setdiff — Return values present in the first input but absent from the second, with MATLAB-compatible ordering modes.
setdiff(A, B) returns unique values (or rows) that appear in A but not in B. It supports MATLAB-compatible 'sorted' and 'stable' ordering modes plus row-wise set differences.
Syntax
C = setdiff(A, B)
C = setdiff(A, B, option...)
[C, ia] = setdiff(A, B)
[C, ia] = setdiff(A, B, option...)Inputs
| Name | Type | Required | Default | Description |
|---|---|---|---|---|
A | Any | Yes | — | First input array. |
B | Any | Yes | — | Second input array. |
option | StringScalar | Variadic | — | Option tokens: 'rows'|'sorted'|'stable'. |
Returns
| Name | Type | Description |
|---|---|---|
C | Any | Values that appear in A but not in B. |
ia | NumericArray | Indices selecting retained values/rows from A. |
Returned values from setdiff depend on how many outputs the caller requests.
Errors
| Identifier | When | Message |
|---|---|---|
RunMat:setdiff:LegacyOptionUnsupported | Legacy compatibility options are requested. | setdiff: the 'legacy' behaviour is not supported |
RunMat:setdiff:ConflictingOrderOptions | Both 'sorted' and 'stable' options are provided. | setdiff: cannot combine 'sorted' with 'stable' |
RunMat:setdiff:UnknownOption | An unsupported option token is provided. | setdiff: unrecognised option |
RunMat:setdiff:RowsColumnMismatch | 'rows' mode is used and column counts differ. | setdiff: inputs must have the same number of columns when using 'rows' |
RunMat:setdiff:UnsupportedInputType | Input values cannot be converted into supported setdiff domains. | setdiff: unsupported input type |
RunMat:setdiff:NumericClassMismatch | Numeric inputs have incompatible nondouble classes. | setdiff: numeric inputs must have the same class, except double may be combined with one nondouble class |
RunMat:setdiff:InvalidArgument | Option arguments are not string-like where required. | setdiff: expected string option arguments |
RunMat:setdiff:Internal | Internal conversion/allocation/provider decode fails. | setdiff: internal operation failed |
How setdiff works
setdiff(A, B)flattens inputs column-major, removes duplicates, subtracts the values ofBfromA, and returns the remaining elements sorted ascending by default.[C, IA] = setdiff(A, B)also returns indices so thatC = A(IA).setdiff(A, B, 'stable')preserves the first appearance order fromAinstead of sorting.setdiff(A, B, 'rows')treats each row as an element. Inputs must share the same number of columns.- Character arrays, string arrays, logical arrays, numeric types, and complex values are all supported.
- Legacy flags (
'legacy','R2012a') are not supported; RunMat always follows modern MATLAB semantics.
Does RunMat run setdiff on the GPU?
setdiff is registered as a residency sink. When tensors reside on the GPU and the active provider does not yet implement a setdiff hook, RunMat gathers them to host memory, performs the CPU implementation, and materialises host-resident results. Future providers can wire a custom hook to perform the set difference directly on-device without affecting existing callers.
Examples
Finding values exclusive to the first numeric vector
A = [5 7 5 1];
B = [7 1 3];
[C, IA] = setdiff(A, B)Expected output:
C =
5
IA =
1Preserving input order with 'stable'
A = [4 2 4 1 3];
B = [3 4 5 1];
[C, IA] = setdiff(A, B, 'stable')Expected output:
C =
2
IA =
2Working with rows of numeric matrices
A = [1 2; 3 4; 1 2];
B = [3 4; 5 6];
[C, IA] = setdiff(A, B, 'rows')Expected output:
C =
1 2
IA =
1Computing set difference for character data
A = ['m','z'; 'm','a'];
B = ['a','x'; 'm','a'];
[C, IA] = setdiff(A, B)Expected output:
C =
m
IA =
1Subtracting string arrays by row
A = ["alpha" "beta"; "gamma" "beta"];
B = ["gamma" "beta"; "delta" "beta"];
[C, IA] = setdiff(A, B, 'rows', 'stable')Expected output:
C =
1x2 string array
"alpha" "beta"
IA =
1Using setdiff with GPU arrays
G = gpuArray([10 4 6 4]);
H = gpuArray([6 4 2]);
C = setdiff(G, H)Expected output:
C =
10Using setdiff with coding agents
Open a RunMat example with live inputs, then ask the agent to explain how setdiff changes the result.
Run a small setdiff example, explain the result, then change one input and compare the output.
FAQ
What ordering does setdiff use by default?⌄
Results are sorted ascending. Specify 'stable' to preserve the first appearance order from the first input.
How are the index outputs defined?⌄
IA points to the positions in A that correspond to each element (or row) returned in C, using MATLAB's one-based indexing.
Can I combine 'rows' with 'stable'?⌄
Yes. 'rows' can be paired with either 'sorted' (default) or 'stable'. Other option combinations that conflict (e.g. 'sorted' with 'stable') are rejected.
Does setdiff remove NaN values from A when they exist in B?⌄
Yes. NaN values are considered equal. If B contains NaN, all NaN entries from A are removed.
Are complex numbers supported?⌄
Absolutely. Complex values use MATLAB's ordering rules (magnitude, then real part, then imaginary part) for the sorted output.
Does GPU execution change the results?⌄
No. Until providers supply a device implementation, RunMat gathers GPU inputs and executes the CPU path to guarantee MATLAB-compatible behaviour.
What happens if the inputs have different classes?⌄
RunMat follows MATLAB's rules: both inputs must share the same class (numeric/logical, complex, char, or string). Mixed-class inputs raise descriptive errors.
Can I request 'legacy' behaviour?⌄
No. RunMat implements the modern semantics only. Passing 'legacy' or 'R2012a' results in an error.
Related Array functions
Sorting Sets
argsort · intersect · ismember · ismembertol · issorted · issortedrows · setxor · sort · sortrows · union · unique
Grouping
accumarray · combinations · discretize · findgroups · groupcounts · grp2idx · splitapply
Shape
blkdiag · cat · circshift · diag · flip · fliplr · flipud · horzcat · ipermute · kron · permute · repelem · repmat · reshape · rot90 · squeeze · toeplitz · tril · triu · vertcat
Open-source implementation
Unlike proprietary runtimes, every RunMat function is open-source. Read exactly how setdiff is executed, line by line, in Rust.
- View the source for setdiff in Rust on GitHub
- Learn how the RunMat runtime works
- Found a bug? Open an issue with a minimal reproduction.
About RunMat
RunMat is an open-source runtime that executes MATLAB-syntax code blazing on any GPU. It is licensed under the Apache 2.0 license.
- RunMat automatically optimizes your math for GPU execution on Apple, Nvidia, and AMD hardware. No code changes needed. Simulations that took hours now take minutes.
- Start running code in seconds. RunMat runs in the browser, on the desktop, or from the CLI. No license server, no IT ticket.