on goto
in [General Programming]
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From: Sjouke Burry on 29 May 2010 16:47 blmblm(a)myrealbox.com wrote: > In article <4be62644$0$14116$703f8584(a)textnews.kpn.nl>, > Sjouke Burry <burrynulnulfour(a)ppllaanneett.nnll> wrote: >> Nick Keighley wrote: >>> On 8 May, 17:46, Tim Streater <timstrea...(a)waitrose.com> wrote: >>>> In article >>>> <5dd9bdc8-4dc4-4537-84a0-885c4c92f...(a)l28g2000yqd.googlegroups.com>, >>>> Nick Keighley <nick_keighley_nos...(a)hotmail.com> wrote: >>>>> On 8 May, 01:41, Lie Ryan <lie.1...(a)gmail.com> wrote: >>> >>>>>>> I've never heard of any programming languages that doesn't support >>>>>>> recursion. >>> [...] >>>>> FORTRAN (in its original form), Coral-66 you had to use a special >>>>> keyword to indicate a function was recursive. Some BASICs probably >>>>> didn't alow recursion. But these all qualify as "ancient" (and maybe >>>>> jocular!) >>>> What FORTRAN are you referring to. I never saw it in any FORTRAN up to >>>> 1978 (the last time I used FORTRAN). >>> lost me. So far as I remember FORTRAN didn't have recursion. Am I >>> wrong? Or are you saying even modern Fortran doesn't have recursion? >> If recursion does not exist, I will have to ditch quite a couple >> of fortran programs. >> There is only 1 hitch, you have to write a 3 line dummy routine, >> to do the calling, so a calls b , b calls a. >> fortran MS 5.1 and later. >> Also Fortran77 for Riscos (arc 320 computer) > > And then there are systems on which this approach would almost > surely fail spectacularly (as you may know -- just sayin'). > In a long-ago job in which one of my responsibilities was finding > and fixing bugs other people's FORTRAN, a particularly nasty > one turned out to be the result of this kind of mutual recursion. > If the system puts the return address for a call to a non-recursive > subroutine in a fixed location, well .... > > I'm curious now about why A-calls-B-calls-A would work but simple > A-calls-A would not. Is it a compiler issue? where the compiler > won't allow the latter, but will allow the former? > a to a gets you an angry compiler. As far as I can remember, the a > b > a advice came from M$. I used recursion for quite some fractal drawing. Those routines themselves were rather small. Because of the excessive use of the stack, you had to tell the linker to use a larger stack. /stack:32768 on the commandline, normally only about ~1024. That stack, together with the "automatic" statement gave reliable recursion.
From: Andy Champ on 30 May 2010 18:03 blmblm(a)myrealbox.com wrote: > > I'm curious now about why A-calls-B-calls-A would work but simple > A-calls-A would not. Is it a compiler issue? where the compiler > won't allow the latter, but will allow the former? > I tried this once upon a time (well back in the last millenium!) and found out that all the variable in the subroutines were what in the C/C++ world are called static - there was only one instance. So when you got to the second instance of A you were using the same local variables as the first instance. The compiler blocked A-calls-A but didn't spot A-calls-B-calls-A - it just let you shoot your own foot. Andy
From: Sjouke Burry on 30 May 2010 19:15 Andy Champ wrote: > blmblm(a)myrealbox.com wrote: >> I'm curious now about why A-calls-B-calls-A would work but simple >> A-calls-A would not. Is it a compiler issue? where the compiler >> won't allow the latter, but will allow the former? >> > > I tried this once upon a time (well back in the last millenium!) and > found out that all the variable in the subroutines were what in the > C/C++ world are called static - there was only one instance. So when > you got to the second instance of A you were using the same local > variables as the first instance. > > The compiler blocked A-calls-A but didn't spot A-calls-B-calls-A - it > just let you shoot your own foot. > > Andy Thats where the "automatic" statement comes in, it makes all variables in the routine temporary, and makes recursion possible.
From: blmblm on 31 May 2010 14:52 In article <CNWdneJy4NoCfZ_RnZ2dnUVZ7o-dnZ2d(a)eclipse.net.uk>, Andy Champ <no.way(a)nospam.invalid> wrote: > blmblm(a)myrealbox.com wrote: > > > > I'm curious now about why A-calls-B-calls-A would work but simple > > A-calls-A would not. Is it a compiler issue? where the compiler > > won't allow the latter, but will allow the former? > > > > I tried this once upon a time (well back in the last millenium!) and > found out that all the variable in the subroutines were what in the > C/C++ world are called static - there was only one instance. So when > you got to the second instance of A you were using the same local > variables as the first instance. > > The compiler blocked A-calls-A but didn't spot A-calls-B-calls-A - it > just let you shoot your own foot. Yup, that's exactly what happened in the story from a previous life that prompted my earlier post. I can't remember now how I (we?) finally tracked *that* bug down, but I seem to remember at some point it may have involved looking at an assembly-language version of the object code produced by the compiler. "Those were the days"? -- B. L. Massingill ObDisclaimer: I don't speak for my employers; they return the favor.
From: Daniel T. on 1 Jun 2010 07:58
James Dow Allen <jdallen2000(a)yahoo.com> wrote: > On Jun 1, 1:31�pm, "g3r...(a)gmail.com" <g3r...(a)gmail.com> wrote: > > > > and the sort of people who hate goto probably design their code > > before they write it > > Count me among the dinosaurs who start writing functions, linearly, > immediately after deciding to. I sometimes even add "goto"'s that I > know probably should be "while ... continue". A minute later, I invest > 5 seconds changing the goto's to continue's, and adding tabs. I'm > typing at good speed and *while typing* it will become clearer whether > I need a "while ... continue" or something slightly more complicated. > (Now watch the pedants try to beat me to a pulp for wasting those 5 > seconds.) No beating from me. All I will say about the above is that I too just start writing, but I don't use 'goto', or 'continue', even in my first draft. My first draft also tends to be SESE. Maybe during the 5 seconds you're talking about, I might change it to have multiple exits, maybe not. Now watch the pendents on the other side beat me to a pulp for "being so dogmatic." :-) |