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CategoryArray: Sorting Sets
GPUYes

intersect — Return common elements or rows across arrays with MATLAB-compatible ordering and index outputs.

intersect(A, B) computes the set intersection of A and B. By default it returns the distinct common values sorted ascending, along with optional index outputs that map each result back to its originating position in the inputs.

How does the intersect function behave in MATLAB / RunMat?

  • intersect(A, B) flattens both inputs column-major, removes duplicates, and returns the sorted intersection.
  • [C, IA, IB] = intersect(A, B) also returns index vectors so that C = A(IA) and C = B(IB).
  • intersect(A, B, 'stable') preserves the first appearance order from A.
  • intersect(A, B, 'rows') treats each row as an element. Inputs must share the same number of columns.
  • Character arrays, string arrays, logical arrays, complex values, and numerical types are fully supported. Mixed classes raise a descriptive error.
  • Legacy flags such as 'legacy' or 'R2012a' are not supported in RunMat.

GPU behavior

intersect registers as a residency sink. When the active acceleration provider offers a dedicated intersect hook, the set logic can execute directly on the device and return GPU-resident results. Until such a hook lands, RunMat automatically gathers GPU tensors to host memory, runs the CPU implementation, and materializes host-side outputs so behavior matches MathWorks MATLAB exactly.

GPU residency

You usually do **not** need to call gpuArray manually. RunMat's planner keeps tensors resident on the GPU when profitable. If a provider lacks an intersect kernel the runtime gathers automatically, so explicit residency management is rarely needed. Explicit calls to gpuArray remain available for compatibility with MathWorks MATLAB workflows.

Examples of using intersect in MATLAB / RunMat

Finding common values in numeric vectors

A = [5 7 5 1];
B = [7 1 3];
[C, IA, IB] = intersect(A, B)

Expected output:

C =
     1
     7
IA =
     4
     2
IB =
     2
     1

Preserving input order with 'stable'

A = [4 2 4 1 3];
B = [3 4 5 1];
C = intersect(A, B, 'stable')

Expected output:

C =
     4
     1
     3

Intersecting matrix rows

A = [1 2; 3 4; 1 2];
B = [2 3; 1 2];
[C, IA, IB] = intersect(A, B, 'rows')

Expected output:

C =
     1     2
IA =
     1
IB =
     2

Working with strings and character arrays

names1 = ["apple" "orange" "pear"];
names2 = ["pear" "grape" "orange"];
[common, IA, IB] = intersect(names1, names2, 'stable')

Expected output:

common =
  1×2 string array
    "orange"    "pear"
IA =
     2
     3
IB =
     3
     1

Using intersect on GPU arrays

G = gpuArray([10 4 6 4]);
H = gpuArray([6 4 2]);
result = intersect(G, H)

Expected output:

result =
     4
     6

FAQ

Does intersect keep duplicate values?

No. MATLAB and RunMat return each common value at most once. Use other logic (such as logical indexing) if you need multiset behavior.

What is the default ordering?

intersect sorts results ascending by default. Specify 'stable' to preserve the input order from A.

Can I intersect rows that contain strings?

Yes. String arrays support both element and row intersections. When using 'rows', the inputs must have the same number of columns.

Are NaN values considered equal?

Yes. NaN values are treated as equal for the purposes of intersection, matching MATLAB.

Is the 'legacy' flag supported?

No. RunMat only implements the modern MATLAB semantics. Passing 'legacy' or 'R2012a' raises an error.

See also

union, unique, sort, sortrows

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