Selecting the longest of two strings using pointers

Question

I am currently studying for an exam and have come across the following question in a past paper:

"Write a function that accepts two strings. Count the number of characters in each, and return a pointer to the long string. You may extend the provided function to complete the solution:"

main()
{
    return 0;
}

This is what I have so far, I am pretty sure I am close but it does not print anything to the stdout, I have also had this problem with other questions so if anyone could point me in the right direction it would help me a lot!

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

int main(void)
{
    printf("The longest is %s", longest("HELLO", "HELLOO"));
    return 0;
}

int longest(char *string1, char *string2)
{
    char longer;
    if (strlen(string1)>strlen(string2)){
        longer=*string1;
    }
    else {
        longer=*string2;
    }
    return longer;
}

Answer 1

You're so close!

Your longest() function intends to return a string, but it's actually returning a single character, and this is where the wheels all fall off.

Instead we redefine the longer variable from char to char *, which is a pointer to a string. Now it's going to return the longer of the two strings.

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

char *longest(char *, char *);

int main(void)
{
    printf("The longest is %s\n", longest("HELLO", "HELLOO"));
    return 0;
}

char * longest(char *string1, char *string2)
{
    char *longer;
    if (strlen(string1) > strlen(string2)){
        longer = string1;
    }
    else {
        longer = string2;
    }
    return longer;
}

This version also adds a bit of housekeeping to declare the longest function before it's used so the compiler knows the types involved.

Turns out you can simplify the function a bit:

char *longest(char *s1, char *s2)
{
    return strlen(s1) > strlen(s2) ? s1 : s2;
}

This version, like yours, decides arbitrarily that if the strings are actually the same length, it gives the win to the first string - it's not clear if this matters to you or not.

EDIT @Vlad has pointed out that this solution does not properly deal with const char * pointers, and of course he's correct. I intentionally left this out because I was not writing a generally-useful function and believe this beyond what the OP was asking about.

But Vlad's right, so I'm including his version here:

char * longest( const char *s1, const char *s2 )
{
    return ( char * )( strlen( s1 ) < strlen( s2 ) ? s2 : s1 );
}

This is one of those wiggy areas in C where it's hard to do it really well: the function longest() does not actually modify any of the pointed-to strings, so it's able to easily handle these const strings, but when it returns one of them, it's removed the const-ness from the pointer, and this is really the best you can do: other library functions work this way.

It would be nice if C had function overloading (one version for const, one for not), or some special unicorn keyboard that carried the const-ness of the parameters to the return value, but - alas - it does not.

This is a more advanced topic than the OP probably cares about, but it's worth noting for the wider audience. Thanks Vlad.

Answer 2

Solution:

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

const char * longest(const char *string1, const char *string2)
{
    const char * longer;
    if (strlen(string1)>strlen(string2)){
        longer=string1;
    }
    else {
        longer=string2;
    }
    return longer;
}

int main(void)
{
    printf("The longest is %s", longest("HELLO", "HELLOO"));
    return 0;
}

Problems in your code:

1. In C, you have to declare (not necessarily implemented, but the signatures need to be defined already) the function before referring to it in your code. See this link;

2. Your return type is int rather than pointer to string requested by the assignment. Therefore, both variable longer and the return type of function longest need to be of type char *.

Good to have:

• Using const assures the caller of the function that no modifications to the strings would be made.

Answer 3

In the assignment there is written

...and return a pointer to the long string.

However your function has return type int and within the function there are assignments that do not make sense

char longer;
//…
longer=*string1;
//…
return longer;

And the function must be declared before its usage.

The function can look the following way

char * longest( const char *s1, const char *s2 )
{
    return ( char * )( strlen( s1 ) < strlen( s2 ) ? s2 : s1 );
}

Here is a demonstrative program.

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

char * longest( const char *s1, const char *s2 )
{
    return ( char * )( strlen( s1 ) < strlen( s2 ) ? s2 : s1 );
}

int main(void) 
{
    char *s1 = "HELLO";
    char *s2 = "HELLOO";

    puts( longest( s1, s2 ) );

    return 0;
}

Its output is

HELLOO

Answer 4

You need to fix several issues with longest function:

int longest(char *string1, char *string2)   // 1) why return int ?
{
    char longer;                            // 2) why char? it should be pointer
    if (strlen(string1)>strlen(string2)){
        longer=*string1;                    // 3) this is just one char, rather than a string
    }
    else {
        longer=*string2;
    }
    return longer;
}

Here's a sample that fixes these issues:

const char* longest(const char *string1, const char *string2)
{
    const char *longer;
    if (strlen(string1)>strlen(string2)){
        longer=string1;
    }
    else {
        longer=string2;
    }
    return longer;
}