cat — Concatenate arrays along a chosen dimension in MATLAB and RunMat.

cat(dim, A1, A2, ...) concatenates arrays along dimension dim, producing a new array whose slices along that dimension come from the inputs. Non-concatenated dimensions must be size-compatible, and ordering follows MATLAB and RunMat column-major semantics.

Syntax

B = cat(dim, A1, A2, An...)
B = cat(dim, A1, A2, An..., "like", prototype)

Inputs

Returns

Errors

How cat works

• dim is 1-based and must be a positive integer.
• Valid unlike-class combinations follow concatenation precedence: string dominates real numeric, character, and logical data; character dominates real numeric data but cannot combine with logical; an integer operand makes the leftmost nonempty integer class dominate real floating and logical operands; otherwise single dominates double, and logical is preserved only when every input is logical.
• Every ordered mixture of the eight real integer classes is accepted. When a nonempty input remains, empty operands are omitted before class selection; otherwise the leftmost integer class determines output class. Other numeric or logical values convert with ordinary rounding and saturation.
• Dimensions other than dim must match exactly; missing higher dimensions are treated as size 1 automatically.
• The size along dim becomes the sum of the corresponding sizes from each input.
• An empty operand is omitted when a nonempty compatible partner remains; compatible all-empty inputs retain the concatenated empty geometry.
• gpuArray inputs stay on the device when an acceleration provider is registered, and real or complex numeric host inputs may be mixed with resident inputs in the same call. Paired complex-integer data retains its exact component class and owning provider.
• RunMat compatibility mode adds an optional trailing "like", prototype pair that can request host or GPU output residency; this pair is not part of the pinned public MATLAB syntax.

Does RunMat run cat on the GPU?

When every supported floating input is a gpuArray, RunMat first calls the active provider's AccelProvider::cat hook. Typed integer data, mixed numeric host/device inputs, runtime empty omission, or providers without the hook use an exact fallback: resident operands are gathered without changing class, empty operands are omitted before class selection when a nonempty input remains, all remaining operands are concatenated on the host in original order, and the result is uploaded to the originating provider. This preserves integer class selection and wide values across the backend boundary. RunMat compatibility mode can additionally use the trailing "like" extension to request output residency.

Examples

Concatenating matrices by stacking rows

A = [1 2; 3 4];
B = [5 6; 7 8];
C = cat(1, A, B)

Expected output:

C =
     1     2
     3     4
     5     6
     7     8

Concatenating matrices by appending columns

left = [1 3; 2 4];
right = [10 30; 20 40];
wide = cat(2, left, right)

Expected output:

wide =
     1     3    10    30
     2     4    20    40

Building a 3-D array from 2-D slices

slice1 = magic(3);
slice2 = eye(3);
cube = cat(3, slice1, slice2)

Concatenating logical masks without type changes

row = logical([1 0 1]);
mask = cat(1, row, ~row)

Expected output:

mask =
   1   0   1
   0   1   0

Joining character arrays into wider text rows

lhs = ['Run' ; 'GPU'];
rhs = ['Mat'; 'Fun'];
words = cat(2, lhs, rhs)

Expected output:

words =
    RunMat
    GPUFun

Concatenating string arrays along rows

names = ["alpha" "beta"];
more = ["gamma" "delta"];
combined = cat(1, names, more)

Appending cell array columns for table-like data

cols1 = {1, 2; 'a', 'b'};
cols2 = {3, 4; 'c', 'd'};
tableCells = cat(2, cols1, cols2)

Keeping gpuArray inputs on the device

G1 = gpuArray(rand(256, 256));
G2 = gpuArray(rand(256, 256));
stacked = cat(3, G1, G2)

Requesting GPU output with the "like" prototype

G = gpuArray(rand(3, 3));
H = cat(3, zeros(3, 3), ones(3, 3), "like", G)

Concatenating complex arrays preserves imaginary parts

z1 = complex([1 2], [3 4]);
z2 = complex([5 6], [7 8]);
joined = cat(2, z1, z2)

Combining empty inputs yields an empty result

emptyRow = zeros(0, 3);
combo = cat(1, emptyRow, emptyRow)

Using cat with coding agents

Open a RunMat example with live inputs, then ask the agent to explain how cat changes the result.

Related Array functions

Open-source implementation

Unlike proprietary runtimes, every RunMat function is open-source. Read exactly how cat is executed, line by line, in Rust.

• View the source for cat 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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