Sunday, October 30, 2011

Program to Test if a Word is a Palindrome.

Problem: "A palindrome is a word that reads identically backward and forward, such as level or noon. Write a predicate method isPalindrome(str) that returns true if the string str is a palindrome. In addition, design and write a test program that calls isPalindrome to demonstrate that it works. In writing the program, concentrate on how to solve the problem simply rather than how to you make your solution more efficient." (Roberts ch 10, problem 6.)

What it looks like:
/* File: PalindromeTester
 * ----------------------------
 * This program lets a user enter in a string, and will respond with confirmation that the word is
 * or is not a palindrome. It uses a private boolean method isPalindrom(). This method goes 
 * halfway down each character of the word and compares to see whether that character and the
 * character at the mirror position on the string equal each other. If they don't, then break and 
 * returnn false, but if they do for all letters in the first half of the string, then return true.

import acm.program.*;
public class PalindromeTester extends ConsoleProgram {
    public void run(){
        String str = readLine("Is it a palindrome? Enter string to test here:");
        if (isPalindrome(str)) {
            println("Yes it is!");
        else println("No it isn't.");

    private boolean isPalindrome(String str) {
        for (int i = 0; i < str.length()/2; i++) {          
            if (str.charAt(i) != str.charAt(str.length()-i-1)) {          
                return false;
        return true;

What made it tough:

When I first wrote this, I had a bug. When checking to see if a word is a palindrome, the general approach is, for all charachters "i", to compare the "ith" character with its mirror character and see if they are equal, for all characters "i". So if your word is "kook", you'd compare the first last one to see if the characters are not the same (break if they're not), see if the two middle ones are the same, and so on.

In my buggy version, this is how I did the mirror check:
"if (str.charAt(i) != str.charAt(str.length()-i))"

This is the right version:
"if (str.charAt(i) != str.charAt(str.length()-i-1))"

Why do you have to subtract the extra 1 when you're trying inspect "i"'s mirror? In short because the "str.length" method counts starting at 1 when it returns how many characters a string contains, and meanwhile the charAt(i) method returns the "ith" character in a string, but counting as if the first letter of that string is 0. So because of this, the character at the second half of the word was actually always one after it should have been.

The funny thing is, when I wrote this and ran into the error, it didn't manifest itself via a warning in the Eclipse editor, or as red text showing up in my program's response. Instead the problem killed my program completely. I'd click to run the program, it would let me load up the interface for the user and enter in a word to test, and then instead of returning a result it would abort.

I'm sure there's a reason why my program reacted to the error in that particular way, but I'm not sure what that is. Thoughts?


  1. When your program compiles, it doesn't know how big the string its gonna be checking is. Therefore, accessing a character that is out of bounds of the string is referred to as a runtime error, because it can only occur during the run of your program. Runtime errors can often occur because of trying to manipulate data that is provided to you in a way that is illegal.

    Compile time errors are errors it can detect in your program that will occur regardless of any input provided. So, this method returns a String but, you're returning an int. The compiler can detect this and throw it's hands up at that point and go "NO GOOD!"

  2. Josh is right: the big difference between runtime errors and compile errors is whether the compiler can detect them before running it. Compile errors usually have to do with the types of values being passed into and returned from methods, or whether a class or method exist. Runtime errors usually have to do with the values being passed around, generally not the types.

    In your case, it was a runtime error because you called the charAt method on the String class with an argument value that was out of range. While the compiler was able to reason that charAt only takes integer values, and you were passing str.length()-i, which will definitely be an integer, the compiler cannot reason about the value of that integer.

    For example, when you enter the word "noon", str.length() is equal to 4 and on the first iteration (i=0), you asked for str.charAt(str.length()-i) which is str.charAt(4) but the 4 characters in "noon" are at positions 0, 1, 2, 3 so charAt threw an exception because the position argument was out of range, but the compiler couldn't know the length of the string until you ran the program. Hence, it must be a runtime error.

    Haskell has something called "quick check" which will run your program with "random" (i.e., arbitrary, but carefully chosen) inputs and attempt to detect exactly these kind of errors. The carefully chosen values are usually boundary values, because they often cause these errors. In your case, the boundary values given to charAt would be -1, 0, 1 (to test the beginning of the string) plus str.length() and str.length()-1 (to test the end).

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