Unique: Difference between revisions

When called on the [[string]] "Mississippi", Unique keeps the first three letters and a later "p", but removes all subsequent occurrences.

       ∪ 'Mississippi'

Misp

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It works on nested arrays as well:

       ∪ v←'CAT' 'DOG' 'CAT' 'DUCK' 'DOG' 'DUCK'

┌───┬───┬────┐

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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:

       ⊢x ← (⍳10)∘.∨2 3 6

1 1 1

The following example shows why we must be careful about how cells are matched when tolerant comparison is involved. Here we produce a vector in which the first and second elements match, as do the second and third, but the first and third elements do not! Unique produces an array which contains an element matching every element of the original array, but a less careful definition (for example, excluding every element which matches an earlier one) would fail to satisfy this property.

       ⎕CT←1e¯14

       ⎕PP←18 ⍝ Print all digits

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.

VecUnique ← {

     u ← 0↑⍵

{{Works in|[[Dyalog APL]]}}

The accumulation above resembles a [[reduction]]—in fact, it is a reverse [[insertion]]. The following implementation written with Reduce works for [[simple]] vectors in [[Nested array model|nested]] APLs because reduction is equivalent to insertion followed by [[Enclose]] on such vectors:

VecUnique ← {⊃∪⍨/⌽⍵}

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{{Works in|[[Dyalog APL]],[[ngn/apl]]}}

It can be modified to obtain a full model for Unique by enclosing enough times, and converting scalars to vectors with [[Replicate]] at the end:

Unique ← {1/↑↑⌽∪/⌽⊂∘⊂⍤¯1⊢⍵}

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