For a class project, my group and I are looking to play around with PCR since we didn't get a chance to dive into much details in class. We were going to look at a few different ways to select the principal components used in PCR beyond the default way R does. One of these that we wanted to look at was just a couple random samples, and seeing how they compare to the default R method. However, I can't find anything in tutorials or the R functions for pcr that let you specify the components themselves - only the number of components used (Right now I'm mainly using the pcr function from the pls package). Is there any package or function that will allow me to specify the components, or will I have to make my own function?