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Message from discussion Postfix to Infix (#148)
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Eric Mahurin  
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 More options Dec 3 2007, 9:42 am
Newsgroups: comp.lang.ruby
From: Eric Mahurin <eric.mahu...@gmail.com>
Date: Mon, 3 Dec 2007 09:42:10 -0500
Local: Mon, Dec 3 2007 9:42 am
Subject: Re: [QUIZ] Postfix to Infix (#148)
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Note:  parts of this message were removed by the gateway to make it a legal Usenet post.

On Dec 3, 2007 8:05 AM, Ken Bloom <kbl...@gmail.com> wrote:

> On Sun, 02 Dec 2007 22:23:23 -0500, Eric Mahurin wrote:

> > Note:  parts of this message were removed by the gateway to make it a
> > legal Usenet post.

> > On Nov 30, 2007 7:28 AM, Ruby Quiz <ja...@grayproductions.net> wrote:

> >> For an added bonus, try to keep the parentheses added to infix
> >> expressions to
> >> the minimum of what is needed.

> > My solution does the above, plus a few more things:

> > * maintains an OO data structure (to do everything below) * further
> > reduce parentheses (and post-fix stack depth) by using some
> > associativity
> > * evaluates the result
> > * gives stack-reduced post-fix form

> > The basic idea of the solution is to have an object for each expression
> > (possibly with sub-expressions as operands) and have methods for
> > applying another operation in either direction (i.e. have both #add and
> > #radd - reverse add).  This allows independent decisions on what each
> > type of expression should do when it is in either operand of another
> > operation.

> > Here are a few examples (result shows internal postfix, infix, and
> > result):

> >>ruby quiz148.rb "2 3 5 + *"
> > 2 3 5 + * => 2*(3 + 5) =>  16
> >>ruby quiz148.rb "56 34 213.7 + * 678 -"
> > 56 34 213.7 + * 678 - => 56*(34 + 213.7) - 678 =>  13193.2
> >>ruby quiz148.rb "1 56 35 + 16 9 - / +"
> > 1 56 35 + 16 9 - / + => 1 + (56 + 35)*(16 - 9) =>  14
> >>ruby quiz148.rb "1 2 3 4 5 + + + +"
> > 1 2 + 3 + 4 + 5 + => 1 + 2 + 3 + 4 + 5 =>  15
> >>ruby quiz148.rb "1 2 3 4 5 - - - -"
> > 1 2 - 3 + 4 - 5 + => 1 - 2 + 3 - 4 + 5 =>  3

> This doesn't look right.
> 3 5 * 5 8 * / => 3*5*(5*8) =>  0

> Thanks Ken,

I obviously didn't test divide.  My previously solution has a stupid typo in
Quotient#to_s.  Should be:

class Quotient < Product
  def to_s
    "#{@left}/#{@right}"  # had a * operator here before, whoops!
  end
  ...

Here's a couple more tests:

>ruby quiz148.rb "3 5 / 5 8 / /"

3 5 / 5 / 8 * => 3/5/5*8 =>  0
>ruby quiz148.rb "3 5 5 8 / / /"

3 5 5 / 8 * / => 3/(5/5*8) =>  0

All the results are zero because it is using ruby's Fixnum#/.  The last form
isn't quite optimal because it preferring to minimize divisions over
minimizing groupings/stack-depth.  If you use the commented code in
Product#rdiv like this:

  def rdiv(other)
    # could do this to reduce grouping and stack depth
    # but this will increase expensive divisions
    @left.rdiv(other).div(@right) # might have more divisions now
    #other.div(Group.new(self))
  end

you'll get this:

>ruby quiz148.rb "3 5 5 8 / / /"

3 5 / 5 * 8 / => 3/5*5/8 =>  0

 
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