Question

This is a PHP code. It is a project of roman converter

function validRoman($str) $romanValid = "/"M{0,4}(CM CD D2C{0,37) (XC XL L?X{0,33) (IX IVV?10,3)/": return (preg_match($romanValid, $str)); function RomanToNum($str) $first = 0; $neighbor = 0; $result = 0; $len = strlen($str); for ($i = 0; $i < $len - 1; $i++) $first = convert($str[$i]); $neighbor = convert($str($i + 1]); if ($first < $neighbor) $result -= $first; else $result += $first; $result += convert($str[$len - 1]); return $result); function convert($val) $arr = array(1 => "I", 5 => "V", 10 => "X", 50 => "L", 100 => "C", 500 => "D", 1000 => "M"); return (array_search($val, $arr));

Can someone explain this statement to me? What does it mean?

$romanValid = "/^M{0,4}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})$/";

and in the second function, what does the first&neighbor stand for? can you explain the logic of this function for me, please?? thank you

Answer 1

Answer)

The statement $romanValid = "/^M{0,4}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})$/"; is a Regular Expression for Roman Number. A regular expression is a sequence of characters which define a pattern for a particular group. For example the above expression match all the roman number when matched with this expression. Thus, the first function validRoman($str) can be used to check if user enters a valid Roman number.

In the second function RomanToNum($str) which accept a roman number as argument and returns to Digit or Number. For example XI is a roman number whose corresponding number is 11.

Now, in order to convert XI to 11 we have three variables $first, $neighbour and $result. $first used to store first character that is X and neighbour stores next character I.

After this we called convert() function which convert X to 10 and I to 1 which are stored back into first and neighbour.

Then, we check if first is less than neighbour that is 10<1 which is not true. Thus, else condition runs in which result get result=result+first that is result=0+10; thus result stores 10.

Now, The loop condition ends as loop as when i=1 loop condition (1 < 1) fails and loop only execute one time. Now after loop we have $result += convert($str[$len - 1]); which means in the result we add last character of roman number. Which in our case is I which is first converted using convert function to number 1 and then added to result. result already have 10 stores after adding 1 result becomes 11. which is returned by RomanToNum() function. which is desired output in Number.

If we consider IX whose number is 9. All the above procedure is same only difference is that if condition becomes true and -1 gets stores in result. After loop finish result= -1 + 10 = 9 is returned.