Recursion to find length of a string in C

Question

I am trying to find the length of a string by recursion using the following code:

#include <stdio.h>
int string_length(char *s, int x);

int main(void)
{
    int length = 0, x;
    char string[] = "This is a string";

    x = string_length(string, length); 
    printf("The length of the string will be: %d\n", x);
    return (0);
}

int string_length(char *c, int x)
{
    int a = 0;
    if (*c != '\0')
    {
        a = a + 1;
        string_length(c + 1, x + 1);
    }
    return (a);
}

But as I run my code, I get the following output:

The length of the string will be: 1

As it can be seen, this is not the correct length. I know that the length of a string is 16. Where did I go wrong.

I have searched for a while, and I have a hint it it has something to do with how I implemented my recursive function. How can I get around my issue?

Answer 1

For starters this function declaration

int string_length(char *s, int x);

does not make a great sense. The second parameter is redundant. Strings have the sentinel value '\0' that forms the basic case of the recursion.

The function always returns either 0 (for an empty string) or 1 because it returns the local variable a

int string_length(char *c, int x)
{
    int a = 0;
    if (*c != '\0')
    {
        a = a + 1;
        string_length(c + 1, x + 1);
    }
    return (a);
}

that does not depend on the recursive call

string_length(c + 1, x + 1);

The function can be defined the following way

size_t string_length( const char *s )
{
    return *s ? 1 + string_length( s + 1 ) : 0;
}

Pay attention to that the type int can be not large enough to be able to store the length of a string. You need to use the type size_t. It is the return type of the standard string function strlen. See the function declaration

size_t strlen(const char *s);

Also as the passed string is not changed within the function then the function parameter should be declared with the qualifier const.

In main you could write

size_t n = string_length( string ); 
printf("The length of the string will be: %zu\n", n);

Here is a demonstration program.

#include <stdio.h>

size_t string_length( const char *s )
{
    return *s ? 1 + string_length( s + 1 ) : 0;
}

int main(void) 
{
    const char *s = "Hello World!";

    printf( "The length of the string \"%s\" is %zu\n", 
            s, string_length( s ) );
}

The program output is

The length of the string "Hello World!" is 12