The percent sign in R

Question

I recently read some source code for a R package called \'pathifier\'. In the source code, it uses the percent sign.

if (0 %in% xs) {
si <- NULL
cat(file = l         


        

Answer 1

The in reserved word can only be used in for loops. The %in% function is different. As noted in the documentation at ?"%in%", is defined as:

"%in%" <- function(x, table) match(x, table, nomatch = 0) > 0

So, it is essentially match. In English, x %in% y returns a vector of logical of the same length as x, with TRUE every time the corresponding element of x exists at least once in y.

The reason why there are % around it is to mark it as a "infix" operator. (I don't know if that is the exact term.)

Answer 2

The useR is given the capacity to create new infix functions and the dispatch mechanism will recognize functions whose names begin and end with %. Say you wanted to make an infix operator that replicated a value n number of times:

 `%rep%` <- function(x,y) rep(x,y)
  10 %rep% 5
  # [1] 10 10 10 10 10

Another example of doing such will be found on the help page for ?match which discusses %in% as well as demonstrates how to make an %w/o% infix operator. The section in the R Language Reference that describes this is 10.3.4: "Special operators".