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CategoryArray: Shape
GPUYes

horzcat — Concatenate inputs side-by-side (dimension 2) just like MATLAB's square-bracket syntax.

horzcat(A1, A2, …) horizontally concatenates its inputs, matching the behaviour of MATLAB's square-bracket syntax [A1 A2 …]. It is the building block for row-wise array construction in RunMat.

How does the horzcat function behave in MATLAB / RunMat?

  • Operates on numeric, logical, complex, character, string, and cell arrays with MATLAB-compatible type checking.
  • All inputs must have the same number of rows (dimension 1). Higher dimensions are padded with singleton sizes where necessary.
  • Scalars act as 1×1 building blocks, so horzcat(1, 2, 3) produces the row vector [1 2 3].
  • Empty inputs participate naturally. If every operand is empty, the result is the canonical 0×0 double.
  • When the trailing 'like', prototype pair is supplied, the output matches the prototype's residency (host or GPU) and numeric category.
  • Mixing gpuArray operands with host operands is an error—convert explicitly using gpuArray or gather.

GPU behavior

horzcat delegates to cat(dim = 2, …). When the active acceleration provider implements the cat hook, the concatenation is executed directly on the GPU without staging data back to the CPU. Providers that lack this hook fall back to gathering the operands, concatenating on the host, and uploading the result so downstream code still sees a gpuArray. This mirrors MATLAB's explicit GPU workflow while keeping RunMat's auto-offload planner informed.

Examples of using horzcat in MATLAB / RunMat

Concatenating matrices by columns

A = [1 2; 3 4];
B = [10 20; 30 40];
C = horzcat(A, B)

Expected output:

C =
     1     2    10    20
     3     4    30    40

Building a row vector from scalars

row = horzcat(1, 2, 3, 4)

Expected output:

row = [1 2 3 4]

Extending character arrays into words

lhs = ['Run' ; 'GPU'];
rhs = ['Mat' ; 'Fun'];
words = horzcat(lhs, rhs)

Expected output:

words =
    RunMat
    GPUFun

Keeping gpuArray inputs resident on the device

G1 = gpuArray(rand(256, 128));
G2 = gpuArray(rand(256, 64));
wide = horzcat(G1, G2)

Preserving empties when all inputs are empty

emptyBlock = zeros(0, 3);
result = horzcat(emptyBlock, emptyBlock)

Matching an output prototype with the 'like' syntax

proto = gpuArray.zeros(5, 1);
combo = horzcat(ones(5, 1), zeros(5, 1), "like", proto)

See also

cat, vertcat, reshape, gpuArray, gather

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