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From: "Matthew Heaney" <mheaney@on2.com>
Newsgroups: comp.lang.ada
Subject: Re: Calculator and Operator Precedence
Date: Tue, 19 Feb 2002 15:00:31 -0500
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Date: 2002-02-19T15:00:31-05:00
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"jon" <jkjw@brighton.ac.uk> wrote in message
news:3C727CCF.505D83E0@brighton.ac.uk...
> i ve built a basic calculator which asks user for expression using a
> console.
>
> eg
>
> Enter expression : 1 + 2 + 3 * 4
>
> now i need to make it so it can handle Operator Precendence

You'll need to define a grammar, something like

 <exp> ::= <factor> [+ <exp>]
<factor> ::= <term> [* <factor>]
<term> ::= (<exp>) | <digit>

I like to use an Interpreter pattern.  As you scan the expression, you can
emit expression objects.  To return a value you "evaluate" the ultimate
expression object:

   Exp : constant Exp_Access := Parse (Line);
   Value : constant Integer := Eval (Exp);

http://www.acm.org/archives/wa.cgi?A2=ind9810&L=patterns&P=R2
http://www.acm.org/archives/patterns.html

If you're worried about memory leaks, you can use "smart"
(reference-counted) pointers or "auto" pointers (a la C++) or "control"
types.