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CategoryMath: Rounding
GPUYes

What does the round function do in MATLAB / RunMat?

round(X) rounds numeric values to the nearest integers using MATLAB-compatible half-away-from-zero semantics. Additional arguments allow rounding to a specified number of decimal digits or significant digits.

How does the round function behave in MATLAB / RunMat?

  • round(X) rounds each element of X to the nearest integer; ties (e.g., ±0.5) round away from zero.
  • round(X, N) rounds to N decimal digits when N is positive and to powers of ten when N is negative.
  • round(X, N, 'significant') rounds to N significant digits. N must be a positive integer.
  • Logical inputs are promoted to double before rounding; round(true) returns 1.
  • Complex inputs are rounded component-wise (round(a + bi) = round(a) + i·round(b)), matching MATLAB.
  • Non-finite values (NaN, Inf, -Inf) propagate unchanged regardless of precision arguments.
  • Character arrays are treated as their numeric code points and return double tensors of the same size.

round Function GPU Execution Behaviour

When a tensor already resides on the GPU, RunMat checks whether the active acceleration provider implements a specialised unary_round kernel. If available, round(X) executes entirely on the device. Advanced modes (round(X, N) and round(X, N, 'significant')) currently gather tensors to the host before rounding to keep semantics aligned with MATLAB. Providers that add digit-aware kernels can extend this path in the future.

Examples of using the round function in MATLAB / RunMat

Rounding values to the nearest integers

X = [-3.5 -2.2 -0.5 0 0.5 1.7];
Y = round(X);

Expected output:

Y = [-4 -2 -1 0 1 2];

Rounding to a fixed number of decimal places

temps = [21.456 19.995 22.501];
rounded = round(temps, 2);

Expected output:

rounded = [21.46 20.00 22.50];

Rounding to negative powers of ten

counts = [1234 5678 91011];
rounded = round(counts, -2);

Expected output:

rounded = [1200 5700 91000];

Rounding to significant digits

measurements = [0.001234 12.3456 98765];
sig3 = round(measurements, 3, 'significant');

Expected output:

sig3 = [0.00123 12.3 98800];

Rounding GPU tensors and gathering the results

G = gpuArray(linspace(-2.5, 2.5, 6));
rounded = round(G);
hostValues = gather(rounded);

Expected output:

hostValues = [-3 -2 -1 1 2 3];

GPU residency in RunMat (Do I need gpuArray?)

You usually do not need to call gpuArray manually. RunMat's planner keeps tensors on the GPU when the provider exposes the required kernels and it is profitable to do so. round takes advantage of this mechanism for the plain round(X) form. When you specify digits or the 'significant' option, RunMat currently gathers data to the host to match MATLAB exactly. Future providers can extend unary_round or add digit-aware kernels to keep those workloads on the device.

FAQ

Does round always round half values away from zero?

Yes. MATLAB and RunMat both use half-away-from-zero semantics, so round(0.5) returns 1 and round(-0.5) returns -1.

Can I round to decimal places and significant digits?

Yes. Use round(X, N) for decimal places and round(X, N, 'significant') for significant digits. Negative N values round to tens, hundreds, and so on.

What happens if I pass a non-integer N?

N must be an integer scalar. RunMat raises a MATLAB-compatible error when N is not an integer or is non-finite.

How are complex numbers handled?

RunMat rounds the real and imaginary components independently, matching MATLAB's component-wise behaviour.

Do NaN or Inf values change when rounded?

No. Non-finite values propagate unchanged for every rounding mode, just like MATLAB.

Will rounding stay on the GPU?

round(X) stays on the GPU when the provider implements unary_round. Rounding with digit arguments currently gathers to the host; providers can override this by adding specialised kernels.

Can I round logical or character arrays?

Yes. Logical values are converted to doubles (0 or 1) and characters are rounded as their numeric code points, returning dense double tensors.

See Also

floor, ceil, fix, gpuArray, gather

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