length — Return largest-dimension length in MATLAB and RunMat.
length(A) returns the size of the largest dimension of A. Scalar/vector/matrix conventions and N-D behavior follow MATLAB semantics.
Syntax
n = length(A)Inputs
| Name | Type | Required | Default | Description |
|---|---|---|---|---|
A | Any | Yes | — | Input value to inspect. |
Returns
| Name | Type | Description |
|---|---|---|
n | NumericScalar | Largest dimension extent of input. |
Errors
| Identifier | When | Message |
|---|---|---|
RunMat:length:UnsupportedTable | The input is a table or timetable. | length: tables and timetables are not supported; use height, width, or size |
How length works
- The result is always a double-precision scalar.
- Empty arrays report
0when every dimension is zero; otherwise the maximum non-zero dimension is reported (e.g.,0×5arrays have length5). - Character arrays and string arrays follow their array dimensions, not the number of code points.
- Cell arrays use their MATLAB array shape (
rows × cols) with the maximum dimension returned. - GPU-resident arrays are inspected entirely through shape metadata already carried by the tensor handle;
lengthnever gathers the payload. - Tables and timetables are rejected; use
height,width, orsizefor tabular values. - Arguments that are not arrays (scalars, logicals, strings, handle objects) are treated as
1×1.
Does RunMat run length on the GPU?
length is a metadata-only query. For a resident tensor, RunMat reads GpuTensorHandle.shape directly and returns a host double scalar. It does not consult the provider, gather or download the payload, launch a kernel, or allocate a device buffer.
Examples
Determine the length of a row vector
row = [1 2 3 4];
n = length(row)Expected output:
n = 4Find the longer side of a rectangular matrix
A = randn(5, 12);
len = length(A)Expected output:
len = 12Handle empty arrays that still have a non-zero dimension
E = zeros(0, 7);
len = length(E)Expected output:
len = 7Measure the length of a character array
name = 'RunMat';
len = length(name)Expected output:
len = 6Inspect the length of a gpuArray without gathering
G = gpuArray(ones(256, 4));
len = length(G)Expected output:
len = 256Using length with coding agents
Open a RunMat example with live inputs, then ask the agent to explain how length changes the result.
Run a small length example, explain the result, then change one input and compare the output.
FAQ
How is length different from size?⌄
length(A) returns the maximum dimension length as a scalar, whereas size(A) returns every dimension in a row vector. Use length when you only care about the longest dimension.
What does length return for scalars?⌄
Scalars are treated as 1×1, so length(scalar) returns 1.
How does length behave for empty arrays?⌄
If all dimensions are zero, length returns 0. If any dimension is non-zero, the maximum dimension is returned. For example, zeros(0, 5) has length 5, but zeros(0, 0) has length 0.
Does length gather GPU data?⌄
No. The runtime reads shape metadata stored in the GPU tensor handle and never accesses the provider or downloads the tensor payload.
Can I use length on cell arrays and structs?⌄
Yes. length examines the MATLAB array shape of the container, so cell arrays and struct arrays return the maximum dimension of their array layout.
Does length count characters or bytes?⌄
For character arrays, the length reflects the array dimensions (rows and columns), not encoded byte length or Unicode scalar counts.
What about string arrays?⌄
String arrays are treated like any other array. String scalars are 1×1, so length("abc") returns 1. Use strlength if you need the number of characters in each element.
Is length safe to use inside fused GPU expressions?⌄
Yes. length never allocates or keeps data on the GPU. It returns a host scalar immediately, so it won't break fusion plans.
Related Array functions
Grouping
accumarray · combinations · discretize · findgroups · groupcounts · grp2idx · splitapply
Sorting Sets
argsort · intersect · ismember · ismembertol · issorted · issortedrows · setdiff · setxor · sort · sortrows · union · unique
Shape
blkdiag · cat · circshift · diag · flip · fliplr · flipud · horzcat · ipermute · kron · permute · repelem · repmat · reshape · rot90 · squeeze · toeplitz · tril · triu · vertcat
Open-source implementation
Unlike proprietary runtimes, every RunMat function is open-source. Read exactly how length is executed, line by line, in Rust.
- View the source for length 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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