C程序检查字符串是否为回文

Question

i have some struggles with my C program! It should check wether a string is a palindrome or not! it should not pay attention to non-alphabetic characters, so the program should recognize this as a palindrome! "He lived as a devil, eh?" That's what i got so far:

#include <stdio.h>
#include <stdlib.h>

int main()
{
    char sentence[39];
    int left = 0;
    int right = 40;

    printf("Enter a message: ");
    fgets(sentence, 40, stdin);

    while(1) {

        while(left < right && !(isalpha(sentence[left])))
            left++;
        while(right > left && !(isalpha(sentence[right])))
            right--;

        if(left >= right)
            break;

        else {

            if(sentence[left] != sentence[right]) {
                printf("Not a Palindrome");
                return 0;
            }

            left++;
            right--;
        }
    }

    printf("Palindrome");

    return 0;
}

It's always printing: NOT A PALINDROME! Even if it is one.

Answer 1

I have made a few changes to your program. Firstly not breaking the array indexing, next using the string length instead of accessing undefined values, third checking same case letters.

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

int main()
{
    char sentence[200];                             // provide plenty of room
    int left = 0;
    int right;                                      // do not assume the length

    printf("Enter a message: ");
    fgets(sentence, sizeof sentence, stdin);        // limit the input
    right = strlen(sentence);                       // now get the length

    while(1) {
        while(left < right && !(isalpha(sentence[left])))
            left++;
        while(right > left && !(isalpha(sentence[right])))
            right--;
        if(left >= right)
            break;
        else {
            if(toupper(sentence[left]) != toupper(sentence[right])) {  // get case the same
                printf("Not a Palindrome\n");
                return 0;
            }
            left++;
            right--;
        }
    }

    printf("Palindrome\n");
    return 0;
}

Program sessions:

Enter a message: He lived as a devil, eh?
Palindrome

Enter a message: palindrome
Not a Palindrome

Answer 2

You could write a separate function that checks whether the inputted sentence is a palindrome.

As for your code then these statements

char sentence[39];
int left = 0;
int right = 40;

printf("Enter a message: ");
fgets(sentence, 40, stdin);

lead to undefined behavior because the array sentence has only 39 elements while you are trying to enter 40 characters. Also the entered string can have kess than 40 characters. You need to determine the length of the string.

Here is a demonstrative program that shows how a corresponding function can be written.

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

int is_palindrome(const char *s)
{
    size_t n = strlen(s);

    const char *first = s, *last = s + n;

    if (n)
    {

        do
        {
            while ( *first && !isalpha((unsigned char)*first)) ++first;
            if (first != last)
            {
                while (!isalpha((unsigned char)*--last));
            }
        } while ( toupper( ( unsigned char )*first ) == 
                  toupper( ( unsigned char )*last ) && 
                  first != last && 
                  ++first != last);
    }

    return first == last;
}

#define N   100

int main()
{
    while (1)
    {
        char s[N];

        printf("Enter a sentence (Enter - exit): ");

        if (!fgets(s, sizeof(s), stdin) || s[0] == '\n') break;

        printf("\nThe sentence is%s palindrome.\n\n",
            is_palindrome(s) ? "" : " not");
    }

    return 0;
}

Its output might look like

Enter a sentence (Enter - exit): He lived as a devil, eh

The sentence is palindrome.

Enter a sentence (Enter - exit):

Answer 3

You should initialize the right as the end of the string:

#include <string.h>

// ...

    right = strlen(sentence) - 1;