[R] closest match in R to c-like struct?

[David Winsemius]

On May 1, 2010, at 3:14 PM, steven mosher wrote:

> maybe I can illustrate the problem by showing how a c programmer  
> might think
> about the problem and the kinds of mistakes 'we' ( I) make when  
> trying to do
> this in R
>
> cstruct<-function(int, bool){
> +
> + myint<- int*2;
> +
> + mybool<-!bool;
> + myvec<-rep(mybool,10)
> + mymat<-matrix(myint*10,nrow=3,ncol=3)
> + myframe<-data.frame(rep(myint,5),rep(bool,5))
> + returnlist<-list(myint,mybool,myvec,mymat,myframe)
> + return(returnlist)
> +
> +
> +
> + }
>
> # so I have a function that returns a list of hetergenous variables.
> # an int, a bool, a vector of bools, a matrix of ints, a dataframe  
> of ints
> and bools
>
>> test<-cstruct(3,T)
>
>
>> test
> [[1]]
> [1] 6
>
> [[2]]
> [1] FALSE
>
> [[3]]
> [1] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
>
> [[4]]
>     [,1] [,2] [,3]
> [1,]   60   60   60
> [2,]   60   60   60
> [3,]   60   60   60
>
> [[5]]
>  rep.myint..5. rep.bool..5.
> 1             6         TRUE
> 2             6         TRUE
> 3             6         TRUE
> 4             6         TRUE
> 5             6         TRUE
>
> # Now I want to access the first element of my list which is an  "an  
> int"
> # first mistake I always make is I just revert to thinking in the
> # 'dot' structure of a c struct.
>
>> test.myint
> Error: object 'test.myint' not found

There is no dot "." accessor function. If the first element were named  
(which ist is not) then you could have used test$myint.

If you wnated to access the elements of htat list with names you need  
to assing to names at the time it is created, eg.:

returnlist<-list(myint=myint, mybool=mybool, myvec-myvec, mymat=mymat,  
myframe=myframe)

As it is you need to do this to get what you later indicate you want,  
an atomic object:

test[[1]]

Double-brackets yield the thing itself, whereas single brackets yield  
a "sub-list".

 > test[4]
[[1]]
      [,1] [,2] [,3]
[1,]   60   60   60
[2,]   60   60   60
[3,]   60   60   60

 > test[[4]]
      [,1] [,2] [,3]
[1,]   60   60   60
[2,]   60   60   60
[3,]   60   60   60
 > class(test[4])
[1] "list"
 > class(test[[4]])
[1] "matrix"

>
> # Then I think its stored like a var in a dataframe, accessed by the $
>> test$myint
> NULL
>
> # then I try to access the first element of the list
>> test[1]
> [[1]]
> [1] 6
>
> # That works.. but the [[1]] confuses me when I eval test[1] I want  
> 6 back
> # again thinking in C.
> # so I try the third element
>
>> test[3]
> [[1]]
> [1] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
>
> # ok I get my vect of bools back. Now I want the first element
> # of that thing
> # well test[3] is that thing.. and I want element 1 of test[3]
>
>> test[3][1]
> [[1]]
> [1] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
>
> #hmm thats not what I expect. I wanted F back.
> # frustrated I try this which i know is wrong
>
>> test[3,1]
> Error in test[3, 1] : incorrect number of dimensions
>
> # crap.. maybe the $ is supposed to be used
>> test$V3
> NULL
>
> # arrg.. how about 'dot"
>> test.myvec
> Error: object 'test.myvec' not found
>
>
> Anyways, That's the kind of frustration. I have a list, third  
> element is a
> matrix
> how do I referernce the 2 row 2 colum of the matrix in my list.. for
> example.


 > str(test)
List of 5
  $ : num 6
  $ : logi FALSE
  $ : logi [1:10] FALSE FALSE FALSE FALSE FALSE FALSE ...
  $ : num [1:3, 1:3] 60 60 60 60 60 60 60 60 60
  $ :'data.frame':	5 obs. of  2 variables:
   ..$ rep.myint..5.: num [1:5] 6 6 6 6 6
   ..$ rep.bool..5. : logi [1:5] TRUE TRUE TRUE TRUE TRUE

 > test[[4]][2,2]
[1] 60



> and so forth..
>
> On Sat, May 1, 2010 at 10:56 AM, Ted Harding
> <Ted.Harding at manchester.ac.uk>wrote:
>
>> On 01-May-10 16:58:49, Giovanni Azua wrote:
>>>
>>> On May 1, 2010, at 6:48 PM, steven mosher wrote:
>>>> I was talking with another guy on the list about this very topic.
>>>>
>>>> A simple example would help.
>>>>
>>>> first a sample C struct, and then how one would do the equivalent  
>>>> in
>>>> R.
>>>>
>>>> In the end i suppose one want to do a an 'array' of these  
>>>> structs, or
>>>> list
>>>> of the structs.
>>>
>>> Or like in my use-case ... I needed a c-like struct to define the  
>>> type
>>> for aggregating the data to return from a function.
>>>
>>> Best regards,
>>> Giovanni
>>
>> Assuming that I understand what you want, this is straightforward
>> and can be found throughout the many functions available in R.
>> The general form is:
>>
>> myfunction <- function(...){
>>   <code to compute objects A1, A2, ... , An>
>>   list(valA1=A1, valA2=A2, ... , valAn=An)
>> }
>>
>> and then a call like
>>
>> myresults <- myfunction(...)
>>
>> will create a list "myresults" with compnents "valA1", ... ,"valAn"
>> which you can access as desired on the lines of
>>
>> myresults$valA5
>>
>> As a simple example, the following is a function which explores
>> by simulation the power of the Fisher Exact Test for comparing
>> two proportions in a 2x2 table:
>>
>> power.fisher.test <- function(p1,p2,n1,n2,alpha=0.05,nsim=100){
>>   y1 <- rbinom(nsim,size=n1,prob=p1)
>>   y2 <- rbinom(nsim,size=n2,prob=p2)
>>   y <- cbind(y1,n1-y1,y2,n2-y2)
>>   p.value <- rep(0,nsim)
>>   for (i in 1:nsim)
>>     p.value[i] <- fisher.test(matrix(y[i,],2,2))$p.value
>>   list(Pwr=mean(p.value < alpha),SE.Pwr=sd(p.value < alpha)/ 
>> sqrt(nsim))
>> }
>>
>> So, given two binomials B(n1,p1) and B(n2,p2), what would be the
>> power of the Fisher test to detect that p1 was different from p2,
>> at given significance level alpha? This is investigated by repeating,
>> nsim times:
>> sample from Bin(n1,p1), sample from Bin(n2.p2)
>> do a Fisher test and get its P-value; store it
>>   in a vector p.value of length nsim
>> and then finally:
>> estimate the power as the proportion Pwr of the nsim cases
>>   in which the P-value was less than alpha
>> get the SE of this estimate
>> return these two values as components Pwr and SE.Pwr of a list
>>
>> As it happens, here each component of the resulting list is of
>> the same type (a single number); but in a different computation
>> each component (and of course there could be more than two)
>> could be anything -- even another list. So you can have lists
>> of lists ... !
>>
>> Thus, instead of the simple returned list above:
>>
>> list(Pwr=mean(p.value < alpha),
>>      SE.Pwr=sd(p.value < alpha)/sqrt(nsim))
>>
>> you could have
>>
>> list(Binoms=list(Bin1=list(size=n1,prob=p1),
>>                  Bin2=list(size=n2,prob=p2))
>>      Pwr=mean(p.value < alpha),
>>      SE.Pwr=sd(p.value < alpha)/sqrt(nsim))
>>
>> thus also returning the details of the Binomials for which the
>> simulation was carried out. You could access these all together as:
>>
>> power.fisher.test(...)$Binoms
>>
>> or separately as
>>
>> power.fisher.test(...)$Binoms$Bin1
>> or
>> power.fisher.test(...)$Binoms$Bin2
>>
>> or even
>> power.fisher.test(...)$Binoms$Bin1$size
>> power.fisher.test(...)$Binoms$Bin1$prob
>> etc.
>>
>> Ted.
>>
>> --------------------------------------------------------------------
>> E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
>> Fax-to-email: +44 (0)870 094 0861
>> Date: 01-May-10                                       Time: 18:56:50
>> ------------------------------ XFMail ------------------------------
>>
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>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
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> and provide commented, minimal, self-contained, reproducible code.

David Winsemius, MD
West Hartford, CT