Thread: a question about recursion

I wrote a recursive function to find if a given string is a palindrome

Code:

#include <stdio.h>

int isPalindromeRec(char *start, char* end);

int main()
{
   char str[] = "abccba";

   //pointers to string
   char* start = str;
   char* end = str;

   while(*end != '\0')
	end++;

   end--;

   if(isPalindromeRec(start, end))
	printf("String is a palindrome");

   else
	printf("Not a palindrome");

   getchar();

   return 0;
}


//recursive function to check if a string is a palindrome
int isPalindromeRec(char *start, char* end)
{
   if(end<=start)
	return 1;
	
  //check if the two characters match
   if(*start==*end)
   {
	start++;
	end--;
	isPalindromeRec(start, end);
   }
   else
	return 0;
}

This code works fine.

I changed the isPalindromeRec function to

Code:

int isPalindromeRec(char *start, char* end)
{
   if(end<=start)
	return 1;
	
   else 
   {
	return(*start==*end && isPalindromeRec((start++), (end--)));
   }
}

which is the same as the function above. However, it doesnt work the same way. On debugging I noticed that the start and end pointers are not incremented inside the isPalindromeRec((start++), (end--)) call.

Why does this happen?

The postfix version of ++ and -- happen after everything else. That means the current versions are passed into the recursive call, then the local copy is incremented. That means you continually call isPalindromeRec with the original values of start and end, you never pass in the next values for them. Try ++start and ++end instead.

Ok thanks this works. I assumed that since I had brackets around the post-incr and post-decr operators, it would happen before the recursive call.

Thanks again anduril462 and itsme86

Parentheses themselves only force that part of an expression to be evaluated before other, unparenthesized parts of the expression. Each parameter is it's own expression, so i++ and (i++) behave the same. The following links might explain a bit:
Sequence point - Wikipedia, the free encyclopedia.
Question 3.8