Not so easy, pal!

Well, it's not a problem about Perl, of course, rather I found it required a quite brainpower for me to write down rules to convert numbers, and I did not search for the web for a

*copy-and-paste*alghoritm. Please note: if you need a rock-solid way to handle conversions, have a look at CPAN that is full of modules for this particular aim.

Here I'm going to discuss the solution I found and how I implemented it. It is not supposed to be the best one, or the faster one, it's just

*my*solution from scratch.

# The program

I split the problem of converting an arabic number into a roman one into three steps, with one dedicated subroutine for each step, so that the main loop reduces to something like the following:`say "$_ = " . $roman_string->( $reassemble->( $disassemble->( $_ ) ) )`

for ( 1..30 );

`1 = I`

2 = II

3 = III

4 = IV

5 = V

6 = VI

7 = VII

8 = VIII

9 = IX

10 = X

11 = XI

12 = XII

13 = XIII

14 = XIV

15 = XV

16 = XVI

17 = XVII

18 = XVIII

19 = XIX

20 = XX

21 = XXI

22 = XXII

23 = XXIII

24 = XXIV

25 = XXV

26 = XXVI

27 = XXVII

28 = XXVIII

29 = XXIX

30 = XXX

The steps must be read from the inner subroutine to the outer, of course, and therefore we have:`disassemble`

that translates an arabic number into roman basis, that is computes how many units, tens, hundreds and thousands are required. In this phase there is no application of roman rules, so numbers are decomposed into a*linear*string of letters. As an example the number`4`

is translated into`IIII`

, which is of course a non-existent roman number.`reassemble`

applies roman rules, in particular*promoting*numbers so that groups are translated, when needed, into higher order letters. For instance`IIII`

is promoted into two groups:`I`

and`V`

.`roman_string`

compose the promoted groups into the final string. The main difficulty of this part is to understand when a letter has to be placed on the right (addition) or on the left (subtraction) of another letter. For instance, having the groups`I`

and`V`

the function must understand if the output have to be`VI`

(6) or`IV`

(4).

*global*hash:

`my $roman = {`

1 => 'I',

5 => 'V',

10 => 'X',

50 => 'L',

100 => 'C',

500 => 'D',

1000 => 'M',

};

`$roman`

when needing to convert from an arabic number to its roman letter. In order to allow method to cooperate together, they accept and return an hash keyed by a roman letter and the number of occurences such letter must appear in the final string. The following is an example of the hash for a few numbers:`# 4 (IV)`

{ 'I' => 1, 'V' => 1 }

# 19 (XIX)

{ 'I' => 1, 'X' => 2 }

# 5 (V)

{ 'V' => 1 }

# 17 (XVII)

{ 'X' => 1, 'V' => 1, 'I' => 2 }

# The `disassemble`

function

The following is the code for the `disassemble`

function, that accepts as only input the arabic number.`# Accepts the arabic number and provides an hash`

# keyed by each letter, with the value of how many times

# such letter should be summed in order to obtain the

# starting number.

my $disassemble = sub{

my ( $number ) = @_;

my $items = {};

# sort the keys, that are arabic thresolds, from

# the greater to the smaller one

for my $current_value ( sort { $b <=> $a } keys $roman->%* ){

my $how_many = int( $number / $current_value );

next unless ( $how_many );

my $letter = $roman->%{ $current_value };

$items->{ $letter } = $how_many;

$number -= $current_value * $how_many;

}

return $items;

};

`$items`

that is what it will return to allow other methods to consume. Each key of the `$roman`

hash is passed ordered by the bigger to the smaller (please note that `sort`

has `$b`

first!). In this way we can surely scompose the number from the thousands, hundreds, tens, and units in this exact order. The `$how_many`

variable contains the integer part of each letter. For example the number `29`

is processed as follows:`29 / 10`

that drives`$how_many`

to be`2`

and the remaining to be a`9`

;`9 / 5`

that makes`$how_many`

to be`1`

and the remaining to be a`4`

;`4 / 1`

that makes`$how_many`

to be`4`

and there's nothing more to do.

`$how_many`

value is inserted into the `$items`

has, that in the above ecample becomes:`# 29 (XIX)`

{ 'X' => 2,

'V' => 1,

'I' => 4

}

# The `reassemble`

method

The `reassemble`

method takes as input the hash produced by `disassemble`

and checks if any letter requires a promotion. Here it is the code:`# Accepts an hash with keys the letters and values the number`

# of times each letter should appear.

# Traverse the hash from the smaller to the greater

# in order to "promote" smaller aggregates. For instance

# 'IIII' (4) is aggregated and therefore the hash is modified

# so there's only an 'I' and another 'V', in such case

# the quantity of the promoted letter is negative to indicate

# it has been promoted.

my $reassemble = sub{

my ( $items ) = @_;

my @sorted_thresolds = sort { $a <=> $b } keys $roman->%*;

for ( my $i = 0; $i < @sorted_thresolds; $i++ ){

my $current_value = $sorted_thresolds[ $i ];

my $key = $roman->%{ $current_value };

my $how_many = $items->%{ $key };

next unless ( $how_many );

my $greater_value = ( $i + 1 > @sorted_thresolds ? 1000 : $sorted_thresolds[ $i + 1 ] );

my $greater_key = $roman->%{ $greater_value };

my $need_to_promote = $how_many == 4

|| ( $greater_value / $current_value == $how_many );

if ( $need_to_promote ){

$items->{ $greater_key }++;

$how_many = $greater_value - $how_many * $current_value;

$items->{ $key } = $how_many * -1;

}

}

return $items;

};

*promotion*must be done from the smaller letter to the greater one, so this time the letters are walked in ascending order (i.e.,

`sort`

has `$a`

first!). Since to promote a letter I need to access the following one, I need a C-style `for`

loop.A letter requires to be promoted if its quantity is

`4`

or /it is 2 and the right bigger value is exactly the double of the current one~, that is while `( $greater_value / $current_value == $how_many )`

. This makes, for instance `IIII`

to be promoted (the quantity is 4), and `VV`

to be promoted into `X`

(because the quantity is 2 and the `X`

is exactly the double of `V`

). The promotion manipulates the hash increasing by one the right bigger letter and leaving a single current letter. In order to flag the promoted letter, I decided to use a negative quantity (where the absolute value is the exact one).So for instance, the 29 hash of the previous paragraph is passed as follows:

`# input to the method`

{ 'X' => 2,

'V' => 1,

'I' => 4

}

# first for step (I)

{ 'X' => 2,

'V' => 2,

'I' => -1 # promoted, keep 1 and increase 'V'

}

# second step (V)

{ 'X' => 3,

'V' => 0, # promoted, increase X by one

'I' => -1

}

`X`

and one `I`

, the point now is to understand how to render them in the correct order. This is the aim of the `roman_string`

method.# The `roman_string`

method

The method accepts the normalized hash (i.e., groups are already formed) and compose the final string placing letter on the left or the right of each other depending on their quantity. The following is the code of the method:`# Do the hard work of composing`

# each group of letters in order to compose the roman string.

my $roman_string = sub {

my ( $items ) = @_;

my @chars;

for my $current_value ( sort { $b <=> $a } keys $roman->%* ){

my $letter = $roman->%{ $current_value };

my $how_many = $items->%{ $letter };

next unless ( $how_many );

if ( $how_many > 0 ){

push @chars, $letter for ( 1 .. $how_many );

}

else{

# this is promoted, so it has to be inserted as last-to-last

# in the previous chain

# example: @chars( X, X ) and here I've 'I' to make XIX (19)

push @chars, ( $letter, pop @chars );

}

}

return join "", @chars;

};

`@chars`

array, that is then `join`

-ed into a single string.Let's suppose we need just to add letters: in this case we need to write letters from the greater to the smaller from left to right, and this is the order I traverse the letters of

`$roman`

(again, note that `sort`

has `$b`

first!). If the quantity of the letter is positive *the letter has not been promoted*and therefore it will not be placed to the left of another letter, so just insert into

`@chars`

the `$letter`

for the `$how_many`

quantity. On the other hand, if `$how_many`

is negative, the letter has been promoted and therefore have to be printed on the left of the last printed letter. This is as easy as doing:`push @chars, ( $letter, pop @chars );`

that inserts into

`@chars`

the `$letter`

and the previous last character that has been removed via `pop`

.With regards to the previous example of

*29*we have that:

`# method input`

{ 'X' => 3,

'I' => -1

}

# first step: prints X

# with quantity 3 (not promoted)

@chars = ( 'X', 'X', 'X' );

# second step: prints I

# that has been promoted

# and must be inserted ascending

# as last-to-last

@chars = ( 'X', 'X' ,

( 'I', # $letter

'X' # pop @chars

) );

# Conclusions

Well, it has been much code that I expected to write. Using an object notation, instead of plain hashes, could surely make the program more robust. I'm pretty sure there's a way to shrink the code down and to avoid that ugly C-style`for`

loop, as well as the promotion part could be simplified keeping in mind that it often reduces to -1 for the current letter and +1 for the greater one. Anyway, it does what I need and seems correct!