| Re: Recursive Function Program in IDL [message #46702 is a reply to message #46701] |
Wed, 14 December 2005 09:46   |
news.qwest.net
Messages: 137
Registered: September 2005
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Senior Member |
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"David Fanning" <davidf@dfanning.com> wrote in message
news:MPG.1e09eff8986d3ab0989ae7@news.frii.com...
> Folks,
>
> Does anyone have a handy recursive function that does something neat?
> Someone is asking, and I don't have time to work on this. He
> (apparently) can't get to the newsgroup.
>
> Thanks,
>
> David
>
> --
> David Fanning, Ph.D.
> Fanning Software Consulting, Inc.
> Coyote's Guide to IDL Programming: http://www.dfanning.com/
An old program in my hackware style of programming.
Honest, I program much better nowadays!!
This finds the prime factors of a number (i.e. for 9, it returns 3, 3).
Very useful for making sure you pass a factorable length time series to
an fft routine ( if time is important to you).
Cheers,
bob
; find the factors of a number
function factors, n,prevfactors=prevfactors
maxfactor = fix(sqrt(n))
if maxfactor le 1 then begin
if keyword_set(prevfactors) then begin
prevfactors = [prevfactors,n]
return,n
endif else begin
return,n
endelse
endif
fac = findgen(maxfactor-1)+2 ; 2 -- sqrt(n)
doloop = 1
factorflag = 0
counter = 0
while doloop do begin
if n mod fac(counter) eq 0 then begin
factorflag = 1
newfactor = fac(counter)
if keyword_set(prevfactors) then prevfactors = [prevfactors,newfactor] $
else prevfactors = newfactor
newnumber = n/newfactor
; to iterate is human, to recurse is divine
r = factors(newnumber,prevfactors=prevfactors)
doloop = 0
endif
counter = counter+1
if counter ge maxfactor-1 then doloop = 0
endwhile
if n_elements(prevfactors) eq 0 then prevfactors = n else begin
; only if n is prime do we add it here
if not(factorflag) then prevfactors = [prevfactors,n]
endelse
return,prevfactors
end
;;;;___________ test code here ___________________
n = 5001
r = factors(n)
print
print
print,'Finished calculating factors_______'
print,'Number: ',n
print,'Factors:'
print,r
end
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