What are the differences between "=" and "<
What are the differences between the assignment operators = and <- in R?
I know that operators are slightly different, as this example shows
x <- y <- 5
x = y = 5
x = y <- 5
x <- y = 5
# Error in (x <- y) = 5 : could not find function "<-<-"
But is this the only difference?
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] 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 = to <- 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 = with <- .
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 = or <- 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.
Google's R style guide simplifies the issue by prohibiting the "=" for assignment. Not a bad choice.
https://google.github.io/styleguide/Rguide.xml
The R manual goes into nice detail on all 5 assignment operators.
http://stat.ethz.ch/R-manual/R-patched/library/base/html/assignOps.html
According to John Chambers, the operator = is only allowed at "the top level," which means it is not allowed in control structures like if , making the following programming error illegal.
> if(x = 0) 1 else x
Error: syntax error
As he writes, "Disallowing the new assignment form [=] in control expressions avoids programming errors (such as the example above) that are more likely with the equal operator than with other S assignments."
You can manage to do this if it's "isolated from surrounding logical structure, by braces or an extra pair of parentheses," so if ((x = 0)) 1 else x would work.
See http://developer.r-project.org/equalAssign.html
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