R help - Apr 2008 - system.time gives error when "=" is used for assignment (R-2.6.0)

Hello list,
I found that system.time works correctly when I used "<-" to assign
a
value to a variable but when I happened to use "=" instead of
"<-", R
gave an error message:
"Error in system.time(your argument here...)". It happened with a few
functions I tried. Is this a bug or is there any circumstances that
"=" cannot be used for assignment? Here is a real simple example.

fn1 <- function(x) x+1

r1 <- system.time(res1=fn1(2))
r2 <- system.time(res2 <- fn1(2))


Thank you.
Kyeongmi
University of Memphis

On Thu, 10 Apr 2008, Kyeongmi Cheon wrote:
> Hello list,
> I found that system.time works correctly when I used "<-" to
assign a
> value to a variable but when I happened to use "=" instead of
"<-", R
> gave an error message:
> "Error in system.time(your argument here...)". It happened with a
few
> functions I tried. Is this a bug or is there any circumstances that
> "=" cannot be used for assignment? Here is a real simple example.
The latter. Although some might say the former, too. And some will no 
doubt flame me for even saying that. The former, I mean.

Arguments in calls to function (aka arglists) can be in the name=object 
form.

So,

 	(function(x,y) x-y)(y=3,x=2)

evaluates to -1, while

 	(function(x,y) x-y)( 3, 2 )

evaluates to 1, as does

 	 (function(x,y) x-y)(me <- 3, you <- 2)
but

 	 (function(x,y) x-y)(me = 3, you = 2)

gives an error when it tries to match 'me' or 'you' to an
argument but
fails to find one.

Hint: often

 	system.time( { result.of.long.compute <- foo(my.args) } )

is what you want.

HTH,

Chuck

p.s. As an exercise for the reader, why does

  	(function(x,y) x-y)({me = 3},{you = 2})

not return an error?
>
> fn1 <- function(x) x+1
>
> r1 <- system.time(res1=fn1(2))
> r2 <- system.time(res2 <- fn1(2))
>
>
> Thank you.
> Kyeongmi
> University of Memphis
>
> ______________________________________________
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> PLEASE do read the posting guide
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>
Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	            UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901

Kyeongmi asks:
> Hello list, 
> 
> I found that system.time works correctly when I used "<-" to
assign
> a value to a variable but when I happened to use "=" instead of
> "<-", R gave an error message: "Error in system.time(your
argument
> here...)". It happened with a few functions I tried. Is this a bug
> or is there any circumstances that "=" cannot be used for
> assignment? Here is a real simple example.
> 
> fn1 <- function(x) x+1
> 
> r1 <- system.time(res1=fn1(2))
> r2 <- system.time(res2 <- fn1(2))
> 
No, it is not a bug.  It is a syntactic trap that using '=' for
assignment will lure you into.  This is one reason why I suggest you
just do not do it.

Think about it.  If the function system.time() had an argument res1,
how would you call it giving a value for this argument?  As you have
done.  So it has nothing to do with the function system.time iteslf,
it applies generally.

Ways around it include

r1 <- system.time((res1 = fn1(2)))  ## if you must!
r1 <- system.time({res1 = fn1(2)})  ## ditto
r1 <- system.time(res1 <- fn1(2))   ## as you discovered.

Bill Venables.