fieldnames — List struct field names, with gated RunMat object introspection.

fieldnames(S) returns a column cell array of character vectors containing the fields stored on a structure. Scalar structures preserve field insertion order. RunMat currently represents structure arrays with cell-backed storage; fieldnames accepts that representation when every element is a scalar structure and returns the sorted union of represented fields. RunMat object-family introspection is available only in RunMat extension mode.

Syntax

names = fieldnames(S)

Inputs

Returns

Errors

How fieldnames works

• Works with scalar structs and RunMat's current cell-backed struct-array representation. A nonempty cell-backed input rejects if any element is not a scalar struct.
• Returns a column cell array (n × 1) whose elements are character vectors (1 × N char arrays).
• Scalar-struct field names preserve the structure's insertion order and remain case-sensitive (name and Name are distinct).
• Cell-backed struct arrays return a deterministic, alphabetically sorted union of their represented field names.
• In RunMat extension mode, value objects, handle objects, and listeners expose RunMat's currently available property metadata. This surface is not claimed as complete MATLAB object introspection.
• Empty scalar structs and empty cell-backed struct arrays yield an empty 0 × 1 cell array. GPU-resident field values nested inside a struct stay on the device because fieldnames only inspects host metadata.
• Integer, logical, floating-point, and resident numeric arrays are not applicable inputs and reject without numeric or text coercion.

Does RunMat run fieldnames on the GPU?

fieldnames is metadata-only. It does not launch kernels or gather data. GPU tensors or handles that live inside structs or objects remain device-resident, and the builtin simply walks the host-side descriptors that remember their names.

GPU memory and residency

You do not need to call gpuArray when using fieldnames. RunMat leaves each value—CPU or GPU—where it already resides. When structs or objects contain GPU handles, those handles remain valid and stay resident on the device throughout the introspection call.

Examples

Listing the fields of a scalar structure

s = struct("name", "Ada", "score", 42);
fields = fieldnames(s)

Expected output:

fields =
    {'name'}
    {'score'}

Inspecting field names before accessing values

stats = struct("min", 1.2, "max", 9.8);
if any(strcmp(fieldnames(stats), "median"))
    disp(stats.median);
else
    disp("median not available");
end

Expected output:

median not available

Gathering represented field names from a struct array

people = struct("name", {"Ada", "Grace"}, "id", {101, 102});
fields = fieldnames(people)

Expected output:

fields =
    {'id'}
    {'name'}

Listing represented object properties

% Save this class in Counter.m before running the example:
% classdef Counter
%     properties
%         Value = 0
%         Step  = 1
%     end
% end
c = Counter;
props = fieldnames(c)

Expected output:

props =
    {'Step'}
    {'Value'}

Discovering fields inside nested structs

config.database = struct("host", "db.local", "port", 5432);
names = fieldnames(config.database)

Expected output:

names =
    {'host'}
    {'port'}

Handling empty structs safely

emptyScalar = struct();
emptyArray = struct("id", {});
fs = fieldnames(emptyScalar);
fa = fieldnames(emptyArray)

Expected output:

fs =
  0x1 empty cell array

fa =
  0x1 empty cell array

Using fieldnames with coding agents

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

FAQ

Related Structs functions

Open-source implementation

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

• View the source for fieldnames in Rust on GitHub
• Learn how the RunMat 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 blazing on any GPU. It is licensed under the Apache 2.0 license.

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