feat: add stats/base/dists/halfnormal/stdev

[bhargava-d16]

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Progresses #9416

Description

What is the purpose of this pull request?

This pull request:

• This PR adds implementation of standard deviation calculation for half-normal distribution.

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• [RFC]: Tracking issue for complete implementation of @stdlib/stats/base/dists/halfnormal package #9416

Questions

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No.

Other

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No.

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• Read, understood, and followed the contributing guidelines.

AI Assistance

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• Test/benchmark generation
• Documentation (including examples)
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I consulted ChatGPT to verify mathematical formulation of half-normal standard deviation function and to confirm expected numerical values

---

@stdlib-js/reviewers

[Planeshifter]

Thanks!

[Planeshifter]

The README example uses 0.1 as the minimum value, but this file uses 0.0. To keep consistency with the README,

Suggested change

	var sigma = uniform( 10, 0.0, 20.0, opts );
	var sigma = uniform( 10, 0.1, 20.0, opts );

[Planeshifter]

This file has two trailing newlines. Per stdlib style, files should end with exactly one trailing newline.

[Planeshifter]

This file has two trailing newlines. Per stdlib style, files should end with exactly one trailing newline.

[Planeshifter]

This file has two trailing newlines. Per stdlib style, files should end with exactly one trailing newline.

[Planeshifter]

See also failing check runs.

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/dists/halfnormal/stdev	$\color{green}168/168$ $\color{green}+0.00%$	$\color{green}8/8$ $\color{green}+0.00%$	$\color{green}2/2$ $\color{green}+0.00%$	$\color{green}168/168$ $\color{green}+0.00%$

The above coverage report was generated for the changes in this PR.

[Planeshifter]

Let's get it in! 🚀

[stdlib-bot]

PR Commit Message

feat: add `stats/base/dists/halfnormal/stdev`

PR-URL: https://github.com/stdlib-js/stdlib/pull/9714
Ref: https://github.com/stdlib-js/stdlib/issues/9416

Co-authored-by: Philipp Burckhardt <[email protected]>
Reviewed-by: Philipp Burckhardt <[email protected]>
Signed-off-by: Philipp Burckhardt <[email protected]>

Please review the above commit message and make any necessary adjustments.

[kgryte]

@bhargava-d16 How did you generate fixture data for this PR? It doesn't appear that the HalfNormal distribution exists in Distributions.jl, so I am curious how you generated the fixture data according to the test fixture runner.

There is a PR adding support for folded distributions, but that has not been merged. Ref: JuliaStats/Distributions.jl#1631

cc @Planeshifter

[bhargava-d16]

Thanks for pointing this out.

You are right that HalfNormal is not currently part of the released Distributions.jl. The fixture values were generated using the analytical expression for the standard deviation of the Half-Normal distribution, not a built-in Julia distribution.

Specifically, for a Half-Normal distribution with scale sigma, the standard deviation is:
stdev = sigma * sqrt(1 - 2/pi)

The current Julia fixture script is therefore misleading, as it suggests use of Distributions.HalfNormal. Please let me know if I need to update the script to compute the standard deviation directly from the formula and I’ll make the requested changes.

[kgryte]

@bhargava-d16 Instead of Julia, you can generate fixtures from SciPy. See https://github.com/stdlib-js/stdlib/tree/develop/tools/snippets/test/fixtures for snippets which can be used accordingly. I suggest making a follow-up PR.

[bhargava-d16]

Thanks for the suggestion!
I’ll generate the fixtures using SciPy following the snippets and open a follow-up PR.

[bhargava-d16]

I’ve opened a follow-up PR as suggested: #9784
This updates the fixture generator to use SciPy instead of Julia.