使用C中的递归函数测试回文结构

Question

I tried to write the program for testing a string if it's a palindrome or not, but I always get the output as it's not one. What's wrong with my code?

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

int is_palindrome(int start, int end, char *str)
{
    if (str[start] != str[end])
        return 0;
    else if (start == end)
        return 1;
    else
        return is_palindrome(++start, --end, str);

    return 0;

}
int main()
{
    char str[20];
    int length,start=0,end=length-1;
    int result;
    printf("Enter the String.\n");
    fgets( str, sizeof( str ), stdin );
    length = strlen(str);

    if(is_palindrome(start,end,str))
        printf("It's a palindrome!\n");
    else
        printf("It's not a palindrome! \n");
    return 0;
}

Answer 1

What happens when ++start and --end pass each other?

else if (start == end)

Should be >= .

Answer 2

You have two main issues,

1) You are initializing end using length without first initializing length :

length = strlen(str);
/* initialize end here */

2) You are not considering the newline you get at the end of the string from fgets :

end = length - 2; /* don't include the newline */

Answer 3

In this is_palindrome() function you must need to check it otherwise it will not work for the even number character of palindrome word

if(start>end)
    return 1;

Answer 4

On the line with if(start==end) there is logical error.

This is caused by the last recursive call, where the value of last and end will always be same ie they both will be at the center of the array. Consequently the function is_palindrome() will always return 1 and the output will always be It's a palindrome!