| Re: Recursive Function Program in IDL [message #46701] |
Wed, 14 December 2005 09:50 |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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R.G. Stockwell writes:
> 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).
Gosh, maybe I'll make a collection of these and post them.
It will probably keep other people from crossing their eyes
and sticking their tongues out the corner of their mouths when
they try to write a recursive function. Thanks!
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
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| 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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| Re: Recursive Function Program in IDL [message #46709 is a reply to message #46708] |
Wed, 14 December 2005 08:36 |
Paolo Grigis
Messages: 171
Registered: December 2003
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Senior Member |
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this computes the binomial coefficients recursively
(the numbers showing up in Pascal's triangle)
FUNCTION binomial,n,j
IF n LT j THEN BEGIN
print,'INVALID INPUT IN BINOMIAL'
RETURN,-1
ENDIF
IF j LE 0 THEN RETURN,1
IF j EQ n THEN $
RETURN,1 ELSE $
RETURN,binomial(n-1,j)+binomial(n-1,j-1)
END
Ciao,
Paolo
David Fanning wrote:
> 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
>
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