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From: "Theodore E. Dennison" <dennison@escmail.orl.mmc.com>
Subject: Re: Ada 83 Style Query
Date: 1996/05/06
Message-ID: <318E6AA4.4487EB71@escmail.orl.mmc.com>#1/1
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Date: 1996-05-06T00:00:00+00:00
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W. Wesley Groleau (Wes) wrote:
> 
> I'm working on some old code that has a long string of elsif checking
> strings that come in from outside against known commands.
> 
> Seeking opinions on the desirability of using an enumerated type with 'VAL
> and a case statement.  The sore spot is the need for an exception handler
> when the incoming string matches none of the literals in the command list.

This is one of my most common text-processing techniques.

The major drawback to doing this is that the text must be a vaild
Ada identifier. That means no commands called "exit" or "end" or "abort" 
(this comes up more often than you'd think), and nothing case-sensitive
or with spaces in the middle. If you don't have control over the command 
names, you may be in trouble here.

I don't see the exception handler being too much of a drawback. It will
only be called when the command isn't in your enumeration. The only 
way you'd even notice the difference is if:
   o  Invalid commands are very common AND
   o  Your compiler has a VERY large exception overhead AND
   o  Your enumeration is very small (only a few values with short
      names).

In many cases, error processing throughput isn't considered that vital.
In most other cases, the time taken to discover that the string isn't
a command will dwarf the time taken in "exception overhead". 

If these commands are comming from a human user, the time taken waiting
for him/her to respond will dwarf both. So why not code it in the 
easiest, most maintainable way?

I don't see any reason other than speed for an exception handler to be
a "sore-spot". It should be way easier to read, understand, and 
maintain than a huge "if..elseif..." statement. Plus you can do nifty
stuff like loop through the list of commands.

-- 
T.E.D.          
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