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Chapter 17: Problem 11

Write a program that uses a stack to determine whether a string is a palindrome (i.e., the string is spelled identically backward and forward). The program should ignore spaces and punctuation.

Short Answer

Expert verified
Check if the cleaned, lowercase string matches its reverse using a stack.

Step by step solution

01

Understand the Problem

We need to determine if a given string is a palindrome by using a stack. A palindrome is a word that reads the same forward and backward, ignoring spaces and punctuation.
02

Prepare Input String

First, transform the string by removing any spaces and punctuation. Also, convert all characters to the same case (lowercase) to ensure case insensitivity. This can be achieved using Python functions `str.lower()`, `str.isalnum()`, and `str.replace()`, or equivalent.
03

Initialize and Populate the Stack

Create an empty stack (using a list in Python) and traverse the prepared string. Push each character onto the stack. This will help us easily reverse the string as stacks operate on a LIFO (Last In First Out) basis.
04

Build the Reverse String

Initialize an empty string for storing the characters popped from the stack. Pop each character from the stack and append it to this empty string. By the end of this loop, you will have a reversed version of the input string.
05

Compare the Input and Reversed Strings

Finally, compare the original prepared string and the reversed string obtained from the stack. If they are identical, the string is a palindrome.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Palindrome
A palindrome is an interesting concept in computer science and mathematics. At its core, a palindrome is a sequence of characters which reads the same forward and backward. For example, the words "radar," "level," and the number "1221" are palindromes. To determine a palindrome, one must ignore certain characters like spaces, punctuation marks, and often even the case of letters. This makes the task of checking a palindrome a little more nuanced.
To check if a given string is a palindrome, you can transform the string by stripping out these ignored characters and then seeing if it is identical to its reverse. In programming, palindromes are a common exercise that helps students practice string manipulation and logical thinking. Finding palindromes has several real-world applications as well, such as in DNA sequencing and data validation.
Stack Data Structure
In the context of programming, a stack is a fundamental data structure. Stacks operate on a Last In, First Out (LIFO) principle. This means that the last element added to the stack will be the first one to be removed. Imagine a stack of plates; you can only take the top plate off the stack.
Stacks are especially useful for reversing items or backtracking problems. When checking if a string is a palindrome, a stack can assist by allowing characters to be reversed very efficiently. As each character is added to the stack, it can later be removed in reverse order, which makes it a handy tool for this kind of problem. This principle can be utilized across many different types of problems in code, making understanding and mastering stacks a worthy endeavor.
String Manipulation
String manipulation is a central theme in many programming tasks. It refers to changing, parsing, or analyzing strings of text in various ways. For the palindrome detection problem, string manipulation includes cleaning and preparing the string. This process involves:
  • Lowercasing all characters to ensure consistency.
  • Removing or ignoring spaces and punctuation, which aren't relevant for checking palindrome properties.
  • Using built-in string methods to automate these tasks.

Understanding string manipulation is crucial because text processing is a common endeavor in software development, affecting everything from user input validation to data formatting for output.
String Comparison
Comparing strings is another vital operation in software development. In this context, once a string and its reversed version have been prepared, they need to be compared. This comparison sees if both strings are identical after all ignored characters have been removed.
There are several ways to compare strings in programming. Most languages offer built-in methods to do this efficiently, even considering different intricacies like case sensitivity.
Effective string comparison is at the heart of many coding tasks such as search functionality, validation, and logic decisions in algorithms. Recognizing when and how to compare strings properly ensures that a program behaves as expected.

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Most popular questions from this chapter

In Exercises \(7.34-7.35,\) we introduced Simpletron Machine Language \((\mathrm{SML}),\) and you implemented a Simpletron computer simulator to execute programs written in SML. In this section, we build a compiler that converts programs written in a high-level programming language to SML. This section "ties" together the entire programming process. You will write programs in this new high-level language, compile them on the compiler you build and run them on the simulator you built in Exercise \(7.35 .\) You should make every effort to implement your compiler in an object-oriented manner. (\text { The Simple Language })\( Before we begin building the compiler, we discuss a simple, yet powerful high-level language similar to early versions of the popular language BASIC. We call the language Simple. Every Simple statement consists of a line number and a Simple instruction. Line numbers must appear in ascending order. Each instruction begins with one of the following Simple commands: rem, input, 1 et, print, goto, if/goto or end (see Fig. 17.22 ). All commands except end can be used repeatedly. Simple evaluates only integer expressions using the \)+,-, *\( and / operators. These operators have the same precedence as in Java. Parentheses can be used to change the order of evaluation of an expression. Our Simple compiler recognizes only lowercase letters. All characters in a Simple file should be lowercase. (Uppercase letters result in a syntax error unless they appear in a rem statement, in which case they are ignored.) A variable name is a single letter. Simple does not allow descriptive variable names, so variables should be explained in remarks to indicate their use in a program. Simple uses only integer variables. Simple does not have variable declarations - merely mentioning a variable name in a program causes the variable to be declared and initialized to zero. The syntax of Simple does not allow string manipulation (reading a string, writing a string, comparing strings, and so on \)) .$ If a string is encountered in a Simple program (after a command other than rem), the compiler generates a syntax error. The first version of our compiler assumes that Simple programs are entered correctly. Exercise 17.29 asks the reader to modify the compiler to perform syntax error checking.

Write a program that inserts 25 random integers from 0 to 100 in order into a linked-list object. The program should calculate the sum of the elements and the floating- point average of the elements.

Write a program that inputs a line of text and uses a stack object to print the words of the line in reverse order.

Write class PostfixEvaluator, which evaluates a postfix expression such as $$62+5=84 /-$$ The program should read a postfix expression consisting of digits and operators into a String- Buffer. Using modified versions of the stack methods implemented earlier in this chapter, the pro- gram should scan the expression and evaluate it (assume it is valid). The algorithm is as follows: a) Append a right parenthesis ')' to the end of the postfix expression. When the right- parenthesis character is encountered, no further processing is necessary. b) When the right-parenthesis character has not been encountered, read the expression from left to right. If the current character is a digit, do the following: Push its integer value on the stack (the integer value of a digit character is its value in the computer鈥檚 character set minus the value of '0' in Unicode). Otherwise, if the current character is an operator: Pop the two top elements of the stack into variables x and y. Calculate y operator x. Push the result of the calculation onto the stack. c) When the right parenthesis is encountered in the expression, pop the top value of the stack. This is the result of the postfix expression. [Note: In b) above (based on the sample expression at the beginning of this exercise), if the operator is '/', the top of the stack is 2 and the next element in the stack is 8, then pop 2 into x, pop 8 into y, evaluate 8/2 and push the result, 4, back on the stack. This note also applies to operator '-'.] The arithmetic operations allowed in an expression are: \+ addition \- subtraction * multiplication / division ^ exponentiation % remainder The stack should be maintained with one of the stack classes introduced in this chapter. You may want to provide the following methods: a) Method evaluatePostfixExpression, which evaluates the postfix expression. b) Method calculate, which evaluates the expression op1 operator op2. c) Method push, which pushes a value onto the stack. d) Method pop, which pops a value off the stack. e) Method isEmpty, which determines whether the stack is empty. f) Method printStack, which prints the stack.

Write a program that merges two ordered list objects of integers into a single ordered-list object of integers. Method merge of class ListMerge should receive references to each of the list objects to be merged and return a reference to the merged list object.
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