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From: "Mike Silva" <mjsilva@jps.net>
Subject: Re: Modular type. What is it and why?
Date: 1999/04/06
Message-ID: <7edk6p$1j7$1@its.hooked.net>#1/1
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References: <7ean3c$79m$1@eol.dd.chalmers.se> <7eb2iq$jc3@hobbes.crc.com>
 <7ede2p$pd1$1@its.hooked.net> <370A4B3A.5EDB3B37@pwfl.com>
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Organization: Whole Earth Networks News
Newsgroups: comp.lang.ada
Date: 1999-04-06T00:00:00+00:00
List-Id: <comp.lang.ada>


Marin David Condic wrote in message <370A4B3A.5EDB3B37@pwfl.com>...
>Mike Silva wrote:
>> Now with Ada I'll be able to write (assuming 'index' is the appropriate
>> modular type):
>>
>> case ( key ) is
>>    when ARROW_DOWN =>
>>       index := index + 1;
>>    when ARROW_UP =>
>>       index := index - 1;
>> end case;
>>
>> Now -that's- what I call progress!
>>
>
>My C is pretty rusty - not having done anything serious in it in at
>least 10 years - but wasn't there some kind of type/operation that
>allowed for wraparound semantics?...

Well, if you're going up you can do:

 index = ( index + 1 ) % NUM_ELEMENTS;    /* where % is the MOD operator */

but you can't do:

 index = ( index - 1 ) % NUM_ELEMENTS;    /* won't handle 0 ->
NUM_ELEMENTS - 1 correctly */

As Ole-Hjalmar Kristensen commented, the nice Ada solution doesn't work if
NUM_ELEMENTS varies at runtime (at least, it appears that a modular type
can't be defined dynamically -- correct?), but that's not the usual case I
run across.

Mike