isscalar — Test whether values are scalars in MATLAB and RunMat.

isscalar(A) returns true when A has exactly one element with unit dimensions. Shape handling across value types follows MATLAB semantics.

Syntax

tf = isscalar(A)

Inputs

Returns

How isscalar works

• isscalar is true if and only if numel(A) == 1 and every entry of size(A) equals 1.
• Numeric, logical, and complex scalars (42, true, 3+4i) satisfy both conditions.
• Row, column, or higher-dimensional vectors (e.g. [1 2 3], zeros(2,1)) are not scalar because at least one dimension exceeds one.
• String scalars ("hello", "") are scalar because they are 1×1 string arrays. Character arrays must be 1×1 to count as scalar—'h' is scalar, while 'runmat' (1×6) is not.
• Cell arrays, structs, objects, and handle objects are scalar when their MATLAB dimensions are 1×1, regardless of the contents they wrap.
• Empty arrays ([], cell(0,1), strings of size 0×1) are not scalar because they contain zero elements even if some dimensions equal one.
• GPU tensors rely on the shape metadata stored in their GpuTensorHandle. If a provider omits that metadata, RunMat gathers once to confirm the dimensions before answering.

Does RunMat run isscalar on the GPU?

isscalar never launches GPU kernels. For gpuArray inputs the builtin first inspects the shape stored in the GpuTensorHandle. When the active acceleration provider omits that metadata, RunMat performs a single gather to compute numel and dimensions on the host. The builtin always returns a host logical scalar, so fusion planning treats it as a metadata query instead of a device-side kernel.

Examples

Checking if a numeric value is scalar

tf = isscalar(42)

Expected output:

tf = logical(1)

Detecting that a row vector is not scalar

tf = isscalar([1 2 3])

Expected output:

tf = logical(0)

Verifying scalar status of string vs char arrays

tf_string = isscalar("hello");
tf_char = isscalar('h');
tf_char_row = isscalar('runmat')

Expected output:

tf_string = logical(1)
tf_char = logical(1)
tf_char_row = logical(0)

Identifying that an empty array is not scalar

tf_empty = isscalar([])

Expected output:

tf_empty = logical(0)

Confirming that single-element cell arrays are scalar

C = {pi};
tf_cell = isscalar(C)

Expected output:

tf_cell = logical(1)

Inspecting a GPU-resident tensor

G = gpuArray(ones(1,1));
tf_gpu = isscalar(G)

Expected output:

tf_gpu = logical(1)

Using isscalar with coding agents

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

FAQ

Related Array functions

Open-source implementation

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

• View the source for isscalar 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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