[R] scoping rules for 'for' [was - Re: for/if loop ]
Duncan Murdoch
murdoch at stats.uwo.ca
Thu Jan 29 17:22:59 CET 2009
On 1/29/2009 11:06 AM, Patrick Burns wrote:
> Certainly not a complete description, but
> 'The R Inferno' talks about this on page 62.
Hmmm, I don't think I agree with that description. There's only one
object named i in that example (which was
for (i in 1:6) {
cat('\n i is', i, '\n\n')
for (i in 1:4) cat('i is', i,'\n')
}
for the lazy). It gets set to 1, then to each of 1:4, then to 2, then
to 1:4, etc. If you print the value after the inner loop, it will
always be 4, you won't go back to the supposed other i. For example,
> for (i in 1:6) {
+ cat('\n i is', i, '\n\n')
+ for (i in 1:4) cat('i is', i,'\n')
+ cat('outer i is', i, '\n')
+ }
i is 1
i is 1
i is 2
i is 3
i is 4
outer i is 4
i is 2
i is 1
i is 2
i is 3
i is 4
outer i is 4
i is 3
i is 1
i is 2
i is 3
i is 4
outer i is 4
Duncan Murdoch
>
>
> Patrick Burns
> patrick at burns-stat.com
> +44 (0)20 8525 0696
> http://www.burns-stat.com
> (home of "The R Inferno" and "A Guide for the Unwilling S User")
>
> davidr at rhotrading.com wrote:
>> I apologize for posting a wrong opinion; I should of course have checked
>> before posting.
>>
>> Henrik's examples illustrate something I had never realized before, and
>> it really surprised me!
>> Where can I read the technical details of this scoping aspect of 'for'
>> as it still presents
>> some subtle puzzles for me. For example:
>>
>>> for (ii in 1:3) { print(ii); if(ii==1){ii <- 20}; print(ii); print(ii
>>>
>> <- ii + 1); print(ii)}
>> [1] 1
>> [1] 20
>> [1] 21
>> [1] 21
>> [1] 2
>> [1] 2
>> [1] 3
>> [1] 3
>> [1] 3
>> [1] 3
>> [1] 4
>> [1] 4
>> Why does R treat ii differently after the 'if' in the first and
>> subsequent iterations?
>> And if the loop variable does not exist before the 'for', why is it
>> created in the parent(?) environment at all?
>> (I.e, if ii did not exist before the for loop, it does and has value 5
>> after. Wouldn't strict
>> scoping mean that ii exists only within the for loop?)
>>
>> I generally don't try to change the loop variable's value inside a loop,
>> but coming from C
>> or other languages, it seems natural to do so in certain circumstances.
>> And the examples are
>> certainly bad programming style. But I remain confused. I assume that
>> 'while' loops have different scoping
>> rules?
>>
>> Thanks,
>>
>> -- David
>>
>>
>> -----Original Message-----
>> From: henrik.bengtsson at gmail.com [mailto:henrik.bengtsson at gmail.com] On
>> Behalf Of Henrik Bengtsson
>> Sent: Wednesday, January 28, 2009 5:08 PM
>> To: David Reiner <davidr at rhotrading.com>
>> Cc: SnowManPaddington; r-help at r-project.org
>> Subject: Re: [R] - Re: for/if loop
>>
>> On Wed, Jan 28, 2009 at 2:41 PM, <davidr at rhotrading.com> wrote:
>>
>>> Well, maybe you are just bad at typing then ;-)
>>>
>>> The lines rr==ii, pp==pp+1, etc. are not setting rr and pp but
>>>
>> comparing
>>
>>> them.
>>> Probably you want rr <- ii and pp <- pp+1, etc.
>>> And the last line of your loop 'ii=ii+1' means that,
>>> since the for statement is already incrementing ii,
>>> you are incrementing it twice and skipping the even indices. Omit this
>>> line probably.
>>>
>>
>> That is actually not the case (because of the scoping rules for for(),
>> I think). Example:
>>
>>
>>> for (ii in 1:5) { print(ii); ii <- ii + 1; }
>>>
>> [1] 1
>> [1] 2
>> [1] 3
>> [1] 4
>> [1] 5
>>
>> Another "counter intuitive" (though it isn't) example:
>>
>> for (ii in 1:3) {
>> cat("Outer ii:",ii,"\n");
>> for (ii in ii:3) {
>> cat(" Inner ii:",ii,"\n");
>> }
>> }
>>
>> Outer ii: 1
>> Inner ii: 1
>> Inner ii: 2
>> Inner ii: 3
>> Outer ii: 2
>> Inner ii: 2
>> Inner ii: 3
>> Outer ii: 3
>> Inner ii: 3
>>
>> My $.02
>>
>> /Henrik
>>
>>
>>> You are also forgetting(?) the operator precedence in
>>> sum(lselb1[rr:ii-1]) and similar lines.
>>> Note that this is equivalent to sum(lselb1[(rr-1):(ii-1)]); is that
>>>
>> what
>>
>>> you meant?
>>> Or did you want sum(lselb1[rr:(ii-1)])?
>>> You are changing some variables but not asking R to print anything as
>>> far as I can see.
>>> To see the results, ask R to print hll.
>>>
>>> HTH,
>>> -- David
>>>
>>> -----Original Message-----
>>> From: r-help-bounces at r-project.org
>>>
>> [mailto:r-help-bounces at r-project.org]
>>
>>> On Behalf Of SnowManPaddington
>>> Sent: Wednesday, January 28, 2009 3:59 PM
>>> To: r-help at r-project.org
>>> Subject: - Re: [R] for/if loop
>>>
>>>
>>> Hi ya, I've revised the code (and finally know what I m doing.. :-D)
>>>
>>> The good news is.. I dont get any error message, but the bad news is
>>>
>> the
>>
>>> following optim generate no results. I still think there is something
>>>
>> to
>>
>>> do
>>> with my loop... can anyone advice? Thanks again!!!
>>>
>>>
>>>
>>> pp=1
>>> rr=1
>>>
>>> for (ii in 1:n){
>>> if (!(panel[ii] == pp)){
>>> hll[pp,1] == sum(lselb1[rr:ii-1])
>>> hll[pp,2] == sum(lselb2[rr:ii-1])
>>> rr==ii
>>> pp==pp+1
>>> }
>>>
>>> if (ii==n){
>>> hll[pp,1] == sum(lselb1[rr:ii])
>>> hll[pp,2] == sum(lselb2[rr:ii])
>>> rr==ii
>>> pp==pp+1
>>> }
>>> ii=ii+1
>>> }
>>>
>>>
>>>
>>>
>>>
>>> pp=1
>>> rr=1
>>>
>>> for (ii in 1:n){
>>> if (!(panel[ii] == pp)){
>>> hll[pp,1] == sum(lselb1[rr:ii-1])
>>> hll[pp,2] == sum(lselb2[rr:ii-1])
>>> rr==ii
>>> pp==pp+1
>>> }
>>>
>>> if (ii==n){
>>> hll[pp,1] == sum(lselb1[rr:ii])
>>> hll[pp,2] == sum(lselb2[rr:ii])
>>> rr==ii
>>> pp==pp+1
>>> }
>>> ii=ii+1
>>> }
>>>
>>>
>>>
>>>
>>>
>>> SnowManPaddington wrote:
>>>
>>>> Hi, it's my first time to write a loop with R for my homework. This
>>>>
>>> loop
>>>
>>>> is part of the function. I wanna assign values for hll according to
>>>>
>>> panel
>>>
>>>> [ii,1]=pp. I didn't get any error message in this part. but then when
>>>>
>>> I
>>>
>>>> further calculate another stuff with hll, the function can't return.
>>>>
>> I
>>
>>>> think it must be some problem in my loop. Probably something stupid
>>>>
>> or
>>
>>>> easy. But I tried to look for previous posts in forum and read R
>>>>
>>> language
>>>
>>>> help. But none can help.. Thanks!
>>>>
>>>>
>>>>
>>>> for (ii in 1:100){
>>>> for (pp in 1:pp+1){
>>>> for (rr in 1:rr+1){
>>>> if (panel[ii,1]!=pp)
>>>> {
>>>> hll(pp,1)=ColSums(lselb1(rr:ii-1,1))
>>>> hll(pp,2)=ColSums(lselb2(rr:ii-1,1))
>>>> rr=ii
>>>> pp=pp+1
>>>> }
>>>> else
>>>> {
>>>> hll(pp,1)=ColSums(lselb1(rr:ii,1))
>>>> hll(pp,2)=ColSums(lselb2(rr:ii,1))
>>>> rr=ii
>>>> pp=pp+1}
>>>> }
>>>> }}}
>>>>
>>>>
>>>> in fact I have the corresponding Gauss code here. But I really don't
>>>>
>>> know
>>>
>>>> how to write such loop in R.
>>>>
>>>> rr=1;
>>>> ii=1;
>>>> pp=1;
>>>> do until ii==n+1;
>>>> if pan[ii,1] ne pp;
>>>> hll[pp,1]=sumc(lselb1[rr:ii-1,1]);
>>>> hll[pp,2]=sumc(lselb2[rr:ii-1,1]);
>>>> rr=ii;
>>>> pp=pp+1;
>>>> endif;
>>>> if ii==n;
>>>> hll[pp,1]=sumc(lselb1[rr:ii,1]);
>>>> hll[pp,2]=sumc(lselb2[rr:ii,1]);
>>>> rr=ii;
>>>> pp=pp+1;
>>>> endif;
>>>> ii=ii+1;
>>>> endo;
>>>>
>>>>
>>>>
>>> --
>>> View this message in context:
>>> http://www.nabble.com/for-if-loop-tp21701496p21715928.html
>>> Sent from the R help mailing list archive at Nabble.com.
>>>
>>>
>>
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>>
>>
>>
>
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