Is `for` a function in R?

Question

At page 307 of Norman Matloff's The Art of R Programming, the author says

for() is, in fact, a function.

The context is to give a remark on a piece of code snippet

for (i in 1:length(x)) z[i] <- x[i] + y[i]

The author comments

Though syntactically the loop looks innocuous, for() is, in fact, a function.

My understanding is that what he is saying is that for() is a function much like + is a function (callable object). Say we may call + in a standard function calling way like

"+"(3 ,5)  # 8

I have background in C/C++ and Python and I have noticed several subtle differences between the languages. For example,

• return is a function, not a statement, in R, so that we must write return(1) with the parentheses.

Going back to my question: is for a function in R?

Thanks for any clarification.

Answer 1

for is a function, but the symbol for is also recognised by the parser as part of the convenient syntax that we can use to call the function for. These are two different things conveniently named the same (note that in is not a function).

is.function(`for`)
#> [1] TRUE

x <- y <- z <- 1:3
for (i in 1:length(x)) z[i] <- x[i] + y[i]
z
#> [1] 2 4 6

x <- y <- z <- 1:3
`for`(i, 1:length(x), z[i] <- x[i] + y[i])
z
#> [1] 2 4 6

^{Created on 2019-05-19 by the reprex package (v0.2.1)}

In a similar fashion the if (cond) foo else bar syntax maps to the function call `if`(cond, foo, bar), but there is no else function.

All other control flow constructs (see ?Control) are also functions.