How do I return a char from a char pointer function in C?

Question

i recently made a function that finds the smallest character in a string. I am not sure how to return the smallest character in a char pointer function.

#include <stdio.h> 
#include <string.h>

char * smallest(char s[]) 
{
    char  small = 'z';

    int i = 0;

    while (s[i] != '\0')
    {
        if (s[i] < small)
        {
            small = s[i];
        }

        i++;
    }

    return small;
}


int main(void)
{
    char s[4] = "dog"; 
    printf("%c",smallest(s));
}

Answer 1

The variable small has the type char according to its declaration

char small = 'z';

//...

return small;

and this variable is returned from the function while the function return type is the pointer type char *.

char * smallest(char s[]) 

Also if the user will pass an empty string to the function then you will try to return the character 'z' as a smallest character though this character is absent in the empty string.

I think in this case you should return a pointer to the terminating zero character '\0'.

The function can be defined the following way

char * smallest( char s[] ) 
{
    char *small = s;

    if ( *s )
    {
        while ( *++s )
        {
           if ( *s < *small ) small = s;
        }
    }

    return small;
}

Or as in C there is no function overloading then the function should be declared and defined like

char * smallest( const char s[] ) 
{
    const char *small = s;

    if ( *s )
    {
        while ( *++s )
        {
           if ( *s < *small ) small = s;
        }
    }

    return ( char * )small;
}

Pay attention to that this assert

assert(smallest(s[4] == 'd'));

is incorrect, It seems you mean

assert( *smallest( s ) == 'd');

Or after you updated your program you need to write

printf("%c\n",*smallest(s));

instead of

printf("%c",smallest(s));

Using this function you can not only to find the smallest character but also to determine its position in the source string.

For example

char *small = smallest( s );

printf( "The smallest character is '%c' at the position %tu\n", 
        *small, small - s ); 

or

char *small = smallest( s );

if ( *small == '\0' )
{
    puts( "The source string is empty" );
}
else
{
    printf( "The smallest character is '%c' at the position %tu\n", 
            *small, small - s ); 
}

Answer 2

There are two problems with your program.

1. Wrong parameters

The function smallest(char[] s)expects to be given an character array but what you are passing in as an argument is s[4] == 'd' which is not a character array. This has nothing to do with the assert() itself.

What you want to do is assert(smallest(s) == 'd').

2. Wrong return type

Your function is declares that it would return *char (= a pointer to a char) but you are trying to return a char. So you should adjust the return type of your function to be char.

The correct program:

#include <stdio.h>

#include <assert.h>
#include <string.h>

char smallest(char s[]) {
    char small = 'z';

    int i = 0;

    while(s[i] != '\0') {
    
        if (s[i] < small) {
            small = s[i];
        }
        i++;
    }
    return small;
}

int main(void) {

    char s[4] = "dog"; 

    assert(smallest(s) == 'd');
    printf("Passed\n");
}