R help - Aug 2009 - Help on a combinatorial task (lists?)

Hello!
I have the following combinatorial problem.
Consider the cumulative sums of all permutations of a given weight vector 
'w'. I need to know how often weight in a certain position brings the 
cumulative sums equal or above the given threshold 'q'. In other words, 
how often each weight is decisive in raising the cumulative sum above
'q'?

Here is what I do:

w <- c(3,2,1)  # vector of weights
q <- 4  # theshold

# computes which coordinate of w is decisive in each permutation
res <- sapply( permn(w), function(x) which(w == x[min(which(cumsum(x) >= 
q))]) )

# complies the frequencies
prop.table( tabulate( res ))


The problem I have is that when the weights are not unique, the which() 
function returns a list as opposed to a vector. I don?t know how to 
proceed when this happens, as tabulate does not work on lists.

The answer, of course, should be that equal weights are ?decisive? equally 
often.


Can you help?
Thanks a lot!

Serguei Kaniovski

Does 'unlist' do it for you:
> w <- c(3,3,2,1)  # vector of weights
> q <- 4  # theshold
>
> # computes which coordinate of w is decisive in each permutation
> res <- unlist(sapply( permn(w), function(x) which(w ==
x[min(which(cumsum(x) >=q))]) ))
>
> # complies the frequencies
> prop.table( tabulate( res ))
[1] 0.4 0.4 0.1 0.1
>

On Tue, Aug 11, 2009 at 7:03 AM, Serguei
Kaniovski<Serguei.Kaniovski at wifo.ac.at> wrote:
> Hello!
> I have the following combinatorial problem.
> Consider the cumulative sums of all permutations of a given weight vector
> 'w'. I need to know how often weight in a certain position brings
the
> cumulative sums equal or above the given threshold 'q'. In other
words,
> how often each weight is decisive in raising the cumulative sum above
'q'?
>
> Here is what I do:
>
> w <- c(3,2,1) ?# vector of weights
> q <- 4 ?# theshold
>
> # computes which coordinate of w is decisive in each permutation
> res <- sapply( permn(w), function(x) which(w == x[min(which(cumsum(x)
>> q))]) )
>
> # complies the frequencies
> prop.table( tabulate( res ))
>
>
> The problem I have is that when the weights are not unique, the which()
> function returns a list as opposed to a vector. I don?t know how to
> proceed when this happens, as tabulate does not work on lists.
>
> The answer, of course, should be that equal weights are ?decisive? equally
> often.
>
>
> Can you help?
> Thanks a lot!
>
> Serguei Kaniovski
>
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>


-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem that you are trying to solve?