Nest

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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[1] using the glyph , and was added to Dyalog APL 16.0 using the glyph .

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.

      EWords{+/'E'¨}
Works in: Dyalog APL

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

      EWords 'I' 'ATE' 'DINNER' 'AND' 'WENT' 'TO' 'SLEEP'  ⍝ ATE, DINNER, WENT, SLEEP
4
Works in: Dyalog APL

But if the user gives only one word, EWords will count E's in each letter instead, giving the wrong answer:

      EWords 'SLEEP'
2
Works in: Dyalog APL

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

      EWords2{+/'E'¨}
      EWords2 'I' 'ATE' 'DINNER' 'AND' 'WENT' 'TO' 'SLEEP'
4
      EWords2 'SLEEP'
1
Works in: Dyalog APL

External links

Documentation

References

  1. "Language Extensions of May 1983". SATN-45, 1983-05-02.


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