apl>

     EWords2←{+/'E'∊¨⊆⍵}
     EWords2 'I' 'ATE' 'DINNER' 'AND' 'WENT' 'TO' 'SLEEP'

4

     EWords2 'SLEEP'

1

</syntaxhighlight>

Works in: Dyalog APL

External links

Documentation

• Dyalog