unique — Return unique elements or rows with MATLAB-compatible ordering and index mappings.
unique(A) removes duplicates and returns distinct elements or rows. It supports MATLAB-compatible sorted/stable ordering plus optional index outputs linking original and unique values.
Syntax
C = unique(A)
C = unique(A, option...)
[C, ia] = unique(A)
[C, ia] = unique(A, option...)
[C, ia, ic] = unique(A)
[C, ia, ic] = unique(A, option...)Inputs
| Name | Type | Required | Default | Description |
|---|---|---|---|---|
A | Any | Yes | — | Input array. |
option | StringScalar | Variadic | — | Option tokens: 'sorted'|'stable'|'rows'|'first'|'last'. |
Returns
| Name | Type | Description |
|---|---|---|
C | Any | Unique values or rows. |
ia | NumericArray | Indices selecting representatives in input A. |
ic | NumericArray | Indices mapping each input element/row to C. |
Returned values from unique depend on how many outputs the caller requests.
Errors
| Identifier | When | Message |
|---|---|---|
RunMat:unique:LegacyOptionUnsupported | Legacy compatibility options are requested. | unique: the 'legacy' behaviour is not supported |
RunMat:unique:ConflictingOrderOptions | Both 'sorted' and 'stable' options are provided. | unique: cannot combine 'sorted' with 'stable' |
RunMat:unique:ConflictingOccurrenceOptions | Both 'first' and 'last' options are provided. | unique: cannot combine 'first' with 'last' |
RunMat:unique:UnknownOption | An unsupported option token is provided. | unique: unrecognised option |
RunMat:unique:RowsRequiresTwoDimensionalInput | 'rows' mode is used with non-2D data. | unique: 'rows' option requires a 2-D matrix input |
RunMat:unique:UnsupportedInputType | Input cannot be converted into a supported unique domain. | unique: unsupported input type |
RunMat:unique:InvalidArgument | Option arguments are not string-like where required. | unique: expected string option arguments |
RunMat:unique:Internal | Internal conversion/allocation/provider decode fails. | unique: internal operation failed |
How unique works
unique(A)flattens numeric, logical, character, string, or complex arrays column-major into a vector of unique values sorted ascending.[C, IA] = unique(A)also returns the indices of the selected occurrences (IA) so thatC = A(IA).[C, IA, IC] = unique(A)providesIC, the mapping from each element ofAto the corresponding index inC.unique(A, 'stable')preserves the first appearance order rather than sorting.unique(A, 'rows')treats each row as an observation and returns a matrix (or char/string array) whose rows are unique.unique(A, 'last')or'first'controls which occurrence contributes toIA(defaults to'first').- Combinations such as
unique(A, 'rows', 'stable', 'last')follow MATLAB's precedence rules; mutually exclusive flags (e.g.'sorted'with'stable') are rejected. - Empty inputs return empty outputs with consistent dimensions.
- Legacy switches such as
'legacy'or'R2012a'are not supported; RunMat always follows the modern MATLAB semantics.
Does RunMat run unique on the GPU?
unique is registered as a residency sink. When the provider exposes the custom unique hook, the runtime can execute the operation entirely on the device and keep results resident. If the active provider does not implement that hook, RunMat gathers the data to host memory, performs the CPU implementation, and returns host-resident outputs so subsequent MATLAB code observes the same values and ordering.
Examples
Getting Sorted Unique Values
A = [3 1 3 2];
C = unique(A)Expected output:
C =
1
2
3Preserving Input Order with 'stable'
A = [4 2 4 1 2];
C = unique(A, 'stable')Expected output:
C =
4
2
1Returning Indices for Reconstruction
A = [7 5 7 3];
[C, IA, IC] = unique(A);
reconstructed = C(IC)Expected output:
C =
3
5
7
IA =
4
2
1
IC =
3
2
3
1
reconstructed =
7
5
7
3Finding Unique Rows in a Matrix
A = [1 3; 1 3; 2 4; 1 2];
[C, IA, IC] = unique(A, 'rows')Expected output:
C =
1 2
1 3
2 4
IA =
4
1
3
IC =
2
2
3
1Selecting Last Occurrences
A = [9 8 9 7 8];
[C, IA] = unique(A, 'last')Expected output:
C =
7
8
9
IA =
4
5
3Working with Empty Arrays
A = zeros(0, 3);
[C, IA, IC] = unique(A, 'rows')Expected output:
C =
IA =
IC =Using unique on GPU Arrays
G = gpuArray([5 3 5 1]);
[C, IA, IC] = unique(G, 'stable')Expected output:
C =
5
3
1
IA =
1
2
4
IC =
1
2
1
3Unique Characters in a Char Array
chars = ['m','z'; 'm','a'];
[C, IA] = unique(chars)Expected output:
C =
a
m
z
IA =
4
1
3Unique Strings with Row Deduplication
S = ["alpha" "beta"; "alpha" "beta"; "gamma" "beta"];
[C, IA, IC] = unique(S, 'rows', 'stable')Expected output:
C =
2x2 string array
"alpha" "beta"
"gamma" "beta"
IA =
1
3
IC =
1
1
2Using unique with coding agents
Open a RunMat example with live inputs, then ask the agent to explain how unique changes the result.
Run a small unique example, explain the result, then change one input and compare the output.
FAQ
Which ordering does unique use by default?⌄
Results are sorted in ascending order unless you pass 'stable', which preserves the first occurrence order.
How are the index outputs defined?⌄
IA indexes into the original data (or rows). IC is a column vector mapping each element (or row) of the input to the position of the corresponding unique value in C.
What do 'first' and 'last' control?⌄
They determine whether IA references the first or last occurrence of each distinct value/row. They do not affect C or IC.
Can I combine 'rows' with 'stable' or 'last'?⌄
Yes. All permutations of 'rows', 'stable'/'sorted', and 'first'/'last' are accepted. The runtime enforces MATLAB's validation rules.
Does unique support complex numbers or characters?⌄
Yes. Complex values use magnitude ordering for the sorted output, and character or string arrays produce results in their native container types (char arrays and string arrays respectively).
How does unique treat NaN values?⌄
All NaN values are considered equal. Sorted outputs place NaNs at the end; stable outputs keep their original relative order.
Are GPU arrays supported?⌄
Yes. When a provider lacks a native kernel, RunMat gathers GPU arrays to host memory and executes the host implementation, guaranteeing MATLAB-compatible output.
Does unique preserve array shape?⌄
Scalar outputs remain scalars. Otherwise, values are returned as column vectors (for element mode) or matrices with the same number of columns as the input (for 'rows').
What happens with empty inputs?⌄
Empty inputs (including empty matrices) return empty outputs with matching dimensions, and index outputs are empty column vectors.
Is unique stable?⌄
Sorting is stable where applicable; ties preserve their relative order. You can also request 'stable' explicitly.
Related Array functions
Shape
cat · circshift · diag · flip · fliplr · flipud · horzcat · ipermute · kron · permute · repelem · repmat · reshape · rot90 · squeeze · tril · triu · vertcat
Open-source implementation
Unlike proprietary runtimes, every RunMat function is open-source. Read exactly how unique is executed, line by line, in Rust.
- View the source for unique in Rust on GitHub
- Learn how the RunMat runtime works
- Found a bug? Open an issue with a minimal reproduction.
About RunMat
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