Roman Numerals – the default dojo option

All code dojos start with roman numerals, it’s the default – it’s not exciting, but it serves a purpose – get your own idea onto the board. As such, I’ve only ever done the roman numerals dojo once, at a work scala dojo while I was attempting to debug some R, it wasn’t a great experience as I didn’t really get to participate.

Today I was woken up at about 6:30 by an annoying cat and, unable to get back to sleep, figured I would do something constructive – have a quick stab at doing the roman numeral thing.

So, my hack looks like this:

This will take us from “MCMLXXXIV” to 1984 – great. I recall in the scala dojo that some people took the option of reversing the sequence then mapping a function over the reverse, this saves the use of an accumulator – for some reason I don’t like doing the reverse then a map … it’s not like this is a chunk of code where performance is critical but …

Here’s an example of the kind of other solutions I’ve seen using reverse, they all use a helper like roman-helper, in the following case ‘add-numeral’

(defn roman [s]
    (reduce add-numeral (map numerals (reverse s))))

Talking to a friend at work, he came up with a slightly different version that removed the recur call – something I like to do. This allowed me to create another solution that I think is nicer – the complete gist can be found here on github.

The updated solution follows:

(defn roman-helper [[a b]]
  (let [curnt (roman-lookup a 0) nxt (roman-lookup b 0)]
  (if (> nxt curnt) (- curnt) curnt)))

(defn roman->decimal [roman]
  (let [as-seq (conj (into [] roman) nil)
        pairs (partition 2 1 as-seq)]
    (apply + (map roman-helper pairs))))

There are a couple of changes here. The first is a small change to the roman-helper function, the aggregate has been removed so the function simply returns it’s value or negative if the current letter is worth less than the next. The roman->decimal function has changed a little bit more, with most of the action happening in the let binding.

The first step

(conj (into [] roman) nil)

turns the string into a vector of characters and then slaps a nil on the end. I am not as bothered by this as the reverse because conj takes essentially constant time – not that this matters at all in this bit of code golf.

into vs (vec “XXX”)

when dealing with big collections, the use of (into [] “xxx”) is about 30% more performant than vec, this is due to the use of transients. Since we aren’t using a big collection here I could’ve written the block as

(conj (vec roman) nil)

but I don’t think it adds much to the clarity of the code.

The second step

(partition 2 1 as-seq)

we use the vector we created in step 1 to create a sequence of overlapping pairs. The 2 indicates the number of elements per partition and the 1 the number of steps we take. Clojure will take all partitions until we exhaust the sequence – without the nil we wouldn’t get the last pair.

The final step

(apply + (map roman-helper pairs))

Convert to numbers and add ’em up.

Going in the reverse direction.
I cheated. Clojure has pprint, which has a format function that allows us to create a partial function. The partial function can be called with a number e.g.
(decimal->roman 1984) and produce “MCMLXXXIV”.


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