[k24bfan]

I just want to know what is wrong with my implementation of my palindrome program. I got it working for simple cases such as "civic" but I also want it to ignore white space,punctuation, and numbers. I am trying to use isalpha(). Also I am trying to properly return the [right] index value if its not a palindrome. Its not working with cases such as "Red rum, sir, is murder!"
Any help is appreciated thanks

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

int palindrome(char * s){
	int right;
	right = (int)strlen(s)-1;
	int left =0;
	
	while(left<right){
		while(!isalpha(s[left])){left++;}
		while(!isalpha(s[right])){right++;}
		if(toupper(s[left])!=toupper(s[right])){return right;}
				
				
				left++;
				right--;}
	return 0;
	}
	
	int main(){
		printf("%d",palindrome("racecar")); return 0;
	}

[JackOfAllTrades]

I would consider copying the string without the offending characters into a new string, and use that to test for the palindrome.

[k24bfan]

Yes that is what I original approach but I was told that was not needed.
So I should just build a function that filters through the string and then check if its palindrome.

[baavgai]

At first glance it looked like it would work. Aside from the fact the boolean is reversed, your s[right] is completely going the wrong direction. And indent style might help.

int palindrome(const char *s){
	int right = strlen(s)-1;
	int left = 0;
	
	while(left<right){
		while(!isalpha(s[left])){left++;}
		while(!isalpha(s[right])){right++;} // think about this
		if(toupper(s[left])!=toupper(s[right])){return right;} // true?  Are you sure?  Don't get too cute.
		left++;
		right--;
	}
	return 0; // if you get here, 1 would be a reasonable result
}

You still run the risk of (left<right) being true after one of those whiles.

[k24bfan]

baavgai, on 10 October 2012 - 03:33 PM, said:

At first glance it looked like it would work. Aside from the fact the boolean is reversed, your s[right] is completely going the wrong direction. And indent style might help.

int palindrome(const char *s){
	int right = strlen(s)-1;
	int left = 0;
	
	while(left<right){
		while(!isalpha(s[left])){left++;}
		while(!isalpha(s[right])){right++;} // think about this
		if(toupper(s[left])!=toupper(s[right])){return right;} // true?  Are you sure?  Don't get too cute.
		left++;
		right--;
	}
	return 0; // if you get here, 1 would be a reasonable result
}

You still run the risk of (left<right) being true after one of those whiles.

changed right++ to right--;

if(toupper(s[left])!=toupper(s[right])){return right;}

What is wrong with it? Purpose is to return the index from the right side is not a palindrome(ignoring spaces, and punctuation). I tested it with some larger strings it seems to work. Thanks for the help.

[baavgai]

k24bfan, on 10 October 2012 - 06:57 PM, said:

What is wrong with it? Purpose is to return the index from the right side is not a palindrome(ignoring spaces, and punctuation).

It does depend on you expectations of the result value. The position at which you fail, at the tail, is one type of result.

However, if you expect a boolean result, 0 is false, !0 is true, and your result would seem counter intuitive.