advice/opinion on "<-" vs "=" in teaching R

On Fri, Jan 15, 2010 at 6:57 AM, Ted Harding
<Ted.Harding at manchester.ac.uk>wrote:

  

There is at least one context where the distinction must be
preserved. Example:

 pnorm(1.5)
 # [1] 0.9331928
 pnorm(x=1.5)
 # Error in pnorm(x = 1.5) : unused argument(s) (x = 1.5)
 pnorm(x<-1.5)
 # [1] 0.9331928
 x
 # [1] 1.5

Ted.


    

 I would regard modifying a variable within the parameters of a function
call as pretty tasteless. What does:


 foo(x<-2,x)
or
 foo(x,x<-3)

do that couldn't be done clearer with two lines of code?
  

The most common use I see that I like is within a conditional test like

if (  !is.null(x <- get("x", somehow)) && length(x) == 1) { dosomething }

The x variable is only used for the test, but since it is used twice 
there, the assignment saves getting it twice.  You could expand it to 
two lines

x <- get("x", somehow)
if ( !is.null(x) && length(x) == 1) { dosomething }

but I find that a tiny bit harder to read. 

On the other hand, I would never use the examples you gave, because I'd 
have no idea what the value of x would be, since it depends on the order 
of evaluation of the arguments.  In R, I don't even know for sure if the 
assignment would be evaluated at all, let alone before the x argument.

Duncan Murdoch

 Remember: 'eschew obfuscation'.

Barry