A String is a String

In Part 8-1 we defined a string as
  1. A string may be an empty string.
  2. A string may be a character followed by a string.
Using that definition of a string we may define the length of a string as:
  1. The length of the empty string is 0.
  2. The length of a non-empty string is 1 plus the length of the string which follows the first character.
From this definition of the length, we may develop a recursive function to compute the length:
int strlen(char *s)
{
   if(*s == '\0')
      return 0;
   else
      return 1 + strlen(s + 1);
}

Here we represent the string as we did before, as a pointer to the first character. (You may not recognize this as how we've represented it before. Earlier we talked about a string being an array of characters and somewhere it must be. But also remember that any time we "pass" an array to a function, we are really passing a pointer to the beginning of the array.) So if the character we're pointing to is the null byte, then we have the empty string and we return 0 according to part 1 of the recursive definition of length. On the other hand if the character we're pointing to is not the null byte then we must have a character followed by a string according to part 2 of the definition of a string. In that case, part 2 of the length definition says that we must return 1 plus the length of the string following that character. We'll we have the function strlen() to find the length of a string. The only question is how we refer to the string that follows this character. Since we're representing the string by a pointer to its first character, we need to determine the place where the first character of the remaining string is located. If s is the address of the whole string, then s+1 must be the address of the character that follows the first one and that is, therefore, the first character of the remaining string.
Using a recursive declaration of a string, we may devise a recursive algorithm for copying a string:
  1. To copy the empty string, copy the null byte.
  2. To copy a non-empty string, copy the first character and then copy the string that follows.
We can implement this by a function like:
void strcpy(char *dest, char *src)
{
   if(*src == '\0')
      *dest = '\0';
   else {
      *dest = *src;
      ...
   }
}
(Note that the strcpy() What should that line be?