Alternative - yes, better - ???

Why use a map - "slices forever !!"

{
  my %rtoa = ( M=>1000, D=>500, C=>100, L=>50, X=>10, V=>5, I=>1 );
  sub roman_to_dec {
    reduce { $a+$b-$a%$b*2 } @rtoa{split//, uc(shift)}
  }
}
[download]

Or double your fun with a double-split.

sub roman_to_dec { reduce { $a+$b-$a%$b*2 }
  @{{split/(\d+)/, 'M1000D500C100L50X10V5I1'}}{split//, uc pop}
}
[download]

Or back to a map...

sub roman_to_dec { reduce { $a+$b-$a%$b*2 }
  map 'M1000D500C100L50X10V5I1' =~ /$_(\d+)/i, split//, pop
}
[download]

Well, that was fun - thanks

you can mimic your semantic in Python by using a generator function acting as closure, defining a nested function roman_to_dec and returning it.

I don't have Python installed and am not a Py expert, so please accept my semantically equal Perl interpolation.

(untested)

sub generator {
    my %rtoa = ( M=>1000, D=>500, C=>100, L=>50, X=>10, V=>5, I=>1 );
    
    my $c_nested = sub {
             reduce { $a+$b-$a%$b*2 } map { $rtoa{$_} } split//, uc(sh
+ift);
         };

    return $c_nested;
}

*roman_to_dec = generator();
[download]

NB: This will look much easier in Python because it's automatically dereferencing.

UPDATE

I suppose most Pythonistas would rather prefer making rtoa a class variable and using roman_to_dec as a method. TIMTOWTDI. ;-)

I suppose most Pythonistas would rather prefer making rtoa a class variable and using roman_to_dec as a method. TIMTOWTDI. ;-)

Ha ha! Yes, despite the Zen of Python aiming for "There should be one -- and preferably only one—obvious way to do it" I could not help but notice when playing code golf in multiple languages that Python sometimes out-TMTOWTDI-ed them all! Also, without TMTOWTDI code golf is no fun at all and, surprisingly, code golf is way more popular in Python than Perl nowadays.

The TMTOWTDI examples below were taken from:

• The golf course looks great, my swing feels good, I like my chances (Part VI)
• Re: Drunk on golf: 99 Bottles of Beer

Consider how to create a "Dear John" string in each of the four languages:

"Dear $name"              # Perl and PHP
"Dear %s" % expr          # Python and Ruby % printf-like operator
"Dear {0}".format(expr)   # Python format string method
"Dear "+`expr`            # Python backticks (TMTOWTDI)
"Dear #{expr}"            # Ruby string interpolation
"Dear @{[expr]}"          # Perl "Baby Cart" string interpolation
[download]

Curiously, Python is the only member of the gang of four languages to allow you to reverse the order of the two string multiply operands:

5 * "X"    also produces "XXXXX" in Python! TMTOWTDI! :)
5 * "X"    ... but not in Ruby (won't compile: type error)
5 x "X"    ... or Perl (produces empty string)
[download]

That is, the string multiply operator is commutative in Python, but not in Perl or Ruby. This language idiosyncrasy makes string multiply based solutions most attractive in Python. To illustrate, note these code snippets from my string multiply based solutions to this game:

$"x(318%$_/9)        Perl
" "*(318%i/9)        Ruby
318%i/9*" "          Python
[download]

This is a very rare example of Python out-golfing both Perl and Ruby.

From my early 195-stroke function-based Python solution:

n=99
z=lambda:`n or 99`+" bottle"+"s of beer on the wall"[n==1:]
while n:y=z();n-=1;print"%s, %s.\n"*2%(y,y[:-12],n and"Take one down a
+nd pass it around"or"Go to the store and buy some more",z())
[download]

notice the expression:

n and"Take one down and pass it around"or"Go to the store and buy some
+ more"
[download]

Shortening the two strings above to "Take" and "Go to" for clarity, let's consider the many and varied ways of doing this in Python (TMTOWTDI):

"Take"if n else"Go to"
(n>0)*"Take"or"Go to"
["Go to","Take"][n>0]
("Go to","Take")[n>0]
n and"Take"or"Go to"
"GToa kteo"[n>0::2]          # "Slice and Dice" wins this golf!
[download]

As you can see, I missed the winning Python "Slice and Dice" tactical trick in this game.

I'm not sure why rtoa has to be in the closure.

Just realised it doesn't thanks to the introduction of state variables in perl v5.10.

use v5.10;
use strict;
use warnings;
use List::Util qw(reduce);

sub roman_to_dec {
  state %rtoa = ( M=>1000, D=>500, C=>100, L=>50, X=>10, V=>5, I=>1 );
  reduce { $a+$b-$a%$b*2 } map { $rtoa{$_} } split//, uc(shift);
}

my @testdata = ( "XLII", "LXIX", "mi" );
for my $r (@testdata) {
  print "$r: ", roman_to_dec($r), "\n";
}
[download]

I don't like state in named subroutines. It makes it hard to reïnitialise the variable and causes hard to debug problems. An anonymous sub is OK, because the state variable is reïnitialised every time the sub is created.

But in this particular case, it's not highly probable you'd ever need to reïnitialise the variable. But, if you plan to convert from different digit systems, you'll quickly see why state is not the solution.

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

> > I'm not sure why rtoa has to be in the closure.

I should have been clearer... I don't understand why you didn't go the easy way of defining my %rtoa either on file or function scope. No problem translating this to python.

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery}