[R] how to do an external validation with R

[beginner]

I would like to do external validation using R software. So far I have used
packages like "Design" and "DAAG". However they perform internal validation
rather than external one. In order to perform external validation I would
have to split my data beforehand into training and test set, leave the test
set on the site and use only the training set to select a model. I would
then test the selected model with the sample set left initially on the site.
I would like to repeat this process several times to make sure that all the
samples are included at least once in a test set. I thought that I need to
use a loop function in R to perform this process automatically. As I am new
to R I don't know how to make a loop. Could you please help me with this or
suggest an R package ? I would be very very grateful for help with this task
!



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