rand — Uniform random numbers on (0, 1) within the MATLAB language.

rand produces uniformly distributed pseudorandom numbers over the open interval (0, 1). RunMat mirrors MATLAB semantics across scalar, vector, matrix, and N-D invocations, including 'like' prototypes that control data type and device residency.

How rand works in RunMat

• rand() returns a scalar double drawn from U(0, 1).
• rand(n) returns an n × n double matrix.
• rand(m, n, ...) returns a dense double array of the requested dimensions.
• rand(sz) accepts a size vector (row or column) and returns a tensor whose shape is sz.
• rand(A) or rand(___, 'like', A) matches the shape and residency of A, including GPU tensors when an acceleration provider is active.
• rand(___, 'double') leaves the output as double precision (default). 'single' returns single-precision results that mirror MATLAB's behaviour.

How rand runs on the GPU

When the prototype lives on the GPU, RunMat first asks the active acceleration provider for a device-side random buffer via random_uniform / random_uniform_like. If the provider lacks those hooks, RunMat generates samples on the host and uploads them to maintain GPU residency. This guarantees MATLAB-compatible behaviour while documenting the extra transfer cost.

GPU memory and residency

You usually do NOT need to call gpuArray yourself in RunMat (unlike MATLAB).

In RunMat, the fusion planner keeps residency on GPU in branches of fused expressions. As such, in the above example, the result of the rand call will already be on the GPU when the fusion planner has detected a net benefit to operating the fused expression it is part of on the GPU.

To preserve backwards compatibility with MathWorks MATLAB, and for when you want to explicitly bootstrap GPU residency, you can call gpuArray explicitly to move data to the GPU if you want to be explicit about the residency.

Since MathWorks MATLAB does not have a fusion planner, and they kept their parallel execution toolbox separate from the core language, as their toolbox is a separate commercial product, MathWorks MATLAB users need to call gpuArray to move data to the GPU manually whereas RunMat users can rely on the fusion planner to keep data on the GPU automatically.

Examples

Creating a 3x3 matrix of random numbers

R = rand(3);         % 3x3 doubles in (0, 1)

Expected output:

R = [0.8147 0.9134 0.1270; 0.9058 0.6324 0.0975; 0.1270 0.0975 0.2785]

Creating a 2x4x3 matrix of random numbers

sz = [2 4 3];
T = rand(sz)

Expected output:

T = [0.8147 0.9134 0.1270 0.9058 0.6324 0.0975 0.1270 0.0975 0.2785; 0.9134 0.6324 0.0975 0.2785 0.0975 0.1270 0.9058 0.6324 0.0975]

Creating a 128x128 matrix of random numbers on a GPU

G = rand(128, 128)

Expected output:

H = [0.8147 0.9134 0.1270 0.9058 0.6324 0.0975 0.1270 0.0975 0.2785; 0.9134 0.6324 0.0975 0.2785 0.0975 0.1270 0.9058 0.6324 0.0975]

FAQ

When should I use the rand function?

Use rand whenever you need to create arrays filled with random numbers over the open interval (0, 1). This is useful for Monte Carlo simulations, generating noise for testing, or creating random initial conditions for optimization.

Does rand produce double arrays by default?

Yes, by default, rand creates dense double-precision arrays unless you explicitly specify a type such as 'single' or use the 'like' argument to match a prototype array.

What does rand(n) return?

rand(n) returns an n × n dense double-precision matrix filled with random numbers over the open interval (0, 1). For example, rand(3) yields a 3-by-3 matrix of random numbers.

How do I create a single precision array of random numbers?

Pass 'single' as the last argument:

S = rand(5, 5, 'single');

This produces a 5x5 single precision matrix of random numbers.

How do I match the type and device residency of an existing array?

Use the 'like', prototype syntax:

A = gpuArray(rand(2,2));
B = rand(2, 2, 'like', A);

B will be a GPU array with the same type and shape as A.

Can I create N-dimensional arrays with rand?

Yes! Pass more than two dimension arguments (or a size vector):

T = rand(2, 3, 4);

This creates a 2×3×4 tensor of random numbers.

How does rand(A) behave?

If you call rand(A), where A is an array, the result is a new array of random numbers with the same shape as A.

Is the output always dense?

Yes. rand always produces a dense array. For sparse matrices of random numbers, use sparse with appropriate arguments.

What if I call rand with no arguments?

rand() returns a scalar double drawn from U(0, 1).

Related functions to explore

These functions work well alongside rand. Each page has runnable examples you can try in the browser.

randn, randi, gpuArray, gather, colon, eye, false, fill, linspace, logspace, magic, meshgrid, ones, randperm, range, true, zeros

Open-source implementation

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

• View rand.rs on GitHub
• Learn how the 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 — faster, on any GPU, with no license required.

• Simulations that took hours now take minutes. RunMat automatically optimizes your math for GPU execution on Apple, Nvidia, and AMD hardware. No code changes needed.
• Start running code in seconds. Open the browser sandbox or download a single binary. No license server, no IT ticket, no setup.
• A full development environment. GPU-accelerated 2D and 3D plotting, automatic versioning on every save, and a browser IDE you can share with a link.

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