A String is a String
In Part 8-1 we defined a string as
- A string may be an empty string.
- 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:
- The length of the empty string is 0.
- 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 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:
- To copy the empty string, copy the null byte.
- 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?
