[R] scoping rules for 'for' [was - Re: for/if loop ]
Patrick Burns
pburns at pburns.seanet.com
Thu Jan 29 17:06:19 CET 2009
Certainly not a complete description, but
'The R Inferno' talks about this on page 62.
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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