[docs][R-package] Update the explanation of num_threads (fixes #4192)

[issactoast]

Hi @jameslamb

Here is the PR related to the issue #4192. Please check out the update!

Best,
Issac

[ghost]

All CLA requirements met.

[jameslamb]

Thanks very much for this!

Sorry I didn't check earlier, but there are two other places with this same bit of documentation. Can you please update these as well?

• LightGBM/R-package/R/lgb.cv.R

  Line 48 in 69d02e3

  #' \item{\code{num_threads}: Number of threads for LightGBM. For the best speed, set this to

• LightGBM/R-package/R/lgb.train.R

  Line 21 in 69d02e3

  #' \item{\code{num_threads}: Number of threads for LightGBM. For the best speed, set this to

[issactoast]

@jameslamb Sure, will do that, but just wonder what's the difference between LightGBM/lightgbm_r and LightGBM/R-package? I know the latter is the r package but what's the lightgbm_r? That folder has the same three files lgb.cv.R, lgb.train.R, and lightgbm.R

[jameslamb]

Oh sure, I can explain. The R package wraps C/C++ code stored in src/, include/, and external_libs/. So to build the R package, you have to copy all that code into the correct location inside the R package's file structure.

To do that, the R build scripts (build-cran-package.sh for autoconf-based builds and build_r.R for CMake-based builds) create a new folder, lightgbm_r, and copy R-package/ plus all those C/C++ files into it at build time. I think it will be clearer if you inspect those build scripts.

lightgbm_r is not actually checked into the repo, as it is ignored by gitignore. So you do not need to change aything in that directory.

Let me know if you have other questions! We'd love to have you contribute more to the R package in the future of you're interested, and understanding this setup might help you do that.

[issactoast]

Thank you for the explanation! Yes, I would like to add some contributions to the LGBM if I could! :) I will ask if I have some questions related to the package!

[jameslamb]

Looks great, thanks very much!

[github-actions]

This pull request has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.