triu — Extract upper-triangular portions of matrices with optional diagonal offsets.

triu(A) keeps elements on and above a selected diagonal and sets elements below that diagonal to zero. Optional k selects the diagonal boundary using MATLAB-compatible semantics.

Syntax

B = triu(A)
B = triu(A, k)

Inputs

Returns

Errors

How triu works

• Works on numeric, logical, and complex arrays.
• Operates on the first two dimensions; trailing dimensions act as independent pages.
• Preserves logical types and complex-valued elements.
• Scalars are treated as 1×1 matrices and honour diagonal offsets (for example triu(5, 1) returns 0).
• gpuArray inputs stay on the device when an acceleration provider supplies a native triu hook; otherwise the runtime gathers, masks on the host, and uploads the result back to the GPU.

Does RunMat run triu on the GPU?

If the active acceleration provider implements a triu kernel the entire operation executes on the GPU.

Without a provider hook, RunMat gathers the tensor to host memory once, applies the mask, uploads the result, and returns a gpuArray so residency is preserved for downstream kernels.

Fallbacks never affect numerical results—only where the computation runs.

GPU memory and residency

G = gpuArray(rand(5));
U = triu(G, -2);
isa(U, 'gpuArray')

Expected output:

ans =
  logical
   1

Examples

Extracting the upper triangular part of a matrix

A = [1 2 3; 4 5 6; 7 8 9];
U = triu(A)

Expected output:

U =
     1     2     3
     0     5     6
     0     0     9

Keeping one sub-diagonal beneath the main diagonal

A = magic(4);
U = triu(A, -1)

Expected output:

U =
    16     2     3     13
     5    11    10     8
     0     7     6    12
     0     0    15     1

Dropping the main diagonal with a positive offset

A = [1 2 3; 4 5 6; 7 8 9];
strict = triu(A, 1)

Expected output:

strict =
     0     2     3
     0     0     6
     0     0     0

Applying triu to every page of a 3-D array

T = reshape(1:18, [3 3 2]);
U = triu(T)

Expected output:

U(:, :, 1) =
     1     2     3
     0     5     6
     0     0     9

U(:, :, 2) =
    10    11    12
     0    14    15
     0     0    18

Preserving gpuArray residency with triu

G = gpuArray(rand(5));
U = triu(G, -2);
isa(U, 'gpuArray')

Expected output:

ans =
  logical
   1

Using triu with coding agents

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

FAQ

Related Array functions

Open-source implementation

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

• View the source for triu 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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