[R] difference between <- and =

[Spencer Graves]

	  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 error first 
replaces column "a" of DF with the trivial vector 1 (of length 1), and 
then interprets that as a logical, which evaluates as TRUE.  Unless the 
"if" statement otherwise generates an error, I must restore column "a" 
from somewhere before I can continue.

	   In addition to specifying function arguments, I also use "=" to 
specify named components of a list or a vector.  That works fine for me. 
  It's only the accidental use of "=" when I mean "==" that creates 
problems.

Best Wishes,
Spencer Graves

Richard A. O'Keefe wrote:
> 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.
> 
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