fieldnames — List struct or object field names in MATLAB and RunMat.

fieldnames(S) returns a column cell array of character vectors containing field names defined on S. Struct arrays return the union of field names across elements, and object/property handling follows MATLAB semantics with deterministic, case-sensitive output.

Syntax

names = fieldnames(S)

Inputs

Returns

Errors

How fieldnames works

• Works with scalar structs as well as struct arrays created by struct, load, or other builtins.
• Returns a column cell array (n × 1) whose elements are character vectors (1 × N char arrays).
• Objects (value or handle) report the union of their public, non-static class properties together with any dynamic properties stored on the instance.
• For struct arrays, the union of field names across all elements is reported.
• Field names are sorted alphabetically and remain case-sensitive (name and Name are distinct).
• Empty structs yield an empty 0 × 1 cell array. GPU-resident field values stay on the device— fieldnames only inspects metadata.

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 field names from a struct array

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

Expected output:

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

Listing the properties of an object

% 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

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