From: Robert Klemme <shortcutter@...>
Date: 2012-12-21T19:34:51+09:00
Subject: Re: Perl to Ruby: regex captures to assignment.

On Thu, Dec 20, 2012 at 7:23 PM, Derrick B. <lists@ruby-forum.com> wrote:
> Robert Klemme wrote in post #1089733:
>>
>> I would choose a completely different approach: I would have a single
>> expression for matching and decide which assignments to make based on
>> the value of one of the capturing groups in the conditional branch:
>>
>> ["1 3/4", "5"].each do |s|
>>   puts s
>>
>>   if %r{\A(\d+)(?:\s+(\d+)/(\d+))?\z} =~ s
>>     w, n, d = $2 ? [$1.to_i, $2.to_i, $3.to_i] : [1, $1.to_i, 0]
>>     printf "%4d %4d %4d\n", w, n, d
>>   else
>>     $stderr.puts "No match #{s}"
>>   end
>> end
>>
>> I also spiced the regexp a bit more to be more restrictive.
>
> I am prompting for input, so I would be parsing individual strings and
> not a list of them.

That was just for demonstration purposes.

>  Though I will remember that for future static
> testing.  Also, I and using the "\S+" to handle negative rational
> numbers: "-1 3/4".

There's still a better way to do that then just match everything non
whitespace.  How would you parse ":s9d2++*3h43" as a number?

["1 3/4", "5", '-23', '-23 -4/4'].each do |s|
  puts s

  if %r{\A([-+]?\d+)(?:\s+([-+]?\d+)/(\d+))?\z} =~ s
    w, n, d = $2 ? [$1.to_i, $2.to_i, $3.to_i] : [1, $1.to_i, 0]
    printf "%4d %4d %4d\n", w, n, d
  else
    $stderr.puts "No match #{s}"
  end
end

Btw, did I mention that I find your variable assignment weird?  I'd rather do

["1 3/4", "5", '-23', '-23 -4/4'].each do |s|
  puts s

  if %r{\A([-+]?\d+)(?:\s+([-+]?\d+)/(\d+))?\z} =~ s
    w, n, d = $2 ? [$1.to_i, $2.to_i, $3.to_i] : [$1.to_i, 0, 1]
    printf "%4d %4d %4d\n", w, n, d
  else
    $stderr.puts "No match #{s}"
  end
end

>> If you like readability then why are you using Perl in the first place?
>> :-)
>
> Because Perl is awesome!

Hm...  It's been a while that I thought that (but I did actually
once).  IMHO Perl has gone over the edge of usefulness long ago.

>  That is why I chose Ruby as my next personal
> choice of languages to learn.

Good choice!

>  I just completed a quarter of Programming
> Language Concepts where I was introduced to Lisp, but was also required
> to use some Python.  I know Python is a fascinatng language, but I am
> not yet a big fan of it.  The final assignment was write a class to work
> with fractions.  It started as a C++ class, then migrated to Perl (for
> fun), and now Ruby (more fun).

:-)

> I attached the two versions, Ruby and Perl.  The Perl version uses a
> "class", also.  Feel free to suggest how to make the Ruby version more
> Ruby-ish!  My goal is to take more advantage of the OO aspects of Ruby,
> and not just have it look like Perl.

I don't have the time right now but you should definitively use class
Rational.  Also your parsing of the command line could be better:

do
  print "Fraction 1: "
  input = gets or break
  f1 = parse_fraction(input)

  print "Fraction 2: "
  input = gets or break
  f2 = parse_fraction(input)

  print "Command: "
  input = gets or break

  case input
  when "A"
    puts f1 + f2
  when "S"
    puts f1 - f2
 ...
 else
   abort "Invalid command: #{input}"
 end

end until cmd == "Q"

puts "Quitting"


You should also not implement #to_string but rather #to_s and then use
string interpolation for constructing your outputs.

Kind regards

robert

--
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/