Thread: Help debug palindrome code

Hi guys!

I am learning C and am writing code to detect palindrome to help me learn to manipulate strings and use pointers and such. I am having trouble debugging it and was hoping you might be able to help.

Code:

void removeChar( char * string, char letter );
char* palindrome(char)

int main(void) {
	char string[100];

	printf("Enter what you think is a palindrome (w/o spaces): ");
	fgets(string, sizeof(string), stdin);
	palindrome(string);
	return(0);
}

int palindrome(string) {
	char string[100];
	int n, *f, *l;					//*f and *l are pointers to the first and last indexed character in string

	f=&string[0];					//do I need the ampersand since it is an array?
	l=&string[n];
	n = strlen(string)-1;
	if(string == " ") 					//Base case for recursion
		printf("Yep! Thats is a palindrome!");

	else if(string[0] == string[n])
		palindrome = removeChar(string, *f);	//Can I use this format instead of a character in single quotes since the
		palindrome = removeChar(string, *l);   //dereferenced pointers point to a singe char?
		return(palindrome(string));			 //check to see if the next outside characters are equal
}

void removeChar( char * string, char letter ) {
	unsigned int i;

	for(i = 0; i < strlen( string ); i++ )
		if( string[i] == letter )
			strcpy( string + i, string + i + 1 );
}

I can either keep my [CODE]printf(yep thats a palindrome!) /CODE] statement and just add a return(0) and keep it declared an int or I could declare the palindrome function to return a char and use

Code:

printf("%s", palindrome(string))

. Either way works.

It seems like it doesn't matter which way I do it. Is there a safer way? Also, when would I declare the palindrome function to return char as opposed to char*? char* function() says that the function points to characters while char function() says that the function returns characters directly. So when is it appropriate to use which?

The calling function obviously has the string you're testing whether it's a palindrome or not, so there is no reason to return the string.

Also, there is no sense in returning a char from palindrome(), because no char is any more important than any other char. Either the string is a palindrome, or it isn't - which is a perfect boolean (0 or 1) type of return.

0 = not a palindrome
1 = it's a palindrome

The general Golden Rule of Returns is never to return the address of any local variable. Although it might work (probably will), depending on your system to refrain from re-using that address for a few lines of code, if very very iffy. You never know just when the local variable memory will be re-used for something else, and cause the whole program to crash ignominiously, when you try to reference that address.

I don't understand the remove_char() function. Why do you have it in your program? Imo, you need main() to get the user's string to be checked, and one function to check the string to see whether it's a palindrome or not.

And that is all you should need. Pseudo code with a bit of C format, might look like this:

Code:

include files

int is_palindrome(char string);  //prototype

int main(void) {
  declare your char array for the string to be checked
  and the int for is_palindrome to return it's value into

  get the string from the user - fgets() is a great choice
  You can either overwrite the newline in the string (fgets 
  always will put one in the string, if there is room for it), 
  or just compare to the last letter before the newline char.

  call ok = is_palindrome(arrayname). 
  if(ok)   
     print affirmative message 
  else
     print negative message 

  return 0
}
int is_palindrome(char arrayname) {
  declare left and right index integers or pointers (whichever you prefer)
  check the array[], either iteratively or via recursion, (whichever you prefer)
  return 1 if it's a palindrome, or zero if not.
}

Hope that helps, without giving it all away.

Okay so I've been working on it and broke it all down and got this which works beautifully. My next step was to add a do-while like:

Code:

	do{
	printf("Enter what you think is a palindrome (w/o spaces): ");
	fgets(string, sizeof(string), stdin);
	}while(string =! "\n");

so that it will continue to prompt the user unless they hit enter. The problem is "string" is a character array and \n I thought would be considered a character but it must be reading it as an int because my error message is that they are "incompatible types". I don't understand how to change this. Insight?

Code:

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

int main(void) {
	char string[100];
	int i, len, pal;

	printf("Enter what you think is a palindrome (w/o spaces): ");
	fgets(string, sizeof(string), stdin);

	len = strlen(string)-1;

	for(i=0; i<=len/2; i++)
		if(string[i] == string[len-1-i]) {
			pal = 0;
		}
		else {
			pal = 1;
		}
	if(pal == 0) {
		printf("Yes it is a palindrome! \n");
	}
	else if(pal == 1) {
		printf("Not a palindrome");
	}
	else {
		printf("Something is wrong");
	}
}

The next step is where I am getting stuck. To finish the program I wanted to put palindrome into a function like below:

Code:

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

int palindrome(char string);

int main(void) {
	char string[100];

	printf("Enter what you think is a palindrome (w/o spaces): ");
	fgets(string, sizeof(string), stdin);

	if(palindrome(string) == 0) {
			printf("Yes it is a palindrome! \n");
		}
		else if(palindrome(string) == 1) {
			printf("Not a palindrome");
		}
		else {
			printf("Something is wrong");
		}
}

int palindrome(char string) {
	int i, len, pal;

	len = strlen(string)-1;
	
	for(i=0; i<=len/2; i++)
		if(string[i] == string[len-1-i]) {
			pal = 0;
		}
		else {
			pal = 1;
		}
	return(pal);
}

but am getting errors that palindrome makes integer from pointer w/o cast. Does my declaration of palindrome need to be char* string to say that string points to a string?

Yes, you need to pass string like it's a string - not a single char:

Code:

int palindrome(char *string); //is your prototype
palindrome(string); //is your call to the function

char's are just a small range integer, which is why you're getting that error message.

Then, in the else statement of the palindrome, you need to have:

Code:

else {

  return 1; 
}

  return 0;

In the else part, you have to return with failure the very first time that the char checking shows the string is NOT a palindrome.

Otherwise the logic can fail if it finds a bad (non-matching) char, but followed by matching chars, in the string array being tested.

You're getting the hang of this!

I see, I see. I understand. Thank you so much for your help!

WHOOO! It works beautifully!

Code:

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

int palindrome(char *string);

int main(void) {
	char string[100];

	printf("Enter what you think is a palindrome (w/o spaces): ");
	fgets(string, sizeof(string), stdin);

	if(palindrome(string) == 0) {
			printf("Yes it is a palindrome! \n");
		}
		else if(palindrome(string) == 1) {
			printf("Not a palindrome");
		}
		else {
			printf("Something is wrong");
		}
	return(0);
}

int palindrome(char *string) {
	int i, len, pal;

	len = strlen(string)-1;

	for(i=0; i<=len/2; i++)
		if(string[i] == string[len-1-i]) {
			pal = 0;
		}
		else {
			pal = 1;
		}
	return(pal);
}