Objectives

• Write code that makes use of command-line arguments.
• Experience writing a recursive solution to a problem.
• Review working with strings.
• Review reading from files.

Step 0 - Set up

• Make a lab12 directory in your 201/labs directory
• Change into your lab12 directory.
• Copy strings.dat from my pub directory into your lab12 directory
  
  cp /afs/umbc.edu/users/b/o/bogar/pub/strings.dat .
  
  
• Copy lab12.py from my pub directory into your lab12 directory
  
  cp /afs/umbc.edu/users/b/o/bogar/pub/lab12.py .
  
  

Overview

Palindromes

• According to Webster's dictionary a palindrome is defined as a word or sentence that is the same whether read forward or backward, coming from the roots palin which means back and drome which means running on.
• "Otto" is a palindrome. It doesn't matter that the first o is captialized and the last o isn't.
• "Madam, I'm Adam" is also a palindrome. Punctuation and whitespace don't matter either.

The Task

• The data file, strings.dat, contains many phrases, some of which are palindromes. Your program should accept the name of the data file as a command-line argument and read in the strings. It should determine whether a given word, phrase or sentence is a palindrome and print out the phrases that are.

strings.dat

What does strings.dat look like?

palindromes
String manipulation and recursion
Otto
Command-line arguments
Anna
file handling
Able was I ere I saw Elba
import sys
Madam, I'm Adam.
Is this a letter?
List of strings?
Was it a cat I saw?
A man, a plan, a cat, a canal: Panama?

Program Outline

Pseudo Code

import sys and string

Make sure correct number of command-line args were entered, exiting if not

Open the file for reading
Read the lines in the file into a list of strings
(don't forget to strip the trailing whitespace)
Close the file

For each of the strings in the list of strings
    Make a temporary string that holds only the letters of the string
    all in upper case

    Determine if the temporary string is a palindrome.
    If it is, print its original string

lab12.py contains some functions that have been written for you, and a commented main() with no code.

You will have to write main() and the function isPalindrome(), which must be a recursive function.

Step 1 - Check number of Command-line Arguments

• In main(), make sure the number of command-line arguments is 2
• If not, print specific running instructions and exit.

Step 2 - Get the list of strings

• Call getListOfStrings() passing it the name of the input file
• It will :
  • open the file,
  • read the lines,
  • strip them of additional whitespace,
  • close the file
  • and return the original list of strings.
  I called the original list of strings origList.

Step 3 - Construct tempStr

• Make a call to the function purgeString() passing it one of the strings in origList
• It will remove all characters from the string except the letters and will make them all capitals.
• The function will return tempStr

Step 4 - Write isPalindrome()

• Now write a function called isPalindrome() which takes tempStr and two indices, left and right, as arguments.
• isPalindrome() must be a recursive function. It must call itself with different indices each time. Since it is a predicate function, it will return either True or False
• How does it work ?
  • Here's the tempString for the original string "Madam, I'm Adam"
    
    "MADAMIMADAM"
  • The first time isPalindrome() is called, it's passed 0 as left and size - 1 as right
  • Since the letters at these indices are the same (they're both M), this phrase might be a palindrome and we should check the letters that are in one more from each end. We do this by calling isPalindrome() again, this time with those new indices for left and right.
  • If it comes to the point where the two letters are not equal, we can just return False.
  • Otherwise, how do we know when we are done and have identified a palindrome ?
    What is the relationship of left and right ?

Sample Output

linuxserver1.cs.umbc.edu[138] python lab12.py strings.dat
The palindromes in the file are:
Otto
Anna
Able was I ere I saw Elba
Madam, I'm Adam.
Was it a cat I saw?
A man, a plan, a cat, a canal: Panama?
linuxserver1.cs.umbc.edu[139]