[R] Changing global variables from functions

[David Winsemius]

On Mar 16, 2010, at 2:43 PM, jtouyz wrote:

>
> Hi David,
> Thank you for your response.
> Yes, num_decks was previously defined in my program, I apologize for  
> the
> confusion as it is an integral value.
> This is only one portion of my program that is being used to  
> simulate card
> counting in blackjack.
>
> The basic idea was to modify deckn without having to return a value  
> from the
> function deck(). That is whenever deck() was run it would write over
> whatever was in deckn (instead of having to input deckn<-deck().)
> It seems from your response that this may not be possible and  
> "return" is
> always required.
> If I have erred, please let me know.

R is "supposed" to be used as a functional language. The use of the  
"<<-" operator is deprecated, so I did not mention that strategy. You  
could also have simply run the loop without the functional wrapper.

-- 
david.
>
> Thanks once again,
> Josh Elliott
>
>
> David Winsemius wrote:
>>
>>
>> On Mar 16, 2010, at 11:04 AM, jtouyz wrote:
>>
>>>
>>> Hey all,
>>> I'm relatively new to the R-environment. I'm having a bit of trouble
>>> with
>>> encapsulation.
>>> I have a globally declared variable that doesn't update it when I
>>> change it
>>> in a function.
>>
>>> For example when I run the following function
>>>
>>>> deckn<-NULL
>>>> deck1<-1  #52 card deck
>>>> deck<-function()
>>> {
>>> #Creating a standard deck
>>> deck1<-c(1:52)
>>> deckn<-deck1
>>>   #Creating n decks
>>>   for (i in 2:num_decks)
>>
>> # could be wrong but it appears that you are expecting the act of
>> putting "num_" in front of "decks" to be interpreted by R as  
>> returning
>> the length of deckn or deck1. That is a higher level of abstraction
>> than is yet available in any computer language with which I am
>> familiar. Or perhaps you were intending to use Greg Snow's soon to be
>> released mind-reading package so that R could know that you wanted it
>> to be 6?.
>>
>> Perhaps instead (depending on what the real problem (unstated as yet)
>> might be:
>>
>>   for (i in 2:length(deck1) )  # or some other object or function
>> that returns a numeric value.
>>
>>>   {
>>>   deckn<-c(deckn,52*i+deck1-1)
>>
>>>   }
>>> }
>>>> deckn
>>
>> You would have needed to assign a "return"-ed value to deckn in the
>> outer environment. The deckn object would have disappeared at the end
>> of the function call, and it would not have needed to be named
>> "deckn", either.
>>
>>
>> Try instead:
>>
>> deckn<-NULL
>> deck<-function(num_decks=6)
>> {
>> #Creating a standard deck
>> deck1<-c(1:52)
>> deckn<-deck1
>>    #Creating n decks
>>    for (i in 2:num_decks)
>>    {
>>    deckn<-c(deckn,52*i+deck1-1)
>>    }; return(deckn)
>> }
>> deckn <- deck()
>> deckn
>>
>> Which has a gap between 52 and 104 because of your logic, not mine.
>>
>>
>>>> NULL
>>>
>>> it returns NULL for deckn instead of a vector of values. Is there an
>>> easy
>>> fix to update deckn in the function so that it outputs a vector of
>>> values (
>>> I don't wish the function to return a value just update the current
>>> one)?
>>
>> You could, of course, explain what you are trying to do.
>>
>>>
>>> Thanks in advance,
>>> Josh Elliott
>>> -- 
>>
>> David Winsemius, MD
>> West Hartford, CT
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>>
>
> -- 
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>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

David Winsemius, MD
West Hartford, CT