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<pre lang="c++" line="1" title="CodeMonkey17673" class="run-this">first convert heximal value into string (or char[])

int HexToDec(string hex)
{
int dec(0);

size_t len = hex.size();
int _16mul = 1;
for(size_t i = len; i > 0; --i) {
char number = static_cast<char>(tolower(hex[i - 1]));
if(number >= '0' && number <= '9')
dec += (number - '0') * _16mul;
else if(number >= 'a' && number <= 'f')
dec += (number - 'a' + 10) * _16mul;
else cout << "wrong symbol: " << number << endl;
_16mul *= 16;
}

return dec;
}</pre><pre title="CodeMonkey17673" input="yes">
</pre>

C#
int HexToDec(string hex)
{
int i = Int32.Parse(hex, NumberStyles.HexNumber);
return i.ToString();
}

int HexDecimalMap(char ch)
{
   if (ch >= '0' && ch <='9')
   {
       return (ch-'0');
   }
   if (ch >= 'a' && ch <= 'f')
   {
       return (10+ (ch-'a');
   }
   else if (ch >= 'A' && ch <= 'F')
   {
       return (10+ (ch-'A');
   }
   else
   {
      return -1;   
   }
}

int HexToDecimal(char *str)
{
   int num = 0;
   while (*str)
   {
      int numMap = HexDecimalMap(str++);
      if (numMap < 0)
      {
         return -1; //error
      }
      num = num * 16 + numMap;
   }
   return num;
}

int i = Int32.Parse(hex, NumberStyles.HexNumber);

}