I had a bug in my last solution.
This one should work.
next.combin <- function(oldcomb,n){
lcomb <- length(oldcomb)
hole.pos <- last.hole.pos(oldcomb,n)
if ((hole.pos == lcomb) & oldcomb[lcomb]==n) {
return(NA)
}
newcomb<-oldcomb
newcomb[hole.pos:lcomb]<-oldcomb[hole.pos]+(1:(lcomb-hole.pos+1))
return(newcomb)
}
last.hole.pos <- function(comb,n){
lcomb <- length(comb)
if (comb[lcomb]<n) {
return(lcomb)
}
diffs <- comb[-1]-comb[-lcomb]
if (max(diffs)==1) {
return(lcomb)
}
diffpos <- which(diffs>1)
return(diffpos[length(diffpos)])
}
demo <- function(n,k){
currCombin <- 1:k
#this is the first possible combination k out if n.
selectedSet <- NULL
while (!any(is.na(currCombin))){
# if(currCombin passes test) {
selectedSet <- rbind(selectedSet,currCombin)
# }
currCombin <- next.combin(currCombin,n)
}
rownames(selectedSet)<-NULL
selectedSet
}
On Mar 6, 2010, at 9:50 PM, Herm Walsh wrote:
> The usage here is exactly what I am looking for, thanks. However this
> function seems to omit some combinations. Continuing with the example below,
> given an always true condition in #***, it will only produce 7 combinations
> (omitting 1,5 1,4 and 2,5).
> Am I overlooking something that makes it produces all of the combinations?
>
>
>
> From: Erich Neuwirth <erich.neuwi...@univie.ac.at>
> To: Herm Walsh <hermwa...@yahoo.com>; r-help <r-help@r-project.org>
> Sent: Sat, March 6, 2010 9:12:13 AM
> Subject: Re: [R] Working with combinations
>
> currCombin <- c(1,2) #this is the first possible combination 2 out if 5.
> Since the vector has length 2, we are doing 2 out of x here.
> selectedSet <- NULL
> while (!is.na(currCombin)){
> if( #*** currCombin passes test ***# ) {
> selectedSet <- rbind(selectedSet,currCombin)
> }
> currCombin <- next.combin(currCombin,5) #here we say that we want 2 out of 5
> }
>
>
>
> On Mar 6, 2010, at 5:54 PM, Herm Walsh wrote:
>
>> Erich-
>> This approach would be great for my context. However, in the code below I
>> do not see how to restrict the output to the set of combinations I am
>> looking for. For example, suppose I am looking for the 10 two element
>> combinations of 1:5. Can you give me some psuedocode that shows how to do
>> this?
>> After putting the code below in a loop I do not see how to restrict the
>> output to containing only numbers from 1:5.
>> thanks very much.
>>
>> From: Erich Neuwirth <erich.neuwi...@univie.ac.at>
>> To: Herm Walsh <hermwa...@yahoo.com>
>> Cc: David Winsemius <dwinsem...@comcast.net>; r-help@r-project.org
>> Sent: Wed, March 3, 2010 2:10:34 PM
>> Subject: Re: [R] Working with combinations
>>
>> The following code takes a combination of type n over k represented by an
>> increasing sequence
>> as input an produces the lexicographically next combinations.
>> So you can single step through all possible combinations and apply your
>> filter criteria
>> before you produce the next combination.
>>
>>
>> next.combin <- function(oldcomb,n){
>> lcomb <- length(oldcomb)
>> hole.pos <- last.hole.pos(oldcomb,n)
>> if ((hole.pos == lcomb) & oldcomb[lcomb]==n) {
>> return(NA)
>> }
>> newcomb<-oldcomb
>> newcomb[hole.pos]<-oldcomb[hole.pos]+1
>> return(newcomb)
>> }
>>
>> last.hole.pos <- function(comb,n){
>> lcomb <- length(comb)
>> diffs <- comb[-1]-comb[-lcomb]
>> if (max(diffs)==1) {
>> return(lcomb)
>> }
>> diffpos <- which(diffs>1)
>> return(diffpos[length(diffpos)])
>>
>> }
>>
>> On Mar 3, 2010, at 7:35 PM, Herm Walsh wrote:
>>
>>> Thanks David for the thoughts. The challenge I have with this approach is
>>> that the criteria I have is defined by a series of tests--which I do not
>>> think I could substitute in in place of the logical indexing.
>>>
>>> In the combinations code I was hoping there is a step where, each new
>>> combination is added to the current list of combinations. If this were the
>>> case, I could put my series of tests in the code right there and then store
>>> the combination if appropriate.
>>>
>>> However, evalutating the code--which uses recursion--I am not sure if this
>>> approach will work. The combinations code is listed below. Is there a
>>> simple place(s) where I could insert my tests, operating on the current
>>> combination?
>>>
>>> function (n, r, v = 1:n, set = TRUE, repeats.allowed = FALSE)
>>> {
>>> if (mode(n) != "numeric" || length(n) != 1 || n < 1 || (n%%1) !=
>>> 0)
>>> stop("bad value of n")
>>> if (mode(r) != "numeric" || length(r) != 1 || r < 1 || (r%%1) !=
>>> 0)
>>> stop("bad value of r")
>>> if (!is.atomic(v) || length(v) < n)
>>> stop("v is either non-atomic or too short")
>>> if ((r > n) & repeats.allowed == FALSE)
>>> stop("r > n and repeats.allowed=FALSE")
>>> if (set) {
>>> v <- unique(sort(v))
>>> if (length(v) < n)
>>> stop("too few different elements")
>>> }
>>> v0 <- vector(mode(v), 0)
>>> if (repeats.allowed)
>>> sub <- function(n, r, v) {
>>> if (r == 0)
>>> v0
>>> else if (r == 1)
>>> matrix(v, n, 1)
>>> else if (n == 1)
>>> matrix(v, 1, r)
>>> else rbind(cbind(v[1], Recall(n, r - 1, v)), Recall(n -
>>> 1, r, v[-1]))
>>> }
>>> else sub <- function(n, r, v) {
>>> if (r == 0)
>>> v0
>>> else if (r == 1)
>>> matrix(v, n, 1)
>>> else if (r == n)
>>> matrix(v, 1, n)
>>> else rbind(cbind(v[1], Recall(n - 1, r - 1, v[-1])),
>>> Recall(n - 1, r, v[-1]))
>>> }
>>> sub(n, r, v[1:n])
>>> }
>>> <environment: namespace:gtools>
>>>
>>>
>>>
>>> **************************************************************************************************************************************************************
>>>
>>>> I am working with the combinations function (available in the gtools
>>>> package). However, rather than store all of the possible combinations I
>>>> would like to check each combination to see if it meets a certain
>>>> criteria. If it does, I will then store it.
>>>>
>>>> I have looked at the combinations code but am unsure where in the
>>>> algorithm I would be able to operate on each combination.
>>>
>>> Logical indexing:
>>>
>>>> combinations(3,2,letters[1:3])[,1]=="a"
>>> [1] TRUE TRUE FALSE
>>>
>>>> combinations(3,2,letters[1:3])[ combinations(3,2,letters[1:3])[,1]=="a", ]
>>> [,1] [,2]
>>> [1,] "a" "b"
>>> [2,] "a" "c"
>>>
>>> --David
>>>
>>>
>>>
>>>
>>> [[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.
>>
>> --
>> Erich Neuwirth
>> Didactic Center for Computer Science and Institute for Scientific Computing
>> University of Vienna
>>
>>
>>
>>
>>
>>
>
> --
> Erich Neuwirth
> Didactic Center for Computer Science and Institute for Scientific Computing
> University of Vienna
>
>
>
>
>
>
--
Erich Neuwirth
Didactic Center for Computer Science and Institute for Scientific Computing
University of Vienna
[[alternative HTML version deleted]]
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