Question

(Palindromes) A palindrome is a sequence of characters that reads the same backward as forward. For example, each of the following five-digit integers is a palindrome: 12321,55555,45554 and \(11611 .\) Write an application that reads in a five-digit integer and determines whether it's a palindrome. If the number is not five digits long, display an error message and allow the user to enter a new value.

Short answer

The application should first validate that the input has five digits. Then, by converting the input to a string, the digits can be compared pairwise from the outside in. If all pairs match, the number is a palindrome.

Step-by-step solution

- Validate Input Length

Check if the entered integer has exactly five digits. If not, display an error message and prompt the user to enter a new value. This can be done by converting the integer to a string and checking the length of the string.

- Convert Integer to String

Convert the input integer to a string to easily access and compare individual digits.

- Compare the First and Last Digits

Check if the first character (digit) of the string is the same as the last character. If they are not equal, the number is not a palindrome.

- Compare the Second and Second-to-Last Digits

Check if the second character is the same as the second-to-last character. If they are not equal, the number is not a palindrome.

- Determine Palindrome Status

If all the corresponding digits match (Steps 3 and 4), then the integer is a palindrome. Otherwise, it is not.

Key concepts

Input Validation

Before diving into any programming task involving user input, such as checking for palindromes, it's crucial to first ensure the input is valid. Input validation is the process of verifying that user input meets specific criteria before application code processes it.

In our exercise, a five-digit integer is required for the palindrome check. Validation involves checking if the input's length exactly equates to five digits. You can achieve this in Java with a simple string conversion and then checking the length:

String inputString = Integer.toString(inputInteger);if (inputString.length() != 5) {  // Display error message}

This step helps to prevent errors that could arise from processing invalid data and informs the user about the acceptable input format. Without proper validation, our program might throw exceptions or produce incorrect results. Ensuring input validity is a best practice in programming that enhances the reliability of the program.

String Conversion

Once the input is deemed valid, the next step is to work with the integer in a way that enables us to check for the palindrome property. String conversion simplifies the comparison of individual digits.

In Java, converting an integer to a string is straightforward using the toString method of the Integer class or the String's valueOf method:

String numString = Integer.toString(inputInteger);// or String numString = String.valueOf(inputInteger);

With the integer converted into a string, each digit is now a separate character within the string. This conversion affords us the ease of accessing each digit using the string's index, which is crucial for the ensuing steps of comparing individual characters.

Character Comparison

Comparing characters is a fundamental part of determining whether an integer is a palindrome. Once our integer is represented as a string, we compare the individual characters at symmetrical positions in the sequence.

If we visualize a palindrome like '12321', we can see that comparing the first and last characters (1 and 1), followed by the second and second-to-last characters (2 and 2), will verify the palindrome property. In Java, this might look like:

if (numString.charAt(0) != numString.charAt(4) ||     numString.charAt(1) != numString.charAt(3)) {  // The number is not a palindrome}

If any of these comparisons fail, we can immediately conclude that the integer is not a palindrome. Character comparison is a simple yet effective way to identify palindromes.

Palindrome Algorithm

The palindrome algorithm is the cohesive procedure that employs string conversion and character comparison to ascertain whether a sequence of characters reads the same forward and backward.

In the context of our palindrome detection task in Java, the algorithm sequentially executes input validation, string conversion, and character comparison steps. It considers all symmetrical characters in the string representation of an integer for matching pairs. The algorithm can be expressed as a combination of the previously described actions:

String numString = Integer.toString(inputInteger);if (numString.length() == 5 &&     numString.charAt(0) == numString.charAt(4) &&     numString.charAt(1) == numString.charAt(3)) {  // It is a palindrome} else {  // It is not a palindrome}

This compact and efficient approach to determine palindromes underpins the logic of many related applications, such as data validation, cryptography, and even game development.