Re: Modular type. What is it and why?

["Mike Silva" <mjsilva@jps.net>]

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