By John Mount
Here is an R programming puzzle. What does the following code snippet actually do? And ever harder: what does it mean? (See here for some material on the difference between what code does and what code means.)
f <- function() { x <- 5 }
f()
In R version 3.2.3 (2015-12-10) -- "Wooden Christmas-Tree" the code appears to call the function f() and return nothing (nothing is printed). When teaching I often state that you should explicitly use a non-assignment expression as your return value. You should write code such as the following:
f <- function() { x <- 5; x }
f()
## [1] 5
(We are showing an R output as being prefixed with ##.)
But take a look at the this:
f <- function() { x <- 5 }
print(f())
## [1] 5
It prints! Read further for what is really going on.
What is going on is: in R in the absence of an explicit return() statement functions always return the value of the last statement executed. Also in R assignment is itself a value returning expression (returning the value assigned). So the original function f <- function() { x <- 5 } is in fact returning a 5. We just don’t see it. The 5 returned is “invisible” (see the “return values” section of Advanced R, Hadley Wickham, CRC 2015 for details).
As we said: R assignments return values. So you can return them and you can chain them like so:
a <- b <- c <- 5 print(a) ## [1] 5
What happens is the assignment x <- 5 returns a value (in this case 5), but that value has an attribute marking it invisible. This is why when you assign a value to a variable in R you don’t see printing as a side …read more
Source:: win-vector.com