cs151 Homework 7 Solution
Fall 1999

Addition and concatenation
--------------------------

Here is my version of the table

             boolean       char       int       String 
boolean      error	   error      error	stringCat
char	     error         int add    int add   stringCat
int          error         int add    int add   stringCat
String       stringCat     stringCat  stringCat stringCat

In other words, a String plus anything yields a String.

Other than that, you can't add booleans to anything.

The only thing in this table I don't like is the way characters
get converted to integer.


isPalindrome
------------

Here's first, last and middle:

  // first returns the first character of a string
  // first only works if s has at least one characters
  public static char first (String s) {
    return s.charAt (0);
  }

  // last returns the last character of a string
  // last only works if s has at least one characters
  public static char last (String s) {
    return s.charAt (s.length() - 1);
  }

  // middle returns a substring that includes all but the
  // first and last; only works if s has at least two characters
  public static String middle (String s) {
    return s.substring (1, s.length()-1);
  }

Here are three versions of isPalindrome for the price of one!
The first one is recursive:

  public static boolean isPalindrome (String s) {
    if (s.length() < 2) {
	return true;
    } else if (first(s) != last(s)) {
	return false;
    } else {
	return isPalindrome (middle (s));
    }
  }

Here is the first iterative one.  It uses two index
variables: i starts at the beginning of the word and
goes forward; j starts at the end and goes backwards.

We quit when they meet in the middle.  At any step
in the traversal, we compare the ith character to the
jth character.  If there are different, we know immediately
that the word in not a palindrome.  If we get all the
way to the end without seeing a mismatch, it must be
a palindrome.

  public static boolean isPalindrome2 (String s) {
      // an iterative version of isPalindrome
    int i = 0;
    int j = s.length()-1;

    while (i < j) {
      if (s.charAt(i) != s.charAt(j)) {
	return false;
      }
      i++;
      j--;
    }
    // if we get all the way through the loop and
    // never found a mismatched pair, it must be true!
    return true;
  }

The next version is also iterative, but instead of
using indices it works by whittling s down until we
get to 0 or 1 characters.  The iterative step is

    s = middle (s);

Eventually that has to result in a very short string.

  public static boolean isPalindrome3 (String s) {
      // another iterative version of isPalindrome

    while (s.length() >= 2) {
      if (first(s) != last(s)) {
	  return false;
      } else {
	  s = middle (s);
      }
    }
    return true;
  }


Names
-----

  public static boolean hasComma (String s) {
    int index = s.indexOf(',');
    return (index != -1);
  }

  public static String convertName (String s) {
    if (hasComma (s)) {
      return s;
    }
    s = s.toLowerCase ();
    int space = s.indexOf(' ');
    int len = s.length();
    String first = s.substring (0, space);
    String last = s.substring (space+1, len);
    return last + ", " + first;
  }
}