Unique

The elements of a collection, each kept once, in first-occurrence order.

✓ checked against that system · ! checked, with disagreements (hover a chip)Wolfram LanguageDeleteDuplicates✓↗

Unique(xs)$xs$ with every repeat of an element dropped, keeping the first occurrence.

Unique(xs, test)as above, but two elements count as duplicates when $test$ holds for them, not just when they're structurally equal.

Typed signatures

Type	Package	Applies
((T) -> T where T: string) & ((collection<T>) -> list<T> where T: any)	compute-engine	always
(collection<any>, function?) -> list<any>	combinatoricsreplaces compute-engine	always

Domain: Collections

Examples

Basic #

live ↗

In$\mathrm{Unique}(\bigl\lbrack1, 2, 2, 3, 1\bigr\rbrack)$Out$\bigl\lbrack1, 2, 3\bigr\rbrack$

live ↗

In$\mathrm{Unique}(\bigl\lbrack a, b, a, c, b\bigr\rbrack)$Out$\bigl\lbrack a, b, c\bigr\rbrack$

Keeps first-occurrence order, without sorting

live ↗

In$\mathrm{Unique}(\bigl\lbrack1, 7, 8, 4, 3, 4, 1, 9, 9, 2, 1\bigr\rbrack)$Out$\bigl\lbrack1, 7, 8, 4, 3, 9, 2\bigr\rbrack$

Removes repeats, keeping first occurrences in order

live ↗

In$\mathrm{Unique}(\bigl\lbrack1, 7, 8, 4, 3, 4, 1, 9, 9, 2\bigr\rbrack)$Out$\bigl\lbrack1, 7, 8, 4, 3, 9, 2\bigr\rbrack$

Scope #

Elements can themselves be lists

live ↗

In$\mathrm{Unique}(\bigl\lbrack\bigl\lbrack1, 2\bigr\rbrack, \bigl\lbrack1, 2\bigr\rbrack, \bigl\lbrack3\bigr\rbrack\bigr\rbrack)$Out$\bigl\lbrack\bigl\lbrack1, 2\bigr\rbrack, \bigl\lbrack3\bigr\rbrack\bigr\rbrack$

live ↗

In$\mathrm{Unique}(\bigl\lbrack\bigr\rbrack)$Out$\bigl\lbrack\bigr\rbrack$

A second argument says when two elements count as duplicates -- here, when they differ by less than 2

live ↗

In$\mathrm{Unique}(\bigl\lbrack1, 2, 3, 4, 5, 6\bigr\rbrack, (\operatorname{\_1}, \operatorname{\_2})↦\vert\operatorname{\_1}-\operatorname{\_2}\vert\lt2)$Out$\bigl\lbrack1, 3, 5\bigr\rbrack$

With a custom comparison, an element is dropped when it relates to an earlier kept one

live ↗

In$\mathrm{Unique}(\bigl\lbrack1, 7, 8, 4, 3, 4, 1, 9, 8, 9, 2\bigr\rbrack, \mathrm{Greater})$Out$\bigl\lbrack1, 7, 8, 9, 9\bigr\rbrack$

Applications #

Count the distinct elements

live ↗

In$\mathrm{Length}(\mathrm{Unique}(\bigl\lbrack1, 2, 2, 3, 3, 3\bigr\rbrack))$Out$3$

The residues mod 3 that occur, in order of first appearance

live ↗

In$\mathrm{Unique}((1..10)\bmod3)$Out$\bigl\lbrack1, 2, 0\bigr\rbrack$

Properties #

Idempotent

live ↗

In$\mathrm{Unique}(\mathrm{Unique}(\bigl\lbrack1, 1, 2\bigr\rbrack))$Out$\bigl\lbrack1, 2\bigr\rbrack$

Possible issues #

A test that is never true treats nothing as a duplicate

live ↗

In$\mathrm{Unique}(\bigl\lbrack3, 3, 3\bigr\rbrack, ()↦\mathrm{False})$Out$\bigl\lbrack3, 3, 3\bigr\rbrack$

About

Examples	12
Checked against	Wolfram Language 12 agree
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