fftshift — Shift zero-frequency components to spectrum centers in MATLAB and RunMat.

fftshift(X) circularly shifts FFT output so the zero-frequency component moves to the center of each shifted dimension. The default all-dimension behavior and optional dimension selection follow MATLAB semantics.

Syntax

Y = fftshift(X)
Y = fftshift(X, DIM)

Inputs

Returns

Errors

How fftshift works

• When called without a dimension list, fftshift shifts along every dimension by floor(size(X, dim) / 2).
• fftshift(X, dims) shifts only the specified dimensions. dims can be a scalar, vector, or logical mask.
• Dimensions of length 0 or 1 are left unchanged.
• Inputs can be real or complex and may already reside on the GPU (gpuArray).

Does RunMat run fftshift on the GPU?

RunMat asks the active acceleration provider to execute fftshift via the circshift hook (with predetermined offsets). If the provider cannot satisfy the request, the tensor is gathered exactly once, shifted on the host, and optionally re-uploaded. Scalars remain on their existing device.

GPU memory and residency

RunMat's auto-offload keeps FFT spectra on the GPU whenever the active provider exposes the circshift hook. In that case fftshift runs entirely on the device and downstream fused operations continue without a gather. If no GPU provider is registered—or it does not expose circshift—RunMat gathers the data once, performs the host shift, and uploads the result back to the device so subsequent work can still benefit from acceleration. You can always call gpuArray explicitly when you need MATLAB compatibility or want to guarantee a specific residency boundary.

Examples

Centering the spectrum of a 1-D FFT result with even length

x = [0 1 2 3 4 5 6 7];
fx = fft(x);
y = fftshift(fx)

Expected output:

y = [-4 -4-1.6569i -4-4i -4-9.6569i 28 -4+9.6569i -4+4i -4+1.6569i]

Handling odd-length vectors

x = 1:5;
y = fftshift(x)

Expected output:

y = [4 5 1 2 3]

Centering both axes of a 2-D FFT

A = [1 2 3; 4 5 6];
C = fftshift(A)

Expected output:

C =
     6     4     5
     3     1     2

Shifting only one dimension of a matrix

A = [1 2 3; 4 5 6];
rowCentered = fftshift(A, 1)   % shift rows only

Expected output:

rowCentered =
     4     5     6
     1     2     3

Applying fftshift to a gpuArray spectrum

G = gpuArray(0:7);
centered = fftshift(G);
H = gather(centered)

Expected output:

H = [4 5 6 7 0 1 2 3]

Using fftshift with coding agents

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

FAQ

Related Math functions

Open-source implementation

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

• View the source for fftshift in Rust on GitHub
• Learn how the RunMat runtime works
• Found a bug? Open an issue with a minimal reproduction.

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