Comparing two functions for speed

[paradoxresolved]

Hi folks,

I'd like your opinion on which of the two functions are faster:

string& Index::Convert(string& word)
{
     string::iterator it;
     it = &word[0];
     while(it != word.end())
     {
          if(*it == '_')
          {
               *it = ' '; // converts '_' into a space.
          }
          ++it;
     }
     return word;
}

VERSUS:

string& Index::Convert(string& word)
{
     double wordlength = word.length();
     double count = 0;
     for (double i = 0; i<=wordlength; i++)
     {
          if (word[i] == '_')
          {
               word[i] = ' '; //converts a '_' into a space
          }
     }
     return word;
}

I'm inclined to believe the first is faster, since it only iterates through the string once, but I'm not sure.