Converting Roman numerals to decimal

Could someone please point out the error(s) in the given code? It was downvoted on Stack Overflow without any explanation, but it seems to be working fine for me:

int value(char roman)
{
   switch(roman)
   {
        case 'I':return 1;
        case 'V':return 5;
        case 'X':return 10;
        case 'L':return 50;
        case 'C':return 100;
        case 'D':return 500;
        case 'M':return 1000;
   }
}

int getdec(const string& input)
{
  int sum=0; char prev='%';
  for(int i=(input.length()-1); i>=0; i--)
  {
    if(value(input[i])<sum && (input[i]!=prev))
    {       sum -= value(input[i]);
            prev = input[i];
    }
    else
    {
            sum += value(input[i]);
            prev = input[i];
    }
  }
  return sum;
}

This was the output received from the code:


I = 1
II = 2
III = 3
IV = 4
V = 5
VI = 6
VII = 7
VIII = 8
IX = 9
X = 10
XI = 11
XII = 12
XIII = 13
XIV = 14
XV = 15
XVI = 16
XVII = 17
XVIII = 18
XIX = 19
XX = 20
XXI = 21
XXII = 22
XXIII = 23
XXIV = 24
XXV = 25
XXVI = 26
XXVII = 27
XXVIII = 28
XXIX = 29
XXX = 30
XXXI = 31
XXXII = 32
XXXIII = 33
XXXIV = 34
XXXV = 35
XXXVI = 36
XXXVII = 37
XXXVIII = 38
XXXIX = 39
XL = 40
MMMMCMXCIX = 4999
CM = 900
XC = 90

If you have only valid Roman numbers you could use a simpler algorithms. (i.e IIV is invalid )

• The symbols "I", "X", "C", and "M" can be repeated three times in succession, but no more. (They may appear more than three times if

they appear non-sequentially, such as XXXIX.) "V", "L", and "D" can
never be repeated. A common exception to this is the use of IIII on
clocks; see below.

• "I" can be subtracted from "V" and "X" only. "X" can be subtracted from "L" and "C" only. "C" can be subtracted from "D" and "M" only.

"V", "L", and "D" can never be subtracted

• Only one small-value symbol may be subtracted from any large-value symbol.

Just (string-)replace IV by IIII, IX by VIIII and so on. Afterwards you just have to sum the numbers from left to right.

StackExchange Code Review Q#22876, answer score: 6