Function in C to check if a string is a palindrome - c

Question

The function should check if an input string is a palindrome. It should not be case sensitive, and must ignore every other character except letters and numbers. The thing I am having problems with is when the string is empty(that means only the spaces are elements of the string), and when the string has a lot of other characters, but no letters or numbers.
This is my code, and it works well, except in the cases stated above.
#include <stdio.h>
#include <string.h>
#include <ctype.h>
int is_it_palindrome(const char* str){
int lenght;
lenght=strlen(str);
const char *start=str+0;
const char *end=str+lenght-1;
while(start<end){
if(!isalnum(*start)){
start++;
}
else if(!isalnum(*end)){
end--;
}
else if(toupper(*start)==toupper(*end)){
start++;
end--;
}
else{
return 0;
}
}
return 1;
}
int main() {
printf ("%d", is_it_palindrome(" "));
printf ("%d", is_it_palindrome("a"));
printf ("%d", is_it_palindrome(",./!\n+_[]{}#"));
printf ("%d", is_it_palindrome(",./!\n+_A[]{}#"));
return 0;
}
The function returns 0 if it is not a palindrome, and 1 if it is a palindrome. So the output here should be 0 1 0 1, but I get 1 1 1 1. I really don't know how to rewrite this program to contain the conditions that I need. I would really appreciate the help.

Answer 1

A string containing no alphanumeric characters is not a palindrome according to your rules yet it doesn't ever reach return 0; cause you skip over non alpha numeric characters.
To check it you have to add a flag that tracks if you have any alpha numeric characters at all.
Additionally to detect the single alnum char I go to start <= end instead of start < end
#include <stdio.h>
#include <string.h>
#include <ctype.h>
int is_it_palindrome(const char* str) {
int lenght;
int has_alnum = 0;
lenght=strlen(str);
const char* start = str + 0;
const char* end = str + lenght - 1;
while (start <= end) {
if (!isalnum(*start)) {
start++;
} else if (!isalnum(*end)) {
end--;
} else if (toupper(*start) == toupper(*end)) {
has_alnum = 1;
start++;
end--;
} else {
return 0;
}
}
return has_alnum;
}
int main(void) {
printf("%d", is_it_palindrome(" "));
printf("%d", is_it_palindrome("a"));
printf("%d", is_it_palindrome(",./!\n+_[]{}#"));
printf("%d", is_it_palindrome(",./!\n+_A[]{}#"));
return 0;
}