Comp 410, Program 2, isPalindrome method

The method header omitted in all code examples.


1)  Two sample solutions 


	int left = 0;
	int right = str.length()-1;
	while( left < right ) {
		if( str.charAt(left) != str.charAt(right))
			return false;
		left++;
		right--;
	}
	return true;


The following is more concise, but less clear and I'm not
sure about the mixing of 'int' and 'double' in determining
the midpoint of the array

	for(int index = 0; index < 0.5*(str.length()); index++ )
		if(str.charAt(index) != 
		   str.charAt(str.length()-1-index))
			return false;
	return true;




2) There is a subtle but jarring asymmetry in the following

	int r = str.length()-1;
	for(int l = 0; l < r; l++ ) {
		if( str.charAt(l) != str.charAt(r))
			return false;
		r--;
	}
	return true;

Note that the indices l and r have exactly the same function
but are delcared, initialized and updated differently.
Code should be symmetrical.  Compare to the symmetrical code

	int l = 0;
	int r = str.length()-1;
	while( l < r ) {
		if( str.charAt(l) != str.charAt(r))
			return false;
		r--;
		l++;
		
	}

Which was written by a master programmer?


As an aside, we don't need an 'else' clause because of the return
statement. However, writing this as 

		if( str.charAt(l) != str.charAt(r))
			return false;
		else {
			l++;
			r--;
		}

is just fine. Its a matter of personal style.



3)  This is correct BUT

	int l = 0;
	int r = str.length()-1;
	boolean palindrome = true;
	while( l < r ) {
		if( str.charAt(l) != str.charAt(r))
			palindrome = false;
		else {
			l++;
			r--;
		}
	}	

	// So far, so good but now comes

	if(palindrome)  SOP("Is plaindrome");
	else SOP("Is not a palindrome");
	return palindrome;

Who said anything about printing?  Thats is not the responsibility
of this method. Suppose (a big stretch) that WORD not only checked
the spelling of each word in a document but used the above code
to check if each word is a palindrome?


4)  The following is correct but what needs to be done in the 
Finish step?

	for(int index = 0; index < str.length; index++ )
		if( str.charAt(index) !=
			str.charAt(str.length()-1-index)
		    return false;
	return true;


5)  Correct but unnecessary is

	boolean palindrome; ..........

	if(palindrome == true) return true;
	else return false;


We could simply write

	if(palindrome) return true;
	else return false;

but more to the point is

	return palindrome;

All 3 of these do exactly the same thing.


6)  Don't repeat yourself. Let your code speak for itself

		// if the characters are not the same then 
		// it is not a palindrome
		if( str.charAt(index) !=
			str.charAt(str.length()-1-index)
		    return false;


7)  I want everyone to write on the blackboard 500 times

	"I will use good names"
	"I will use good names"


Compare the following

	boolean loopRunning = true;
	while( loopRunning && left < right ) {
		if( str.charAt(index) !=
			str.charAt(str.length()-1-index)
		    loopRunning = false;
		.....

	boolean done = false;
	while( ! done && left < right ) {
		if( str.charAt(index) !=
			str.charAt(str.length()-1-index)
		    done = true;
		.....


	boolean palindrome = true;
	while( palindrome && left < right ) {
		if( str.charAt(index) !=
			str.charAt(str.length()-1-index)
		    palindrome = false;;
		.....

and all these are pretty good code. There were lots of variables
named: i1,i2,temp,x,y,........


Here is an attempt, but the code lies

	int firstChar = 0;
	int lastChar = str.length()-1;
	while( ... )
		if(str.charAt(firstChar) !=
		   str.charAt(lastChar) ) ......


Two things wrong with this:

i) At the time of the initialization, yes, they denote
the first and last characters of str, but, after initialization,
they never again are the first and last - only the one on the
left and the one on the right.

ii) More importantly, firstChar is not a character at all.  It is
an index into a String.  str.charAt(firsChar) is a character.


8)  Inexact is


	boolean palindrome = true;
	while( palindrome && left < right ) {
		if( str.charAt(index) !=
			str.charAt(str.length()-1-index)
		    palindrome = false;;
		left++;
		right--;
	}
	return palindrome;

This will get the correct answer, but if we may know the answer
to the question being asked after the very first comparison.
Why keep looping?



9) What is wrong with the following? Hint, its not the indexing.

	boolean isPalindrome = false;
	for(int index = 0; index < str.length; index++) {
		if(str.charAt(index) == 
			str.charAt(str.length()-1-index))
		   isPalindrome = true:
		else isPalindrome = false;
	}
	return isPalindrome;