The Assignment Operator in R: Does "

In R, I have come across an example in which both = and <- are valid for assignment. e.g.

> y = c("hello", "world")
> y
[1] "hello" "world"
> y2 <- c("hello", "world")
> y2
[1] "hello" "world"

I have also come across an example in which = is invalid and <- is valid. e.g.

> quote(y[1] <- 1)
y[1] <- 1
> quote(x[1] = 1)
Error: unexpected '=' in "quote(x[1] ="

My question is, are there any cases in which the vice versa is true? i.e. <- is invalid whilst = is valid?

Reason for asking this question is to understand whether to stick with =, <-, or either (depending on circumstances) when performing assignment operation in R.

This will really help me setting up my mindset when performing coding in R.

Thank you!

Answer

R has several assignment operators. Per the documentation

The operator <- can be used anywhere, whereas the operator = is only allowed at the top level (e.g., in the complete expression typed at the command prompt) or as one of the subexpressions in a braced list of expressions.

The only place I am aware of where you must use the <- operator is naming items of a list in attach.

This does not work:

> attach(what <- list(foo <- function(x) print(x)))

but this does:

> attach(what <- list(foo = function(x) print(x)))

I don't actually know why this is. If anyone else knows I'd love to learn why.

I am also compelled to discourage use of any of the blasphemous right assignment operators.