Unique: Difference between revisions

{{Primitive|∪|Unique}}, or '''Nub''', is a [[monadic]] [[set function]] which removes duplicate [[major cells]] from an array. It returns only the distinct cells (those which do not [[match]] an earlier distinct cell) in the order they appeared in the array.

In some languages, such as [[Dyalog APL]] and [[J]], Unique applies to the [[major cells]] of an array, including those with rank greater than 1:

The array matching is subject to [[tolerant comparison]]. In the intolerant case, that is, when equality is [https://en.wikipedia.org/wiki/Transitive_relation transitive], a cell is included if and only if it does not match any other cell which appears earlier in the array. That's because all the discarded duplicate cells must have matched an earlier included cell; if equality is transitive then a cell which matches the duplicate would also match the earlier cell.

For [[Vector|vectors]] the following implementation based on [[Union]] can be used. It repeatedly adds cells of the argument to an accumulated unique vector <code>u</code>, using Union so that duplicate cells are never added.

While the model <code>{((⍳≢⍵)=(⍵⍳⍵))⌿⍵}</code>, based on [[Nub Sieve]], is often described as an implementation of Unique, it does not correctly handle [[tolerant comparison]]. A correct implementation of Nub Sieve could be used to implement Unique, but writing such an implementation is no easier than implementing Unique directly.

* [http://help.dyalog.com/latest/Content/Language/Primitive%20Functions/Unique.htm Dyalog]

* [https://www.jsoftware.com/help/dictionary/d221.htm J Dictionary], [https://code.jsoftware.com/wiki/Vocabulary/tildedot J NuVoc] (as <code>~.</code>)