calling a recursive function in fortran 77 ?
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From: ajay on 26 Mar 2010 08:24 On Mar 25, 11:59 pm, Aris Tofanis <aris.tofa...(a)6zbp6h.invalid> wrote: > m_b_metcalf <michaelmetc...(a)compuserve.com> wrote: > > On Mar 25, 6:55=A0am, ajay <ajay.rawa...(a)gmail.com> wrote: > >> how can we implement this in fortran 77. > >> i know there is intrinsic recursive function in fortran 90/95. > > >> function func(x,y) > >> ... > >> ... > >> if ( y.lt.1.0d0 ) then > > >> func = ........ > > >> elseif ( y.gt.1.0d0 ) then > > >> func = something/func(x,-y) > > >> end if > > >> end function > > >> waiting for the reply..... > >> thanking you..... > > > Basically, you can't. But if you just add the recursive keyword and > > use a modern compiler, it will work. > > I have never seen problems with g77 when creating a wrapper routine, > which only calls the routine you want to call recursively: > > function wrapfunc(x,y) > wrapfunc = func(x,y) > end > > and call wrapfunc inside func. But I also never felt quite comfortable > with this solution. I came across very intersting example from : http://www.esm.psu.edu/~ajm138/fortranexamples.html PROGRAM MAIN INTEGER N, X EXTERNAL SUB1 COMMON /GLOBALS/ N X = 0 PRINT *, 'Enter number of repeats' READ (*,*) N CALL SUB1(X,SUB1) END SUBROUTINE SUB1(X,DUMSUB) INTEGER N, X EXTERNAL DUMSUB COMMON /GLOBALS/ N IF(X .LT. N)THEN X = X + 1 PRINT *, 'x = ', X CALL DUMSUB(X,DUMSUB) END IF END
From: Colin Watters on 26 Mar 2010 15:20 "ajay" <ajay.rawat83(a)gmail.com> wrote in message news:5225f53a-d8a2-487a-8426-1f5b4c0835e2(a)l24g2000prh.googlegroups.com... On Mar 26, 9:34 am, glen herrmannsfeldt <g...(a)ugcs.caltech.edu> wrote: > Phred Phungus <Ph...(a)example.invalid> wrote: > > Why can't I emulate a pdp on ubuntu, other than not having > > any idea how to do so? > > You can do that, but you can also get free accounts > on PDPplanet. > > -- glen I came across very intersting example from : http://www.esm.psu.edu/~ajm138/fortranexamples.html PROGRAM MAIN INTEGER N, X EXTERNAL SUB1 COMMON /GLOBALS/ N X = 0 PRINT *, 'Enter number of repeats' READ (*,*) N CALL SUB1(X,SUB1) END SUBROUTINE SUB1(X,DUMSUB) INTEGER N, X EXTERNAL DUMSUB COMMON /GLOBALS/ N IF(X .LT. N)THEN X = X + 1 PRINT *, 'x = ', X CALL DUMSUB(X,DUMSUB) END IF END ....From the text accompanying this example: 'I've read in some texts that indirect reference is not allowed, but ask yourself "How can SUB1 possibly know that it's actually calling itself when it calls the dummy subroutine?" It can't, and thus this must work. In fact, on every system I've tried so far, this routine works perfectly.' ....Clearly, the author never tried it on a CDC mainframe. The opsys and Fortran (FORTRAN, in fact) compiler on these beasts was notable for not using a stack. Recursion on such machines works fine up until the point where you are finished calling, and start returning. -- Qolin Email: my qname at domain dot com Domain: qomputing
From: Richard Maine on 26 Mar 2010 15:45 Colin Watters <boss(a)qomputing.com> wrote: [cute example of recursion elided] > ...From the text accompanying this example: > > 'I've read in some texts that indirect reference is not allowed, but ask > yourself "How can SUB1 possibly know that it's actually calling itself when > it calls the dummy subroutine?" It can't, and thus this must work. In fact, > on every system I've tried so far, this routine works perfectly.' > > ...Clearly, the author never tried it on a CDC mainframe. The opsys and > Fortran (FORTRAN, in fact) compiler on these beasts was notable for not > using a stack. Recursion on such machines works fine up until the point > where you are finished calling, and start returning. The author also clearly completely misses the point of the restriction against recursion. That restriction isn't just in order to intentionally make life difficult. It is there because recursion might not work correctly without a lot of special effort on some systems. To argue that something must work just because the compiler can't diagnose it is simply nonsense. And, as I mentioned earlier in the thread, testing could quite plausibly fail to find the problems. -- Richard Maine | Good judgment comes from experience; email: last name at domain . net | experience comes from bad judgment. domain: summertriangle | -- Mark Twain
From: mecej4 on 26 Mar 2010 17:02 On 3/26/2010 2:45 PM, Richard Maine wrote: > Colin Watters<boss(a)qomputing.com> wrote: > [cute example of recursion elided] > >> ...From the text accompanying this example: >> >> 'I've read in some texts that indirect reference is not allowed, but ask >> yourself "How can SUB1 possibly know that it's actually calling itself when >> it calls the dummy subroutine?" It can't, and thus this must work. In fact, >> on every system I've tried so far, this routine works perfectly.' >> >> ...Clearly, the author never tried it on a CDC mainframe. The opsys and >> Fortran (FORTRAN, in fact) compiler on these beasts was notable for not >> using a stack. Recursion on such machines works fine up until the point >> where you are finished calling, and start returning. > > The author also clearly completely misses the point of the restriction > against recursion. That restriction isn't just in order to intentionally > make life difficult. It is there because recursion might not work > correctly without a lot of special effort on some systems. > > To argue that something must work just because the compiler can't > diagnose it is simply nonsense. Or, putting that in slightly different words, a belief that if one can hoodwink the compiler the resulting code will work correctly if the compiler is "good". > And, as I mentioned earlier in the > thread, testing could quite plausibly fail to find the problems. > -- mecej4
From: Tobias Burnus on 27 Mar 2010 05:53
Am 25.03.2010 19:59, schrieb Aris Tofanis: > I have never seen problems with g77 when creating a wrapper routine, > which only calls the routine you want to call recursively: > > function wrapfunc(x,y) > wrapfunc = func(x,y) > end > > and call wrapfunc inside func. But I also never felt quite comfortable > with this solution. At least with gfortran but also with some other compilers it might fail sometimes; gfortran puts large local arrays into static memory; but if you use recursion (or call a procedure simultaneously vis OpenMP), all the local variables of a routine need to be on the stack otherwise they might override another. Thus, using such a work around might lead to wrong results; and the bug might be difficult to find (e.g. it only appears after one had increased a local array by a small amount). See "-frecursive", "-fmax-stack-var-size=" and "-fno-automatic" in the gfortran manual. Tobias |