Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!ukc!dcl-cs!gdt!ccsdhd
From: ccsdhd@gdt.bath.ac.uk (Dennis Davis)
Newsgroups: comp.unix.shell
Subject: Re: SUMMARY Re: sh loop variable and "double indirection"
Message-ID: <1991Feb12.173749.24052@gdt.bath.ac.uk>
Date: 12 Feb 91 17:37:49 GMT
References: <1991Feb5.003613.21081@shibaya.lonestar.org> <5081@acorn.co.uk>
Organization: Bath University Computing Services, Bath, England
Lines: 56

In article <5081@acorn.co.uk> steve@acorn.co.uk (Steve "daffy" Hunt) writes:
>In article <1991Feb5.003613.21081@shibaya.lonestar.org> afc@shibaya.lonestar.org (Augustine Cano) writes:
>>
>>
>>I didn't anticipate the great response I got to my questions.  This
>>newsgroup is an important resource.  Many thanks to the following people,
>>whose summary of responses follow:
>
>... more deleted...
>
>I only just subscribed to this newsgroup, so this is rather late, but
>my favourite solution to the loop index problem is the following shell
>function (just inline it if your shell does not support functions).
>
># usage: range start end [step]
>
>range () {
>	echo "[li p ${3-1} + d si $2!<a] sa ${1}silax" | dc
>}
>
>I don't guarantee that your version of dc will be happy with that, but
>it works for me!  Making it count backwards, etc, is left as an exercise.
>
>							Steve
>
>-- 
>			Steve Hunt	steve@acorn.co.uk

I've always found that languages such as 'dc' are very much
write-only languages.  This is probably a reflection on me rather
than on the language itself.  I'd prefer to write the above more
verbosely using 'bc', as something like:

range ()
{ set `echo $@ | tr '-' '_'`
  echo "if (${3-1} < 0)
        { for (i=$1; i>=$2; i=i+${3-1}) i }
        if (${3-1} > 0)
        { for (i=$1; i<=$2; i=i+${3-1}) i }" | bc
}

The 'set' command is there as a sop to syntactic sugar.  'dc' and
'bc' require that negative numbers are preceeded by a underline
instead of a minus sign.  The 'set' command enables you to write
them more naturally using a minus sign, eg 'range -5 -10 -1'.

The above has (for me) the overwhelming advantages that I can
understand it both now *and* in the future.

... all this is purely academic of course.  No doubt there are far
more elegant solutions using awk, perl etc etc ...
-- 
Dennis Davis            JANET: D.H.Davis@UK.AC.BATH
University of Bath       UUCP: ...!mcsun!ukc!gdr!D.H.Davis
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