`unique`

Distinct values of an array, sorted, with optional index maps.

Category: Language Fundamentals > Data Types

Syntax

C = unique(A)

Returns the sorted distinct values of A.

C = unique(A, 'stable')

Keeps the order of first appearance instead of sorting.

[C, ia, ic] = unique(A)

Also returns index vectors: C = A(ia) and A = C(ic).

Description

unique removes repeated values. Numbers are sorted ascending and text alphabetically. Several NaN values are all counted as different, so each NaN is kept.

The third output ic is especially useful: accumarray(ic, 1) counts how many times each distinct value occurs.

Input arguments

A: Numeric, logical, char, string array, cell array of character vectors, categorical or table.
'stable': Return values in the order they first appear.

Output arguments

C: The distinct values. For a row vector input the result is a row; for a cell array of text it is a column.
ia: Indices of the first occurrence of each value of C in A.
ic: Indices that rebuild A from C, one per element of A. It is also a way to number groups.

Example

Values, indices and stable order

[u, i, j] = unique([3 1 1 2 3])
unique({'b','a','b'})
unique([3 1 1 2 3], 'stable')

Output

u = 1×3

     1     2     3
i = 3×1

     2
     4
     1
j = 5×1

     3
     1
     1
     2
     3
ans = 2×1 cell

    {'a'}
    {'b'}
ans = 1×3

     3     1     2

unique