How do I prevent source() R code being included more than once?

Question

I have lots of R source files. For example, in both A.R and C.R files, B.R is loaded via source(). Now I'd like to use the functions in both A.R and C.R, how can I avoid sourcing B.R repeatedly? Is there any include guard mechanisms similar in C/C++?

Answer 1

If B.R already has a function or data set in it with a rather unique name, you could use it as a guard. Otherwise, just define one:

B.R

B_is_loaded <- TRUE
# The normal B stuff

A.R:

if(!exists("B_is_loaded")) source("B.R")
# Now you can call the stuff in B