R help - Oct 2009 - Stranger Behavior -maybe not

Hello everyone,

When I run a for loop I noticed that the looping variable gets assigned the
last value in the loop:

For example, if I let h=200, and I run

for(i in 1:h),

i gets the value 200. What's wrong here?

-- 
-Tony

	[[alternative HTML version deleted]]

On 03/10/2009 8:53 PM, Antonio Paredes wrote:
> Hello everyone,
> 
> When I run a for loop I noticed that the looping variable gets assigned the
> last value in the loop:
> 
> For example, if I let h=200, and I run
> 
> for(i in 1:h),
> 
> i gets the value 200. What's wrong here?
Nothing?

Duncan Murdoch

Yes of course.   I was sticking to R.

On 3-Oct-09, at 7:42 PM, hadley wickham wrote:
>> The point Duncan was making was
>>
>> "What other value would you expect it to have after incrementing  
>> from 1 to
>> 200 and stopping"?
>
> Well, depending on the scoping rules of the language you are used to,
> you might expect i at the top-level to remain undefined.
>
> Hadley
>
> -- 
> http://had.co.nz/
Don McKenzie, Research Ecologist
Pacific WIldland Fire Sciences Lab
US Forest Service

Affiliate Professor
School of Forest Resources, College of the Environment
CSES Climate Impacts Group
University of Washington

desk: 206-732-7824
cell: 206-321-5966
dmck at u.washington.edu
donaldmckenzie at fs.fed.us

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Antonio Paredes
> Sent: Saturday, October 03, 2009 5:53 PM
> To: r-help at r-project.org
> Subject: [R] Stranger Behavior -maybe not
> 
> Hello everyone,
> 
> When I run a for loop I noticed that the looping variable 
> gets assigned the
> last value in the loop:
> 
> For example, if I let h=200, and I run
> 
> for(i in 1:h),
> 
> i gets the value 200. What's wrong here?
That is the way for loops work in R.  After the loop
is done 'i' remains defined to be the last value it had
in the loop, just as any other variable defined in the
loop would remain defined.

In S+ (and S) the 'i' in the loop is defined only within
the loop and disappears after the loop is
done.  If another 'i' were defined before the start of
the loop then it would be masked by the loop's 'i'
until the loop was done and the pre-loop value of 'i'
would then reappear.  I.e., the for loop acted a bit like
a new frame, but unlike the usual frames in S+.  (C++
acts like this, doesn't it, if the 'i' is declared in the for
statement.  R and S don't have declarations so the analogy
isn't good.)

E.g., 
   RS> i<-7
   RS> for(i in 1:3)if(i==2)break
   RS> i
   S+: [1] 7
   R : [1] 2

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com 
> 
> -- 
> -Tony
> 
> 	[[alternative HTML version deleted]]
> 
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