Calling a void function?

Question

I'm trying to write a program that can recognize a palindrome.

I've made a function for recognizing if it is a palindrome and used it in main successfully.

However, I also want to remove any spaces or numbers or special characters as those could interfere with recognizing it as one.

I've tried calling on it in main with inputstring(isalphabet) but it won't compile at all. Also tried changing isalphabet to char instead of void, and giving it return inputstring but it won't work. Any and all suggestions appreciated

#include <stdio.h>

int isPalindrome(char inputString[]);

int main() {
    char inputString[100];   
    int ret;

    scanf("%s", inputString);
    
    ret = isPalindrome(inputString);

    printf("%d", ret);
}

int isPalindrome(char inputString[]) {
    int count = 0;
    int result;

    while (inputString[count] != '\0') {
        count++;
    }

    char stcp[100];
    int i = count, j = 0;

    while (i != 0) 
        stcp[j++] = inputString[--i];

    stcp[j] = 0;

    if (strcmp(inputString, stcp) == 0) {
        result = 1;
    }
    else {
        result = 0;
    }

    return result;   
}

void isalphabet(char inputString[], char nystring[]) {
    int count, i, ny_i = 0;
    count = strlen(inputString);

    for (i = 0; i < count; i++) {
        if (isalpha(inputString[i]) != 0) {
            nystring[ny_i] = inputString[i];
            ny_i++;
        }

        nystring[ny_i] = '\0';  
    }
}

If you ask questions about build errors, then please copy-paste the *full* and *complete* error output into the question itself. Also please add a comment on the lines where you get the errors. — Some programmer dude, Oct 03 '21 at 19:56
*inputstring(isalphabet)* - did you literally call it like that? inputstring is a variable and isalphabet is a function, so it should be something like isalphabet(inputstring, nystring) — jps, Oct 03 '21 at 19:59
@notmenotyou Move this statement nystring[ny_i] = '\0'; outside the for loop. — Vlad from Moscow, Oct 03 '21 at 20:02
Side note: you need to include header `` to use `isalpha()` function; also, you need to put the prototype of `isalphabet()` (i.e.: `void isalphabet(char[], char[]);`) before `main()` as you did with `isPalindrome()`. — Luca Polito, Oct 03 '21 at 20:02
Instead of modifying and copying the string to a new string, it's probably easier to just use pointers, one at the start going forwards, one at the end going backwards, you can check `isalpha` and move them more if it's not, until the pointers collide. — Neil, Oct 03 '21 at 21:42

Vlad from Moscow · Accepted Answer · 2021-10-03T20:30:27.133

Using the auxiliary array with the magic number 100

char stcp[100];

is a bad idea. The user can pass to the function a string of an arbitrary length that can be greater than 100.

It seems what you need is something like the following.

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

int isPalindrome( const char *s, int cmp( int ) )
{
    int palindrome = 1;
    
    const char *p =  s + strlen( s );
    
    while ( palindrome && s < p )
    {
        if ( !cmp( ( unsigned char )*s ) )
        {
            ++s;
        }
        else if ( !cmp( ( unsigned char )*--p ) )
        {
            ;
        }
        else
        {
            palindrome = *s++ == *p;    
        }
    }
    
    return palindrome;
}

int main(void) 
{
    printf( "%d\n", isPalindrome( "1a12b123c1234b12345a123456", isalpha ) );
    printf( "%d\n", isPalindrome( "1a2bc3def2gh1i", isdigit ) );
    
    return 0;
}

The program output is

1
1

Calling a void function?

1 Answers1