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Re: how to calculate an index and vice versa

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I'm not entirely sure what it is you're asking, but if I interpret your question correctly
then one way to do this is to create a list with all the subsets of length 4 of {1,2,...,90}
and use Position[] to find the index, e.g.

n = 90; k = 4;
lst = Subsets[Range[n], {k}];

index[s_] := Position[lst, s, 1, 1][[1, 1]]

Then index[{a,b,c,d}] will give you the index of subset {a,b,c,d}. Note that in this example I
start counting from 1, so index[{1,2,3,4}]==1 (and index[{87,88,89,90}]==2555190 which is
Binomial[90,4] as it should be).

lst[[k]] will give you the subset with index k.

Heike.

On 28 May 2011, at 12:20, ferradini wrote:

> given n numbers (eg. 1 to 90)
> I can build the following list of numbers k (in this case 4)
>
> 1, 2, 3, 4 associated index -> 0
> 1, 2, 3, 5 associated index -> 1
> 1, 2, 3, 6 associated index -> 2
> 1, 2, 3, 7 associated index -> 3
> 1, 2, 3, 8 ................... ....
> 1, 2, 3, 9 .......................
> .......................
> .......................
> 10, 41, 60, 61 associated index -> 954722
> .......................
> .......................
> 87, 88, 89, 90 associated index -> 2555190
> I need to build an algorithm to assign eg 4 elemnti can obtain its index
> in the list
> (1, 2, 3, 7 return 3)
>
> The second algorithm is 'inverse: given a number such as 954722 I get
> the four elements 87; 88, 89, 90.
> The elemnti in the list are always sorted values.
>
> I thank those who will send me information on the subject to follow
> Thank ferradini
>

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Re: how to calculate an index and vice versa Heike Gramberg <heike.gramberg@gmail.com> - 2011-05-30 10:33 +0000

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