How to convert a Roman numeral to integer in PHP?

Question

Using PHP, I'd like to convert a string containing a Roman number into its integer representation. I need this because I need to make calculations on them.

Wikipedia on Roman numerals

It would suffice to only recognize the basic Roman numeral characters, like:

$roman_values=array(
    'I' => 1,
    'V' => 5,
    'X' => 10,
    'L' => 50,
    'C' => 100,
    'D' => 500,
    'M' => 1000,
);

That means the highest possible number is 3999 (MMMCMXCIX). I will use N to represent zero, other than that only positive integers are supported.

I cannot use the PEAR library for Roman numbers.

I found this great question on SO on how to test whether the string contains a valid Roman numeral:

How do you match only valid roman numerals with a regular expression?

What would be the best way of coding this?

Answer 1

How about this:

$romans = array(
    'M' => 1000,
    'CM' => 900,
    'D' => 500,
    'CD' => 400,
    'C' => 100,
    'XC' => 90,
    'L' => 50,
    'XL' => 40,
    'X' => 10,
    'IX' => 9,
    'V' => 5,
    'IV' => 4,
    'I' => 1,
);

$roman = 'MMMCMXCIX';
$result = 0;

foreach ($romans as $key => $value) {
    while (strpos($roman, $key) === 0) {
        $result += $value;
        $roman = substr($roman, strlen($key));
    }
}
echo $result;

which should output 3999 for the supplied $roman. It seems to work for my limited testing:

MCMXC = 1990
MM = 2000
MMXI = 2011
MCMLXXV = 1975

You might want to do some validation first as well :-)

Answer 2

I am not sure whether you've got ZF or not, but in case you (or any of you who's reading this) do here is my snippet:

$number = new Zend_Measure_Number('MCMLXXV', Zend_Measure_Number::ROMAN);
$number->convertTo (Zend_Measure_Number::DECIMAL);
echo $number->getValue();

• Zend_Measure_Number on framework.zend.com

Answer 3

This is the one I came up with, I added the validity check as well.

class RomanNumber {
    //array of roman values
    public static $roman_values=array(
        'I' => 1, 'V' => 5, 
        'X' => 10, 'L' => 50,
        'C' => 100, 'D' => 500,
        'M' => 1000,
    );
    //values that should evaluate as 0
    public static $roman_zero=array('N', 'nulla');
    //Regex - checking for valid Roman numerals
    public static $roman_regex='/^M{0,3}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})$/';

    //Roman numeral validation function - is the string a valid Roman Number?
    static function IsRomanNumber($roman) {
         return preg_match(self::$roman_regex, $roman) > 0;
    }

    //Conversion: Roman Numeral to Integer
    static function Roman2Int ($roman) {
        //checking for zero values
        if (in_array($roman, self::$roman_zero)) {
            return 0;
        }
        //validating string
        if (!self::IsRomanNumber($roman)) {
            return false;
        }

        $values=self::$roman_values;
        $result = 0;
        //iterating through characters LTR
        for ($i = 0, $length = strlen($roman); $i < $length; $i++) {
            //getting value of current char
            $value = $values[$roman[$i]];
            //getting value of next char - null if there is no next char
            $nextvalue = !isset($roman[$i + 1]) ? null : $values[$roman[$i + 1]];
            //adding/subtracting value from result based on $nextvalue
            $result += (!is_null($nextvalue) && $nextvalue > $value) ? -$value : $value;
        }
        return $result;
    }
}

Answer 4

Quick idea - go through the Roman number from right to left, if value of $current (more to the left) is smaller than $previous, then subtract it from the result, if larger, then add it.

$romanValues=array(
    'I' => 1,
    'V' => 5,
    'X' => 10,
    'L' => 50,
    'C' => 100,
    'D' => 500,
    'M' => 1000,
);
$roman = 'MMMCMXCIX';

// RTL
$arabic = 0;
$prev = null;
for ( $n = strlen($roman) - 1; $n >= 0; --$n ) {
    $curr = $roman[$n];
    if ( is_null($prev) ) {
        $arabic += $romanValues[$roman[$n]];
    } else {
        $arabic += $romanValues[$prev] > $romanValues[$curr] ? -$romanValues[$curr] : +$romanValues[$curr];
    }
    $prev = $curr;
}
echo $arabic, "\n";

// LTR
$arabic = 0;
$romanLength = strlen($roman);
for ( $n = 0; $n < $romanLength; ++$n ) {
    if ( $n === $romanLength - 1 ) {
        $arabic += $romanValues[$roman[$n]];
    } else {
        $arabic += $romanValues[$roman[$n]] < $romanValues[$roman[$n+1]] ? -$romanValues[$roman[$n]] : +$romanValues[$roman[$n]];
    }
}
echo $arabic, "\n";

Some validation of roman number should also be added, though you said that you already have found how to do it.