The difference in assignment operators is clearer when you use them to set an argument value in a function call. For example:
median(x = 1:10) x ## Error: object 'x' not found
In this case,
x is declared within the scope of the function, so it does not exist in the user workspace.
median(x <- 1:10) x ##  1 2 3 4 5 6 7 8 9 10
In this case,
x is declared in the user workspace, so you can use it after the function call has been completed.
There is a general preference among the R community for using
<- for assignment (other than in function signatures) for compatibility with (very) old versions of S-Plus. Note that the spaces help to clarify situations like
x<-3 # Does this mean assignment? x <- 3 # Or less than? x < -3
Most R IDEs have keyboard shortcuts to make
<- easier to type. Ctrl + = in Architect, Alt + - in RStudio (Option + - under macOS), Shift + - (underscore) in emacs+ESS.
If you prefer writing
<- but want to use the more common assignment symbol for publicly released code (on CRAN, for example), then you can use one of the
tidy_* functions in the
formatR package to automatically replace
library(formatR) tidy_source(text = "x=1:5", arrow = TRUE) ## x <- 1:5
The answer to the question "Why does
x <- y = 5 throw an error but not
x <- y <- 5?" is "It's down to the magic contained in the parser". R's syntax contains many ambiguous cases that have to be resolved one way or another. The parser chooses to resolve the bits of the expression in different orders depending on whether
<- was used.
To understand what is happening, you need to know that assignment silently returns the value that was assigned. You can see that more clearly by explicitly printing, for example
print(x <- 2 + 3).
Secondly, it's clearer if we use prefix notation for assignment. So
x <- 5 `<-`(x, 5) #same thing y = 5 `=`(y, 5) #also the same thing
The parser interprets
x <- y <- 5 as
`<-`(x, `<-`(y, 5))
We might expect that
x <- y = 5 would then be
`<-`(x, `=`(y, 5))
but actually it gets interpreted as
`=`(`<-`(x, y), 5)
This is because
= is lower precedence than
<-, as shown on the
?Syntax help page.