sub2ind — Convert N-D subscript coordinates into MATLAB-style column-major linear indices.

sub2ind(sz, s1, s2, ...) maps row/column or higher-dimensional subscripts into linear indices using MATLAB column-major ordering. The size vector sz defines target extents and each dimension requires a matching subscript input.

Syntax

ind = sub2ind(sz, I1, In...)

Inputs

Returns

Errors

How sub2ind works

• Subscripts can be scalars or arrays. When arrays are provided, they must share the same size. Scalars broadcast to that common shape.
• All subscripts must be positive integers within the corresponding dimension's range.
• The size vector can be a row or column vector. Each element must be a positive integer.
• Complex, NaN, or infinite values are rejected.
• The result uses the same shape as the subscript arrays. Scalars produce a scalar double.
• When any input is a GPU tensor, RunMat computes on the host (to reuse integer semantics) and uploads the resulting indices back to the GPU so fusion and downstream kernels keep operating on device.

Does RunMat run sub2ind on the GPU?

When a WGPU-backed provider is active, sub2ind executes entirely on the GPU. The shader mirrors MATLAB's validation rules: it rejects non-finite values, non-integer subscripts, and out-of-range indices, surfacing the same diagnostic messages as the CPU path. Providers that do not yet implement the hook fall back to the host implementation; after the indices are computed they are uploaded back to the active provider so downstream fused kernels continue operating on device data.

GPU memory and residency

rows = gpuArray((1:100)');
cols = gpuArray(ones(100, 1) * 4);
idx = sub2ind([100 4], rows, cols);

Expected behavior:

% idx remains a gpuArray containing the column-major indices.
disp(gather(idx(1:5)));
% Output:
%    301
%    302
%    303
%    304
%    305

Examples

Converting a single matrix subscript to a linear index

idx = sub2ind([3 4], 2, 3)

Expected output:

idx = 8

Mapping multiple subscripts into one-dimensional indices

rows = [1; 2; 3];
cols = [3; 3; 3];
idx = sub2ind([3 5], rows, cols)

Expected output:

idx =
     7
     8
     9

Handling higher-dimensional array subscripts

row = [1 1];
col = [2 3];
page = [1 2];
idx = sub2ind([2 3 4], row, col, page)

Expected output:

idx = [3 11]

Broadcasting scalar subscripts across array inputs

rows = [1 2 3];
idx = sub2ind([3 4], rows, 4)

Expected output:

idx = [10 11 12]

Retaining GPU residency for batched index conversions

rows = gpuArray((1:100)');
cols = gpuArray(ones(100, 1) * 4);
idx = sub2ind([100 4], rows, cols)

Expected output:

% idx remains a gpuArray containing the column-major indices.
disp(gather(idx(1:5)));
% Output:
%    301
%    302
%    303
%    304
%    305

Detecting invalid out-of-range subscripts

try
    idx = sub2ind([3 4], 4, 1);
catch ME
    disp(ME.message);
end

Expected output:

Index exceeds the number of rows in dimension 1.

Using sub2ind with coding agents

Open a RunMat example with live inputs, then ask the agent to explain how sub2ind changes the result.

FAQ

Related Array functions

Open-source implementation

Unlike proprietary runtimes, every RunMat function is open-source. Read exactly how sub2ind is executed, line by line, in Rust.

• View the source for sub2ind 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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