Question

I am doing online work using CodeLab for C++ and am not sure what’s wrong with my code. Here is the question:

Write a recursive, int-valued function, len, that accepts a string and returns the number of characters in the string.
The length of a string is:
0 if the string is the empty string (“”).
1 more than the length of the rest of the string beyond the first character.

And here’s my code:

int len(string s)
{
  if (s.length()==0)
    return 0;
 else
 {
    return 1+(len(s)-1);
 }
}

It says I have a run-time error.
Any help?

Thanks.

Answer 1

Well here:

     return 1+(len(s)-1);

The length of the string never decreases. So you will eventually have a stackoverflow because you never hit your base case (s.length() == 0). You need to get a substring where the length of s decreases by 1:

     return 1+(len(s.erase(0,1))); // erases 1 char from beginning then recurses

Hopefully this is purely academic, because std::string has a length method that will run in constant time. (Not to mention erasing from the front of a string is probably horribly inefficient– see the other answers that work with char *)