R help - Aug 2003 - difference between <- and =

Hi,

I'm somewhat new to R and I'm trying to figure out the difference 
between the operators <- and =.

I've noticed that <- cannot be used to bind arguments to values in 
function definitions and calls.  That is, f(x <- 2) sets x to 2 in the 
calling frame and then calls f(2) because the expression x <- 2 returns 
a value of 2, whereas f(x = 2) sets x = 2 in the evaluation frame and 
not in the calling frame.  From what I can tell, = is only different 
than <- in the context of function definitions and calls.

Is there any reason for using one operator over the other outside of 
function definitions and calls?  Can someone point me to a precise 
specification of what each operator does?  Any help would be appreciated!

Levi

Levi Larkey schrieb:
> Hi,
> 
> I'm somewhat new to R and I'm trying to figure out the difference 
> between the operators <- and =.
<- is an assignement operator
=  is primarily used for named arguments

Some thoughts about the use of = as assignement operator can be found on:

http://developer.r-project.org/equalAssign.html

Thomas

Levi Larkey <larkey at mail.utexas.edu> wrote:

a perfect illustration of why allowing "=" as an alternative
spelling for "<-" was always an obviously really bad idea.

	Is there any reason for using one operator over the other outside of 
	function definitions and calls?

You should never use "=" for assignment unless you are suffering such
severe "C" withdrawal symptoms that the doctor just had to give you a
shot to calm you down.  It is best to keep different symbols for
different purposes.  (I note that Ada uses ":=" for assignment and
"=>" for keyword arguments.)

Something that would be quite useful would be an option asking R to
warn you
    - if you have any assignment operator in a function argument
      (thanks to lazy evaluation this probably *won't* do what you
      expect)
    - if you have "=" anywhere except for binding a name to an
argument.
Perhaps these could be two options.

Spencer Graves <spencer.graves at PDF.COM> wrote:
	A common, punishing error for me, with DF being a data frame, is the 
	following:
	
		  if(DF$a = 1) ...
	
	 where I intended to write "if(DF$a == 1)...".

This is a syntax error in R.

    > d <- data.frame(a=2,b=3,c=4)
    > if (d$a=1) cat("foo!")
    Error: syntax error

If I've understood the R documentation correctly, "=" is only
allowed
as a synonym for "<-" at "statement" level.

Of course, that's one more reason not to use "=" when you DO want
assignment; "<-" has no such restriction.