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CategoryIntrospection

What does the isstring function do in MATLAB / RunMat?

tf = isstring(x) returns a logical scalar true when x is a MATLAB string array (including string scalars and empty string arrays) and logical false otherwise. Use it when you need to validate arguments, branch on string support, or distinguish strings from other text representations.

How does the isstring function behave in MATLAB / RunMat?

  • String scalars such as "RunMat" and string arrays like ["alpha", "beta"] both return logical(1).
  • Empty string arrays produced by strings(0) or string.empty(...) are still recognised as the string type, so they return logical(1).
  • Character arrays created with single quotes ('text'), cell arrays of character vectors, numeric tensors, logical masks, structs, tables, and other non-string containers return logical(0).
  • gpuArray inputs currently store numeric or logical tensors in RunMat, so isstring(gpuArray(...)) returns logical(0), matching MATLAB’s behaviour.
  • The result is always a host logical scalar, making it easy to use for argument validation, control-flow guards, or input parsing.

isstring Function GPU Execution Behaviour

isstring runs entirely from metadata. When you pass a gpuArray, RunMat inspects the handle and returns logical(0) because device buffers do not yet store string data. No kernels are launched, and no host↔device transfers occur, so the device allocation remains resident for subsequent operations.

GPU residency in RunMat (Do I need gpuArray?)

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, calling isstring on a gpuArray result leaves the device allocation untouched while still answering the question using metadata only.

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 of using the isstring function in MATLAB / RunMat

Checking if a string scalar is recognised

tf = isstring("RunMat");

Expected output:

tf = logical(1)

Detecting string arrays with multiple elements

values = ["alpha", "beta", "gamma"];
tf = isstring(values);

Expected output:

tf = logical(1)

Distinguishing strings from character vectors

tf_string = isstring("hello");
tf_char   = isstring('hello');

Expected output:

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

Confirming that empty string arrays are still strings

emptyStrs = strings(0, 2);
tf = isstring(emptyStrs);

Expected output:

tf = logical(1)

Recognising that numeric and logical arrays are not strings

numbers = [1 2 3];
mask = true(1, 3);
tf_numbers = isstring(numbers);
tf_mask = isstring(mask);

Expected output:

tf_numbers = logical(0)
tf_mask = logical(0)

Validating cell arrays of character vectors

cells = {'a', 'b', 'c'};
tf = isstring(cells);

Expected output:

tf = logical(0)

Inspecting gpuArray inputs without transfers

G = gpuArray(ones(2, 2));
tf = isstring(G);

Expected output:

tf = logical(0)

Using isstring in argument validation

function greet(name)
    if ~isstring(name)
        error("Name must be provided as a string.");
    end
    disp("Hello " + name + "!");
end

greet("RunMat");
% greet(42); % throws "Name must be provided as a string."

Expected output:

Hello RunMat!

FAQ

Does isstring return true for empty string arrays?

Yes. Empty string arrays are still of type string, so isstring(strings(0)) returns logical(1).

How is isstring different from ischar?

isstring recognises MATLAB’s modern string type introduced in R2016b. ischar recognises legacy character arrays created with single quotes. Only isstring returns true for double-quoted values.

What about string arrays stored inside cell arrays?

Cell arrays are containers, so isstring({'a', 'b'}) returns logical(0). Use iscellstr or all(isstring(cells)) to validate the contents instead.

Can gpuArray hold string data in RunMat?

Not currently. gpuArray values hold numeric or logical tensors, so isstring always reports logical(0) for device-resident data.

Does isstring inspect array contents element by element?

No. It only checks the value’s type metadata, so the cost is constant regardless of array size.

See Also

ischar, isa, string, convertStringsToChars, gpuArray

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