|Perl: the Markov chain saw|
Rosetta PGA-TRAMby eyepopslikeamosquito (Chancellor)
|on Jun 13, 2009 at 11:48 UTC||Need Help??|
After my recent "discovery" of the PGA-TRAM algorithm, I couldn't resist coding it up in a variety of languages. I'm doing this as a rosetta code node because I find it fun; hopefully, it'll provide some interesting insights into a variety of programming languages. Please note that this meditation is not about golf, but about implementing an arbitrary, specific algorithm in the most "natural" way in a variety of languages.
Here's the spec:
Write a function, roman_to_dec, to convert a "modern" Roman Numeral to its decimal equivalent. The function takes a single string argument and returns an integer. The string argument may be assumed to always contain a valid Roman Numeral in the range 1-3999. Error handling is not required. For example, roman_to_dec("XLII") should return 42.
Let's get started with a Perl version:
Running this program produces:
To me, this is the most natural way to express the PGA-TRAM algorithm in Perl. It uses a variety of common Perl idioms:
For those folks unfamiliar or uncomfortable with reduce, note that it can be eliminated like this:
Though I don't mind that at all, I do prefer the reduce version.
As you might expect, my "most natural" Python solution looks very similar to the Perl one above:
That this solution is essentially identical to the Perl one shows how similar these two languages are. Indeed, I'm fond of claiming that Perl, Python and Ruby are "essentially equivalent". :) Since Python lacks Perl's simple lexical scoping mechanism, I chose to "data hide" the rtoa hash by making it a default function argument. Note that Python default function arguments are initialized once only. Though I could have employed the Python map function (with a lambda), I chose instead to use a lazily evaluated Python generator list comprehension, (__rtoa[c] for c in r.upper()), because that seemed more "naturally Pythonic" to me.
Here's my Haskell solution, using GHC:
Instead of using a rtoa hash, natural in Perl and Python, a rtoa function using simple pattern matching seemed the most Haskelly way of expressing this algorithm.
Finally, my ANSI C++ solution:
The Perl reduce function is called "accumulate" in ANSI C++. Implementing the lookup with the admittedly long-winded romtab table felt like natural C++ to me because it seeks high performance. Indeed, if you peek inside <ctype.h> you will likely see toupper() implemented in a similar vein. This makes the rtoa(toupper(c)) above ridiculously fast, costing only two (constant) array lookups, not requiring even a single C function call! Also of note in this solution is that I didn't bother with map, instead simply applying the ultra fast rtoa(toupper(c)) combo twice in the accumulator function.
Update: As indicated below, we can eliminate the call toupper(), while eliminating any memory faults on invalid input, by adjusting romtab as shown below:
I'd love to see some sample implementations in other languages, especially Perl 6. So please feel free to respond away. :)
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