| Re: Selecting groups of 5 coords from a set of n (nC5) [message #47834 is a reply to message #47833] |
Fri, 03 March 2006 09:19  |
Paolo Grigis
Messages: 171
Registered: December 2003
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Senior Member |
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Some time ago I needed combinations and I came up with
this function. Example:
IDL> print,pgcomb(5,3)
0 1 2
0 1 3
0 1 4
0 2 3
0 2 4
0 3 4
1 2 3
1 2 4
1 3 4
2 3 4
It might not be the most efficient solution, but at least
I understand the (quite simple) underlying algorithm.
Be careful that no check is made for bad inputs, it's
up to the user to make sure that 1<=j<=n.
Ciao,
Paolo
FUNCTION pgcomb,n,j
;;number of combinations of j elements chosen from n
nelres=long(factorial(n)/(factorial(j)*factorial(n-j)))
res=intarr(j,nelres);array for the result
res[*,0]=indgen(j);initialize first combination
FOR i=1,nelres-1 DO BEGIN;go over all combinations
res[*,i]=res[*,i-1];initialize with previous value
FOR k=1,j DO BEGIN;scan numbers from right to left
IF res[j-k,i] LT n-k THEN BEGIN;check if number can be increased
res[j-k,i]=res[j-k,i-1]+1;do so
;if number has been increased, set all numbers to its right
;as low as possible
IF k GT 1 THEN res[j-k+1:j-1,i]=indgen(k-1)+res[j-k,i]+1
BREAK;we can skip to the next combination
ENDIF
ENDFOR
ENDFOR
RETURN,res
END
Olivia wrote:
> Dear All,
>
> I am trying to write a loop to perform a calculation on all possible
> sets of 5 coordinates from a group of n. The test case I am working on
> has a total number of coordinates of 8, so there will be 8c5=56 unique
> solutions. At the moment, I am thinking of using 5 for loops as an
> extension of a similar problem I worked on choosing 3 points. The 3
> point code ran like this:
>
> ;Select groups of 3 boundary points
> ;for p=1, n, 1 do begin
> ; for k=1, n, 1 do begin
> ; for m=0, n-1, 1 do begin
> ; ellipse_points=[[bx[m], by[m]],$
> ; [bx[m+k], by[m+k]],$
> ; [bx[m+k+p], by[m+k+p]]]
>
> On reflection, I was wondering if there might be a better way of doing
> this. This is probably only a fear as my code is already looking really
> complicated and I am worried about putting rubbish in and getting
> rubbish out whilst being completely unaware. If anyone has any ideas I
> would be really grateful to hear them. Thank you very much,
>
> Olivia
>
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