Thank you Tamas. I added return a and restructured the code (I think some
formatting was lost when pasting). Based on the examples I tested, it
appears to work. Can you recommend an efficient method of inputing an array
of numbers. For example:
a = [10,12]
p = perm_matrix(a,2)
p = [10 10; 10 12; 12 10; 12 12]
I could probably hack something together that maps elements of a to the
elements of p (0 through n-1), but you might have a more efficient and
elegant solution.
Regarding the second question, the combinations function (I believe this is
what you were referring to) would treat (1,2) and (2,1) as
indistinguishable. For example, the code below yields only three unordered
pairs rather than six ordered pairs
In [9]:
collect(combinations([1;2;3],2))
Out[9]:
3-element Array{Array{Int64,1},1}:
[1,2]
[1,3]
[2,3]
Thanks again
On Friday, July 31, 2015 at 5:35:22 AM UTC-4, Tamas Papp wrote:
>
> On Fri, Jul 31 2015, Christopher Fisher <[email protected] <javascript:>>
>
> wrote:
>
> > I was wondering if there is a function for enumerating all of
> > the permutations of size m from n elements, with repetitions
> > allowed. For example, 3 permutations of [1 0] would be [ 1 1
> > 1;1 1 0;1 0 1;0 1 1;1 0 0; 0 1 0;0 0 1; 0 0 0]. (Analogous to
> >
> http://www.mathworks.com/matlabcentral/fileexchange/11462-npermutek/content/npermutek.m)
>
>
>
> Perhaps something like (not tested extensively, check for bugs)
>
> @doc "All length `n` sequences of `1:v` in a matrix, with
> repetitions."-> function perm_matrix{T}(v::T, n)
> m = v^n a = zeros(T, m, n) for i = 1:m
> k = i-1 j = n while ((k > 0) && (j > 0))
> (d,r) = divrem(k,v) a[i,j] = r k = d j -= 1
> end
> end a
> end perm_matrix(3, 2)
>
> > Along similar lines, I was wondering if there is a function for
> > permutations without repetitions, such as 2 elements from [1 2
> > 3] is [1 2;1 3;2 3;2 1;3 1;3 2]. I see that there is a
> > permutations function but it only enumerates permutations of
> > the same size as the original set.
>
> Did you have a look at the function Base.combinations? Unless I
> misunderstand your question, it does what you are asking for.
>
> Best,
>
> Tamas
>
