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Re: Limitations and workarounds to expressions defining size of an array

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On 09/05/2012 03:56 AM, Francois Grieu wrote:
...
> I was hoping for
> // select temporary type
> #include <limits.h>
> #ifdef ULLONG_MAX
> typedef unsigned long long uw;
> #else
> typedef unsigned long      uw;
> #endif
> #define UTMP static const uw
> 
> #define EXPECTED 15000
> UTMP kExpected = EXPECTED;
> UTMP kRoot2 = 12*12*kExpected;
> // we want ceil(sqrt(12*12*kExpected)), use Newton/Rapson
> UTMP kExTmp0 = (uw)3<<(kRoot2>>30 > 0 ? 15 :
>                        kRoot2>>24 > 0 ? 12 :
>                        kRoot2>>18 > 0 ?  9 :
>                        kRoot2>>12 > 0 ?  6 : 3);
> UTMP kExTmp1 = (kExTmp0+kRoot2/kExTmp0)/2;
> UTMP kExTmp2 = (kExTmp1+kRoot2/kExTmp1)/2;
> UTMP kExTmp3 = (kExTmp2+kRoot2/kExTmp2)/2;
> UTMP kExTmp4 = (kExTmp3+kRoot2/kExTmp3)/2;
> UTMP kExTmp5 = (kExTmp4+kRoot2/kExTmp4)/2;
> UTMP kExTmp6 = (kExTmp5+kRoot2/kExTmp5)/2;
> UTMP kExTmp7 = (kExTmp6+kRoot2/kExTmp6)/2;
> UTMP kExTmp8 = (kExTmp7+kRoot2/kExTmp7)/2;
> UTMP kExTmp9 = (kExTmp8+kRoot2/kExTmp8)/2;
> UTMP kExTmpA = (kExTmp9+kRoot2/kExTmp9)/2;
> UTMP kExTmpB = (kExTmpA+kRoot2/kExTmpA)/2;
> UTMP kExTmpC = (kExTmpB+kRoot2/kExTmpB)/2;
> UTMP kRoot = kExTmpC+(kExTmpB>kExTmpC?1:0);
> 
> struct foo * bar[kExpected+kRoot];
> 
> 
> This is not valid C99 (the definition of kRoot2 is invalid);
> is that valid in C11? <ot>Note: it seem valid in C++</ot>.

Ah - I see the point you're raising now. Sorry for missing it in your
previous message - I didn't notice your use of 'static' in the
#definition of UTMP. Initializers for an object of static storage
duration must be constant expressions. In C++, a const variable
initialized with a constant expression can itself be used as a constant
expression, but in C it is not allowed. You'd have to re-write this as a
single constant expression, and you'll have to be careful to avoid
running into line length limitations if you do so. I haven't checked to
be sure, but you might have settle for fewer iterations.

If you remove the 'static' and define these inside a function, there
should be no problem except with the definition of bar itself, which
requires VLA support. Of course, if you can use a VLA, you wouldn't need
to use such a round-about method to do this, you could simply calculate
the sqrt() directly.
-- 
James Kuyper

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Limitations and workarounds to expressions defining size of an array Francois Grieu <fgrieu@gmail.com> - 2012-09-04 19:14 +0200
  Re: Limitations and workarounds to expressions defining size of an array James Kuyper <jameskuyper@verizon.net> - 2012-09-04 14:08 -0400
    Re: Limitations and workarounds to expressions defining size of an array Francois Grieu <fgrieu@gmail.com> - 2012-09-05 09:56 +0200
      Re: Limitations and workarounds to expressions defining size of an array James Kuyper <jameskuyper@verizon.net> - 2012-09-05 07:33 -0400
        Re: Limitations and workarounds to expressions defining size of an array Francois Grieu <fgrieu@gmail.com> - 2012-09-05 23:39 +0200
          Re: Limitations and workarounds to expressions defining size of an array Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-09-05 18:11 -0400
    Re: Limitations and workarounds to expressions defining size of an array James Kuyper <jameskuyper@verizon.net> - 2012-09-05 07:17 -0400
  Re: Limitations and workarounds to expressions defining size of an array Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-09-04 19:18 +0100
  Re: Limitations and workarounds to expressions defining size of an array Kaz Kylheku <kaz@kylheku.com> - 2012-09-04 18:41 +0000
    Re: Limitations and workarounds to expressions defining size of an array "BartC" <bc@freeuk.com> - 2012-09-04 20:22 +0100
      Re: Limitations and workarounds to expressions defining size of an array Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-09-04 17:07 -0400
        Re: Limitations and workarounds to expressions defining size of an array James Kuyper <jameskuyper@verizon.net> - 2012-09-04 17:25 -0400
          Re: Limitations and workarounds to expressions defining size of an array Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-09-04 18:24 -0400
            Re: Limitations and workarounds to expressions defining size of an array Keith Thompson <kst-u@mib.org> - 2012-09-04 17:29 -0700
        Re: Limitations and workarounds to expressions defining size of an array army1987 <army1987@ask-for-it.invalid> - 2012-09-04 21:26 +0000
          Re: Limitations and workarounds to expressions defining size of an array army1987 <army1987@ask-for-it.invalid> - 2012-09-04 21:31 +0000
            Re: Limitations and workarounds to expressions defining size of an array James Kuyper <jameskuyper@verizon.net> - 2012-09-04 17:41 -0400
              Re: Limitations and workarounds to expressions defining size of an array army1987 <army1987@ask-for-it.invalid> - 2012-09-05 20:45 +0000
                Re: Limitations and workarounds to expressions defining size of an array James Kuyper <jameskuyper@verizon.net> - 2012-09-05 17:23 -0400
                Re: Limitations and workarounds to expressions defining size of an array army1987 <army1987@ask-for-it.invalid> - 2012-09-05 21:30 +0000
                Re: Limitations and workarounds to expressions defining size of an array Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-09-05 23:04 +0100
                Re: Limitations and workarounds to expressions defining size of an array Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-09-05 18:17 -0400
                Re: Limitations and workarounds to expressions defining size of an array James Kuyper <jameskuyper@verizon.net> - 2012-09-05 18:29 -0400
            Re: Limitations and workarounds to expressions defining size of an array Kaz Kylheku <kaz@kylheku.com> - 2012-09-04 21:44 +0000
      Re: Limitations and workarounds to expressions defining size of an array Kaz Kylheku <kaz@kylheku.com> - 2012-09-04 21:39 +0000
        Re: Limitations and workarounds to expressions defining size of an array "BartC" <bc@freeuk.com> - 2012-09-04 22:58 +0100
          Re: Limitations and workarounds to expressions defining size of an array Kaz Kylheku <kaz@kylheku.com> - 2012-09-04 22:12 +0000
          Re: Limitations and workarounds to expressions defining size of an array Lowell Gilbert <lgusenet@be-well.ilk.org> - 2012-09-04 18:55 -0400
      Re: Limitations and workarounds to expressions defining size of an array Keith Thompson <kst-u@mib.org> - 2012-09-04 16:51 -0700
        Re: Limitations and workarounds to expressions defining size of an array Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-09-04 20:58 -0400
          Re: Limitations and workarounds to expressions defining size of an array Francois Grieu <fgrieu@gmail.com> - 2012-09-05 10:04 +0200
          Re: Limitations and workarounds to expressions defining size of an array Tim Rentsch <txr@alumni.caltech.edu> - 2012-09-07 04:47 -0700
            Re: Limitations and workarounds to expressions defining size of an array Keith Thompson <kst-u@mib.org> - 2012-09-07 14:04 -0700
              Re: Limitations and workarounds to expressions defining size of an array Casey Carter <casey_at_carter_dot_net@nospam.invalid> - 2012-09-07 16:40 -0500
              Re: Limitations and workarounds to expressions defining size of an array Tim Rentsch <txr@alumni.caltech.edu> - 2012-09-07 19:54 -0700
                Re: Limitations and workarounds to expressions defining size of an array Keith Thompson <kst-u@mib.org> - 2012-09-08 14:15 -0700
                Re: Limitations and workarounds to expressions defining size of an array lawrence.jones@siemens.com - 2012-09-17 12:09 -0400
        Re: Limitations and workarounds to expressions defining size of an array Tim Rentsch <txr@alumni.caltech.edu> - 2012-09-07 04:53 -0700
          Re: Limitations and workarounds to expressions defining size of an array "BartC" <bc@freeuk.com> - 2012-09-07 13:07 +0100
            Re: Limitations and workarounds to expressions defining size of an array Keith Thompson <kst-u@mib.org> - 2012-09-07 14:08 -0700
            Re: Limitations and workarounds to expressions defining size of an array Tim Rentsch <txr@alumni.caltech.edu> - 2012-09-07 19:41 -0700
  Re: Limitations and workarounds to expressions defining size of an array "BartC" <bc@freeuk.com> - 2012-09-04 20:26 +0100
  Re: Limitations and workarounds to expressions defining size of an array Barry Schwarz <schwarzb@dqel.com> - 2012-09-04 14:14 -0700
  Re: Limitations and workarounds to expressions defining size of an array Tim Rentsch <txr@alumni.caltech.edu> - 2012-09-07 04:41 -0700
  Re: Limitations and workarounds to expressions defining size of an array army1987 <army1987@ask-for-it.invalid> - 2012-09-07 13:32 +0000

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