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 ) ;
      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 is 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 had if the character we're pointing to is no 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 do 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 them 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() studied earlier doesn't have a void return type; it actually returns the first argument.) The ... represents a recursive call. What should that line be?