how to configure R! environment

Where/how to configure R start-up environment
There are several approaches can be used to customize the R working environment such as options and library directory etc. at R start-up:

• Modify the R original profile file directly. The "Rprofile.site" is under the directory ".\R directory name\etc\". At both startup and end, the R will use the "Rprofile.site" file, then looks for the user-defined ".Rprofile" file in the current working directory (run "getwd()" to find the current location of working directory) or in the user's R home directory (run "R.home()" or "Sys.getenv("R_HOME")"to find where it is). You can edit the "Rprofile.site" file or create a ".Rprofile" file to customize the startup. For more information see Initialization at start of an R session and Customizing startup. I am using R-Portable and prefer to create a ".Rprofile" in the same directory of "R-Portable.exe" file. In such way, I don't need to dig deep and edit the R original setting.
• to lists all the options can be set, run "names(options())"
• to show the value of an item, run "options("option name")", for example:
•  "options("digits")" shows "$digits, [1] 7", which means the number will be shown 7 digits.
• "options("defaultPackages")" shows the packages attached by default when R starts up
• to modify the values of an option item, run "options(xxx=yyy), for example:
• "options(digits=15)" changes the digit number into 15. Notes: this is for setting full length of number but not number of decimal places. To set the number of decimal, try such as "round(4/3, digits=2)" with 2 decimal places but not in "options()" unfortunately.
• to set the directory of personal R library, create a ".Rprofile" file in the working directory and include ".libPaths(c(.libPaths(),"c:/myRlib directory name")", save it.
• or, edit "Rprofile.site", add line: Add line: ".libPaths(c(.libPaths(),"c:/myRlib directory name")"
• When use RStudio as the IDE, modify the options file ("Options.R") under the ".\Rstudio directory name\R\". The option setting overwrites the option setting in R profiles both "Rprofile.site" and ".Rprofile".
• to set the directory of personal R library, edit file "Options.R", add line: ".libPaths(c(.libPaths(),"c:/myRlib directory name")",  then save the "Options.R".
• or, to use ".Rprofile", this file needs be in the working directory when not in a project (to set this master working directory using RStudio GUI: tools -> Global options... -> change the "Default working directory(when not in a project):"). Also you can change R.home() under the "R version:".
• By the way, the options and the directory of package library can also be changed after the start-up of R.
• de Vries (2015).Best practices for handling packages in R projects
• Gillespie. R startup