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.hard...@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.hard...@manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 01-May-10 Time: 18:56:50
------------------------------ XFMail ------------------------------
______________________________________________
R-help@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.
[[alternative HTML version deleted]]
______________________________________________
R-help@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
______________________________________________
R-help@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.
