strange arithmetic

strange arithmetic

[Louis-David Mitterrand]

(using 3.0.5)

% i=0;while [[ $[++i] < 900 ]];do;echo $i;done
% 1
% 2
[...]
% 90

90 ?? I asked for 900 and it stops at 90. On the other hand the
following:

% i=0;while ! [[ $[++i] = 900 ]];do;echo $i;done
% 1
% 2
[...]
% 899

Is this normal?

-- 

     Louis-David Mitterrand
     http://www.aparima.com
     mito@aparima.com

Re: strange arithmetic

[Andrew Main]

Louis-David Mitterrand wrote:
>% i=0;while [[ $[++i] < 900 ]];do;echo $i;done

"<" does a string comparison.  For a numeric comparison, you want "-lt".

>% i=0;while ! [[ $[++i] = 900 ]];do;echo $i;done

"=" (or the preferred "==") also does string comparison.  You should
really use "-eq" here.

I've just noticed that this is exactly the opposite convention from Perl.
I can even see why the difference was historically inevitable.

-zefram

Re: strange arithmetic

[Alain Caron]

Louis-David Mitterrand wrote:

> (using 3.0.5)
>
> % i=0;while [[ $[++i] < 900 ]];do;echo $i;done
> % 1
> % 2
> [...]
> % 90
>
> 90 ?? I asked for 900 and it stops at 90. On the other hand the
> following:
>
> % i=0;while ! [[ $[++i] = 900 ]];do;echo $i;done
> % 1
> % 2
> [...]
> % 899
>
> Is this normal?
>
> --
>
>      Louis-David Mitterrand
>      http://www.aparima.com
>      mito@aparima.com

  This is, I presume, because ">" and "=" are doing a lexicographic
comparison.  For a numeric
comparison, use "-gt" and "-eq" operators.

Alain Caron
alainc@sangacorp.com

Re: strange arithmetic

[Louis-David Mitterrand]

On Fri, Nov 28, 1997 at 03:16:37PM -0500, Alain Caron wrote:
> > % 90
> >
> > 90 ?? I asked for 900 and it stops at 90. On the other hand the
> > following:
> >
> > % i=0;while ! [[ $[++i] = 900 ]];do;echo $i;done
> > % 1
> > % 2
> > [...]
> > % 899
> >
> > Is this normal?
> 
>   This is, I presume, because ">" and "=" are doing a lexicographic
> comparison.  For a numeric
> comparison, use "-gt" and "-eq" operators.

Thanks to you and zefram, I guess I've been exposed to Perl for a little
too long (the time I could put off learning shell scripting ;-).

And to think I thought I had discovered a bug in mighty zsh ..

-- 

     Louis-David Mitterrand
     http://www.aparima.com
     mito@aparima.com

Re: strange arithmetic

[Dan Nelson]

In the last episode (Nov 28), Louis-David Mitterrand said:
> > > % i=0;while ! [[ $[++i] < 900 ]];do;echo $i;done
> > > % 1
> > > % 2
> > > [...]
> > > % 90
> > >
> > > Is this normal?
> 
> Thanks to you and zefram, I guess I've been exposed to Perl for a little
> too long (the time I could put off learning shell scripting ;-).
> 
> And to think I thought I had discovered a bug in mighty zsh ..

I'm not sure what Perl has to do with your confusion, unless Perl uses
[[ .. ]] to signify arithmetic evaluation?  I'd check the Perl man
pages, but as they are split into 35 (!!!!!) sections, I won't.  Thank
goodness for "man zshall".

I think [[ ]] is a builtin version of the test command, with more
testing capabilities.

You probably wanted to use the (( )) construct in zsh, to do arithmetic
evaluation of your test.

	-Dan Nelson
	dnelson@emsphone.com

Re: strange arithmetic

[Christopher Croughton]

Louis-David Mitterrand wrote:
> 
> (using 3.0.5)
> 
> % i=0;while [[ $[++i] < 900 ]];do;echo $i;done
> % 1
> % 2
> [...]
> % 90
> 
> 90 ?? I asked for 900 and it stops at 90. On the other hand the
> following:

It happens here with 3.1.2 as well.  I can't see any logical reason...

Chris C

Re: strange arithmetic

[Louis-David Mitterrand]

On Fri, Nov 28, 1997 at 08:04:44PM -0600, Dan Nelson wrote:
> In the last episode (Nov 28), Louis-David Mitterrand said:
> > > > % i=0;while ! [[ $[++i] < 900 ]];do;echo $i;done
> > > > % 1
> > > > % 2
> > > > [...]
> > > > % 90
> > > >
> > > > Is this normal?
> > 
> > Thanks to you and zefram, I guess I've been exposed to Perl for a little
> > too long (the time I could put off learning shell scripting ;-).
> > 
> > And to think I thought I had discovered a bug in mighty zsh ..
> 
> I'm not sure what Perl has to do with your confusion, unless Perl uses
> [[ .. ]] to signify arithmetic evaluation?  I'd check the Perl man
> pages, but as they are split into 35 (!!!!!) sections, I won't.  Thank
> goodness for "man zshall".
> 
> I think [[ ]] is a builtin version of the test command, with more
> testing capabilities.
> 
> You probably wanted to use the (( )) construct in zsh, to do arithmetic
> evaluation of your test.

Aha! Right. I hadn't ventured deep engough in zsh's man pages. Sorry for
taking bandwidth with "should have RTFM'd" questions.

Right now I'm trying to understand bitwise manipulations and every day
that passes I thanks zsh for being such a wonderful prototyping tool. No
need to compile stuff, just type away and here comes the result... The
$[2#10010101] stuff comes real handy there to understand binary format.

Again, congratulations to zsh's developers. It's an awesome concentrate
of intelligence sitting behind that modest '%' prompt.

-- 

     Louis-David Mitterrand
     http://www.aparima.com
     mito@aparima.com