lt — Element-wise less-than comparison for scalars, arrays, strings, and gpuArray inputs.

lt(A, B) (or the infix A < B) performs an element-wise less-than comparison. The result is a logical scalar when the broadcasted shape contains one element, or a logical array otherwise.

How lt works in RunMat

• Numeric, logical, and character inputs compare element-wise using MATLAB's implicit expansion rules.
• Character arrays compare by Unicode code point; mixing them with numeric arrays behaves like comparing numeric codes ('A' < 66).
• String scalars and string arrays compare lexically; implicit expansion works across string dimensions.
• Complex inputs are not supported, matching MATLAB's behaviour.
• Mixed numeric/string inputs raise MATLAB-compatible type errors.

How lt runs on the GPU

When both operands are gpuArray values and the active acceleration provider implements the elem_lt hook, RunMat executes the comparison entirely on the device and returns a gpuArray logical result. If the provider does not expose this hook, the runtime gathers the inputs to host memory automatically and performs the CPU comparison instead of failing.

GPU memory and residency

You usually do **not** need to call gpuArray explicitly. RunMat's native auto-offload planner keeps intermediate results on the GPU when fused expressions benefit from device execution. Explicit gpuArray and gather calls remain available for compatibility with MATLAB code that manages residency manually.

Examples

Determine Whether One Scalar Is Less Than Another

flag = lt(17, 42)

Expected output:

flag =
     1

Filter Matrix Elements Below A Threshold

M = [1 2 3; 4 5 6];
mask = lt(M, 3)

Expected output:

mask =
  2×3 logical array
     1     1     0
     0     0     0

Apply Less-Than With Implicit Expansion

v = [1 3 5 7];
mask = lt(v, [2 6])

Expected output:

mask =
  1×4 logical array
     1     0     0     0

Compare Character Data To Numeric Codes

letters = ['A' 'B' 'C'];
isBeforeC = lt(letters, 67)

Expected output:

isBeforeC =
  1×3 logical array
     1     1     0

Compare String Arrays Lexicographically

names = ["alice" "charlie" "bob"];
earlier = lt(names, "bob")

Expected output:

earlier =
  1×3 logical array
     1     0     0

Run lt Directly On gpuArray Inputs

G1 = gpuArray([1 4 7]);
G2 = gpuArray([2 4 8]);
deviceResult = lt(G1, G2);
hostResult = gather(deviceResult)

Expected output:

deviceResult =
  1×3 gpuArray logical array
     1     0     1
hostResult =
  1×3 logical array
     1     0     1

FAQ

Does lt return logical values?

Yes. Scalars return true or false. Arrays return logical arrays, and gpuArray inputs return gpuArray logical outputs.

How are NaN values treated?

Any comparison involving NaN returns false, matching MATLAB behaviour.

Can I compare complex numbers with lt?

No. MATLAB does not define relational ordering for complex numbers, so RunMat raises a MATLAB-compatible error when complex inputs are supplied.

How are strings compared?

String scalars and arrays compare lexicographically using Unicode code points, with full support for implicit expansion against scalar strings.

Are character vectors treated as numbers or text?

Character arrays participate as numeric code points when compared to numeric inputs, and they are converted to strings when compared against string scalars or arrays.

Do I need to gather results manually after a GPU comparison?

No. When both inputs are gpuArray values and the provider supports elem_lt, the result stays on the GPU. Otherwise, RunMat gathers inputs transparently and returns a host logical array.

Does implicit expansion apply to string arrays?

Yes. String arrays support MATLAB-style implicit expansion, so you can compare against scalar strings without manual replication.

Can I fuse lt inside GPU expressions?

Yes. The builtin registers element-wise fusion metadata so the planner can fuse comparisons with surrounding GPU-friendly operations.

Related functions to explore

These functions work well alongside lt. Each page has runnable examples you can try in the browser.

gt, ge, eq, ne, isequal, le

Open-source implementation

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

• View lt.rs on GitHub
• Learn how the runtime works
• Found a bug? Open an issue with a minimal reproduction.

About RunMat

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