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

[Duncan Murdoch]

Barry Rowlingson wrote:
> 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
>
>