What does the repmat function do in MATLAB / RunMat?

repmat(A, reps) tiles the array A so that it repeats according to reps. RunMat mirrors MATLAB semantics, supporting scalars, matrices, tensors, logical, string, char, cell, and GPU-resident arrays.

How does the repmat function behave in MATLAB / RunMat?

• repmat(A, m, n) or repmat(A, [m n]) repeats A m times along rows and n times along columns.
• A size vector [r1 r2 … rN] can be given as a row or column; trailing dimensions default to 1, letting you replicate only the axes you care about.
• Supplying a scalar replication factor (e.g., repmat(A, k)) duplicates A across every dimension, ensuring the first two dimensions are replicated even when the input is a row vector.
• Zero replication factors produce empty dimensions, e.g., repmat(A, 0, 3) yields an empty array with shape [0 size(A,2)].
• Char and cell arrays follow MATLAB by supporting row/column tiling; additional dimensions must be 1, while numeric, logical, complex, and string arrays support full N-D replication.
• Replication factors must be finite, non-negative integers; RunMat raises descriptive errors for negative, fractional, or excessively large sizes.
• GPU arrays remain on the device when the acceleration provider implements the tiling hook; otherwise, RunMat gathers to host memory, tiles in software, and re-uploads the replicated tensor so downstream GPU work keeps residency.

repmat Function GPU Execution Behaviour

RunMat first calls AccelProvider::repmat, giving the backend a chance to tile directly on the device. Providers that have not yet implemented this hook fall back to a safe path that gathers the tensor, performs tiling on the host, and uploads the replicated data back to the GPU. The fallback preserves correct behaviour today while enabling backend authors to drop in optimized kernels as they become available.

Examples of using the repmat function in MATLAB / RunMat

Tiling a matrix across rows and columns

A = [1 2; 3 4];
B = repmat(A, 2, 3);

Expected output:

B =
     1     2     1     2     1     2
     3     4     3     4     3     4
     1     2     1     2     1     2
     3     4     3     4     3     4

Using a scalar replication factor

row = 1:4;
Tiled = repmat(row, 3);
size(Tiled)

Expected output:

ans =
     3    12

Replicating into three dimensions

A = reshape(1:6, [1 3 2]);
T = repmat(A, [2 1 4]);
size(T)

Expected output:

ans =
     2     3     8

Repeating logical masks with zero dimensions

mask = logical([1 0 1]);
emptyMask = repmat(mask, 0, 3);
size(emptyMask)

Expected output:

ans =
     0     3

Replicating string scalars into string arrays

name = "runmat";
names = repmat(name, 2, 2);

Expected output:

names =
  2x2 string array
    "runmat"    "runmat"
    "runmat"    "runmat"

Replicating data that lives on the GPU

G = gpuArray(magic(3));
T = repmat(G, [2 1]);
result = gather(T);

Expected behaviour: result is a 6-by-3 matrix that tiles the original magic(3) matrix twice along the row dimension without requiring explicit gpuArray calls in user code.

GPU residency in RunMat (Do I need gpuArray?)

You usually do not need to call gpuArray directly. RunMat's planner keeps values on the GPU when it detects that further operations benefit from staying there. Explicit gpuArray calls remain available for compatibility with legacy MATLAB code and when you want to steer residency manually.

FAQ

Are replication factors required to be integers?

Yes. RunMat follows MATLAB and requires every replication factor to be a non-negative integer. Non-integers raise the descriptive error repmat: replication factor <n> must be an integer.

What happens when I pass a scalar replication factor?

RunMat duplicates the input along every dimension, ensuring at least the first two dimensions receive the factor so matrices tile both rows and columns.

Can I request zero replication along a dimension?

Yes. Zero factors produce empty dimensions while preserving the remaining sizes, which is useful when constructing placeholder tensors or short-circuiting loops.

Does repmat work with char, string, or cell arrays?

Yes. Char arrays and cell arrays tile across rows and columns (extra dimensions must currently be 1), while string arrays, numeric tensors, logical arrays, and complex tensors support full N-D replication.

How does repmat handle GPU tensors today?

The runtime asks the provider for a device implementation. If none exists (for example, the in-process test provider), RunMat gathers to the host, tiles there, and re-uploads the result so downstream GPU kernels still see the replicated tensor.

Does the result reuse backing storage from the input?

No. repmat always creates a new array so modifying the result never mutates the original input.

Can replication overflow memory?

RunMat checks for overflow when multiplying shape dimensions. If the requested size cannot fit in native address space, the builtin raises a descriptive error before attempting allocation.

Does repmat preserve data ordering?

Yes. Column-major ordering is maintained for numeric, logical, string, and complex arrays, while char and cell arrays respect their row-major storage conventions within RunMat.

See Also

reshape, permute, squeeze, zeros, gpuArray, gather

Source & Feedback

• Implementation: crates/runmat-runtime/src/builtins/array/shape/repmat.rs
• Found a behavioural difference? Open an issue.