can someone help me figure out this recursive algorithm?

[david]

Recursive algorithm for an integer
If numDigiths <= 1 return true
Else check first and last digits  num/10 numDigits-1 and num % 10
if they do not match return false
If they match, check more digitsApply algorithm recursively to:num % 10 numDigits-1 and numDigits - 2

what is the nuumdigits-1 after the 10.

[Alvaro]

Else check first and last digits  num/10 numDigits-1 and num % 10
[...]
what is the nuumdigits-1 after the 10.

It should be "num/(10^(numDigits-1))", where the ^ means "raised to a power", not "xor". That's just how to define what "the first digit" is. For example, take num=7652. numDigits=4. 10^(numDigits-1)=10^3=1000. num/1000 = 7652/1000 = 7 (if the division is truncating, as it normally is in C/C++).

Good luck.

[david]

would this be correct

Code: Select all

bool palindrome(int z)
{
   int firstdigit = 0;
   int lastdigit = 0;

   if (digitcounter(z) <= 1)
   {
      return true;
   }
   else
   {
      firstdigit = z /= (power(10,digitcounter(z) - 1));
      lastdigit = z % 10;
      if(firstdigit != lastdigit)
      {
         return false;
      }
      else
      {
         palindrome((z % power(10,digitcounter(z)-1) / (10 + digitcounter(z) -2)));
      }
   }
}

[Alvaro]

You don't need to put things in an `else' block if the body of the `if' ends in a `return'.

Where you said `/=' it should just be `/'.

The recursive call should be `return palindrome(...'.

You are calling `digitcounter' many times. Just call it once at the beginning and save the result in a local variable.

You don't need to initialize local variables to values that are never used. Also, you can delay the declaration until the place where they are used.