← CS50 Problems

Problem 3

Palindromes

Count how many words in a sentence read the same forwards and backwards.

The word "racecar" with a faded, upside-down reflection of itself underneath, like a word reflected in still water racecar racecar
Racecar reads the same forwards and backwards, like its own reflection.

Background

A palindrome is a word that reads the same forwards and backwards, like “mom” or “racecar.” In this problem, you’ll write a program that reads in a sentence and counts how many of its words are palindromes.

This problem was originally written for this class’s C track by a past student, and is adapted here for Python.

This is just a single-word checker to build your intuition. Your program needs to handle a whole sentence at once, which is a bit more work; see the specification below.

Getting Started

Log into cs50.dev, click on your terminal window, and run:

cd
mkdir palindromes
cd palindromes
code palindromes.py

That creates a new folder called palindromes, moves into it, and opens a new, empty file called palindromes.py for you to edit.

Starter Code

Paste this into palindromes.py to start from. clean_word and is_palindrome are stubbed out with pass and doctest examples; run python3 -m doctest palindromes.py from inside your palindromes folder to check them. They’ll fail until you replace pass with real code.

main is left as a comment on purpose, same as it was in Lineup: the sentence handling is its own job, and a docstring can’t check a program that reads input the way it can check a pure function.

# Student Initials:
"""
Palindromes

Slug: porttack/cs50/problems/py/palindromes
Doctests: python3 -m doctest palindromes.py
"""


def main():
    # Read a sentence with input(), then split it into words. Call
    # clean_word on each word. Skip any cleaned word shorter than 2
    # letters. Count the words that are palindromes, then print the
    # count.
    pass


def clean_word(word):
    """
    Returns word in lowercase with everything that isn't a letter
    removed.

    >>> clean_word("Civic.")
    'civic'
    >>> clean_word("don't")
    'dont'
    """
    pass


def is_palindrome(word):
    """
    Takes a word that is already lowercase and letters only. Returns
    True if it reads the same forwards and backwards, False otherwise.

    >>> is_palindrome("racecar")
    True
    >>> is_palindrome("cat")
    False
    """
    pass


if __name__ == "__main__":
    main()

Specification

Implement a program, in palindromes.py, that reads a sentence from the user and prints how many of its words are palindromes.

  • Prompt the user for a sentence with input().
  • A word counts as a palindrome if it reads the same forwards and backwards, ignoring case. Mom and mom are the same word for this purpose.
  • Ignore any character that isn’t a letter. civic. and kayak? should be treated as civic and kayak, and don't as dont.
  • A single letter, like a or I, does not count as a palindrome, even though it trivially reads the same both ways. Only words of two or more letters count.
  • Words are separated by whitespace.
  • Print the total number of palindromic words in the sentence, followed by a newline.
  • is_palindrome must compare characters by index, working from both ends of the word toward the middle, using a loop. It must not build a reversed copy of the word, so no slicing, reversed(), or join. check50 can’t test for this, but your code will be read for it.

Usage

Your program should behave per the examples below. The underlined text is what a user has typed.

$ python palindromes.py
Sentence: My mom has a cat!
1

$ python palindromes.py
Sentence: Do you want to kayak with my mom and me?
2

$ python palindromes.py
Sentence: no palindromes here
0

Hints

  • .split() breaks a sentence into a list of words, wherever there’s whitespace.
  • A word’s punctuation isn’t part of the word, so you’ll need to strip it off before comparing. str.isalpha() can help you check which characters are letters and which aren’t.
  • Every character in a word has an index, and you can ask for one with word[i]. The first character is at index 0. The last character is at len(word) - 1. Which other pair of characters would have to match in a palindrome?
  • Converting each word to the same case, with .lower(), takes care of “Mom” vs. “mom” for you.

To Get Full Credit

It’s more important that you submit a working solution than that you do everything below. Submit early, then keep improving and resubmit as many times as you like.

  • Fill in clean_word and is_palindrome with real code, and add one more >>> example of your own to each docstring covering a case not already shown (a word with an even number of letters for is_palindrome, for instance). Don’t try to doctest main; a program that reads input doesn’t fit the format.
  • At the end of the program, add a comment describing any challenges you ran into or what you’d improve if you did this again. A sentence or two is fine.

Style and Submission

Run these one at a time, from inside your palindromes folder.

Check your style:

style50 palindromes.py

Check your correctness:

check50 porttack/cs50/problems/py/palindromes

Submit your work:

submit50 porttack/cs50/problems/py/palindromes

Glossary

  • string — A type that represents sequences of characters.
  • substring — A contiguous portion of a string.
  • index — An integer value used to select an item in a sequence, such as a character in a string. In Python indices start from 0.
  • boolean expression — An expression whose value is either True or False.