| writing recursive functions [message #15645] |
Thu, 03 June 1999 00:00 |
Martin Downing
Messages: 136
Registered: September 1998
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Senior Member |
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Hi all,
My aim is to write a function that (recursively) call itself, that can be
compiled on the fly as with standard code.
So - whats the problem, well if you _could_ write a function for factorials
using recursion as:
------------------------------------------------------------
; fact.pro : take 1
; example of recursion (I know there's a factorial function!!)
function fact , n=n
res = float(n)
if res gt 1 then $
res = res * fact(n=res-1)
return, res
end
; (I used a keyword as otherwise it compiles thinking fact() is a variable)
------------------------------------------------------------
...This will not compile
It appears that the compiler needs something equivalent to a declaration
statement (as in C code), so I tried to just write a dummy empty function
above the real code.
e.g.:
------------------------------------------------------------
;fact.pro : take2
; dummy definition I.e. declaration
function fact , n
print, "this should not be called!"
return, n
end
; the real code
; example of recursion (I know there's a factorial function!!)
function fact , n
res = float(n)
if res gt 1 then $
res = res * fact(res-1)
return, res
end
----------------------------------------------------------
However, unless this is forcibly compiled (.compile fact.pro), the
auto-compile will stop after the initial definition.
This leaves two options - permanently compiling the code or calling by
another routine:
------------------------------------------------------------
; call_fact.pro
@fact.pro
function call_fact, n
return, fact(n)
end
------------------------------------------------------------
Any suggestions welcome
Martin,
Aberdeen, Scotland
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| Re: writing recursive functions [message #15646 is a reply to message #15645] |
Wed, 02 June 1999 00:00  |
William Daffer
Messages: 34
Registered: February 1999
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Member |
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"Martin Downing" <m.downing@abdn.ac.uk> writes:
Are you using PVWave? fact.pro (take 1) worked fine in IDL 5.1.1.
William
> Hi all,
>
> My aim is to write a function that (recursively) call itself, that can be
> compiled on the fly as with standard code.
> So - whats the problem, well if you _could_ write a function for factorials
> using recursion as:
>
> ------------------------------------------------------------
> ; fact.pro : take 1
> ; example of recursion (I know there's a factorial function!!)
> function fact , n=n
> res = float(n)
> if res gt 1 then $
> res = res * fact(n=res-1)
> return, res
> end
> ; (I used a keyword as otherwise it compiles thinking fact() is a variable)
> ------------------------------------------------------------
>
> ....This will not compile
> It appears that the compiler needs something equivalent to a declaration
> statement (as in C code), so I tried to just write a dummy empty function
> above the real code.
>
> e.g.:
>
> ------------------------------------------------------------
> ;fact.pro : take2
> ; dummy definition I.e. declaration
> function fact , n
> print, "this should not be called!"
> return, n
> end
>
> ; the real code
> ; example of recursion (I know there's a factorial function!!)
> function fact , n
> res = float(n)
> if res gt 1 then $
> res = res * fact(res-1)
> return, res
> end
> ----------------------------------------------------------
> However, unless this is forcibly compiled (.compile fact.pro), the
> auto-compile will stop after the initial definition.
>
> This leaves two options - permanently compiling the code or calling by
> another routine:
>
> ------------------------------------------------------------
> ; call_fact.pro
> @fact.pro
> function call_fact, n
> return, fact(n)
> end
> ------------------------------------------------------------
>
>
> Any suggestions welcome
>
> Martin,
> Aberdeen, Scotland
>
>
>
>
>
>
>
--
Outside of a dog, a book is man's best friend
Inside of a dog, it's too dark to read.
Groucho Marx.
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| Re: writing recursive functions [message #15649 is a reply to message #15645] |
Wed, 02 June 1999 00:00 |
davidf
Messages: 2866
Registered: September 1996
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Senior Member |
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Martin Downing (m.downing@abdn.ac.uk) writes:
> My aim is to write a function that (recursively) call itself, that can be
> compiled on the fly as with standard code.
> So - whats the problem, well if you _could_ write a function for factorials
> using recursion as:
>
> ------------------------------------------------------------
> ; fact.pro : take 1
> ; example of recursion (I know there's a factorial function!!)
> function fact , n=n
> res = float(n)
> if res gt 1 then $
> res = res * fact(n=res-1)
> return, res
> end
> ; (I used a keyword as otherwise it compiles thinking fact() is a variable)
> ------------------------------------------------------------
>
> ...This will not compile
How about this, using Forward_Function to indicate
that FACT is a function and not a variable. You could
go back to using n as a positional parameter. :-)
function fact , n=n
forward_function fact
res = float(n)
if res gt 1 then $
res = res * fact(n=res-1)
return, res
end
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting
Phone: 970-221-0438 E-Mail: davidf@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155
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