How can I convert chars(pointer) to strings in C - to be used in strcmp

Question

I am trying to create a function that takes a string as an input and compares each character to each other to ensure they don't repeat.

The string consists of 26 alpha characters The goal is to compare the 26 characters to each other to ensure that none repeat. To do that, I want to use strcmp (because I know not of any function that compares chars).

To do that, I first need to convert the chars in my code to strings in order to compare them. How can I go about doing that?

This is my function:

bool is_valid_key(string verify)
int a = 0;
    int b = 0;
    string s1 = verify[a], s2 = verify[b];
        for ( a = 0; a < verify[a]; a++)
    {
        if (strcmp( s1, s2) == 0)
        return 0;
    }
   return 1;

Try what happens with `if('a'=='a')` and `if('a'=='b')`. It is that simple. You are right however, that comparing strings (or null-terminated char sequences, to be precise) does require a function. — Yunnosch, Oct 16 '22 at 18:55
@Yunnosch - could you please elaborate further. I am a begginer at this and I am not quite following what you've written — Sizwe, Oct 16 '22 at 19:00
I provided code examples for comparing characters without a function. I thought you need that info, because you state that you are unaware of a function to do so. Try the conditionals I showed, experiment with them. You will see and then not need to turn characters into pointers to strings anymore. — Yunnosch, Oct 16 '22 at 19:02

chqrlie · Accepted Answer · 2022-10-16T19:07:48.600

You do not need to compare strings for this test, just single char values. char values can be compared with == and != so you could write 2 nested loops:

bool is_valid_key(const char *key) {
    for (int a = 0; key[a] != '\0'; a++) {
        for (int b = 0; b < a; b++) {
            // if 2 characters match, the key is not valid
            if (key[a] == key[b])
                return false;
        }
    }
    // all characters are different: key is valid
    return true;
}

The problem with this approach is you do not test the key length, nor that it is only composed of letters.

Here is a different approach:

// check that key contains 26 lowercase letters without duplicates
bool is_valid_key(const char *key) {
    unsigned long mask = 0;
    for (int i = 0; key[i] != '\0'; i++) {
        if (key[i] < 'a' || key[i] > 'z') {
            // invalid character
            return false;
        }
        // this assumes that letters are contiguous, which is true for ASCII
        unsigned long bit = 1UL << (key[i] - 'a');
        if (mask & bit) {
            // duplicate character
            return false;
        }
        mask |= bit;
    }
    // check that all 26 letters have been set
    return mask == (1UL << 27) - 1;
}

Vlad from Moscow · Answer 2 · 2022-10-16T19:19:39.027

These declarations and the following for loop

string s1 = verify[a], s2 = verify[b];
    for ( a = 0; a < verify[a]; a++)
    //...

do not make sense.

For example the declared s1 and s2 has the pointer type string (char *) while the initialized expressions have the type char.

Toy need to compare characters.

The function can look the following way

bool is_valid_key( string verify )
{
    bool unique = true;

    for ( ; unique && *verify != '\0'; ++verify )
    {
        string next = verify + 1;

        while ( *next != '\0' && *next != *verify ) ++next;

        unique = *next == '\0';
    }

    return unique;
}

If you need to check that all elements of the string are alpha symbols then you can write

#include <ctype.h>

//...

bool is_valid_key( string verify )
{
    bool valid = true;

    for ( ; valid && *verify != '\0'; ++verify )
    {
        if ( ( valid = isalpha( ( unsigned char )*verify ) ) )
        {    
            string next = verify + 1;

            while ( isupper( ( unsigned char )*next ) && *next != *verify ) ++next;

            valid = *next == '\0';
        }
    }

    return valid;
}

How can I convert chars(pointer) to strings in C - to be used in strcmp

2 Answers2