compose — Format values into string arrays using printf-style placeholders in MATLAB and RunMat.

compose(formatSpec, A1, ..., An) substitutes data into % placeholders and returns a string array. It combines sprintf-style formatting with string-array expansion behavior matching MATLAB and RunMat.

Syntax

S = compose(formatSpec)
S = compose(formatSpec, A...)

Inputs

Returns

Errors

How compose works

• formatSpec must be text: a string scalar, string array, character vector, character array, or cell array of character vectors.
• If formatSpec is scalar and any argument array has more than one element, RunMat broadcasts the scalar specification over the array dimensions.
• When formatSpec is a string or character array with multiple elements, the output has the same shape as the specification. Each element uses the corresponding row or cell during formatting.
• Arguments can be numeric, logical, string, or text-like cell arrays. Non-text arguments are converted using MATLAB-compatible rules (logical values become 1 or 0, complex numbers use the a + bi form).
• When you omit additional arguments, compose(formatSpec) simply converts the specification into a string array, preserving the original structure.
• Errors are raised if argument shapes are incompatible with the specification or if format specifiers are incomplete.

Does RunMat run compose on the GPU?

compose is a residency sink. When inputs include GPU-resident tensors, RunMat gathers the data back to host memory using the active acceleration provider before performing the formatting logic. All formatted strings live in host memory, so acceleration providers do not need compose-specific kernels.

Examples

Formatting A Scalar Value Into A Sentence

msg = compose("The answer is %d.", 42)

Expected output:

msg = "The answer is 42."

Broadcasting A Scalar Format Spec Over A Vector

result = compose("Trial %d", 1:4)

Expected output:

result = 1×4 string
    "Trial 1"    "Trial 2"    "Trial 3"    "Trial 4"

Using A String Array Of Formats

spec = ["max: %0.2f", "min: %0.2f"];
values = compose(spec, [3.14159, 0.125])

Expected output:

values = 1×2 string
    "max: 3.14"    "min: 0.12"

Formatting Each Row Of A Character Array

C = ['Row %02d'; 'Row %02d'; 'Row %02d'];
idx = compose(C, (1:3).')

Expected output:

idx = 3×1 string
    "Row 01"
    "Row 02"
    "Row 03"

Combining Real And Imaginary Parts

Z = [1+2i, 3-4i];
txt = compose("z = %s", Z)

Expected output:

txt = 1×2 string
    "z = 1+2i"    "z = 3-4i"

Using A Cell Array Of Format Specs

specs = {'%0.1f volts', '%0.1f amps'};
readings = compose(specs, {12.6, 3.4})

Expected output:

readings = 2×1 string
    "12.6 volts"
    "3.4 amps"

Formatting GPU-Resident Data

G = gpuArray([10 20 30]);
labels = compose("Value %d", G)

Expected output:

labels = 1×3 string
    "Value 10"    "Value 20"    "Value 30"

Using compose with coding agents

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

FAQ

Related Strings functions

Open-source implementation

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

• View the source for compose 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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