[R] difference between <- and =

[Peter Dalgaard BSA]

Spencer Graves <spencer.graves at pdf.com> writes:

> 	  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.

Eh?

> a <- list(x=2)
> if (a$x = 1) 5
Error: syntax error

I think you're referring to another R-like language....

 
> 	   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.

(Actually, that's the same thing. list() and c() are function calls like
(almost) everything else.)

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
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