Problem 3

Palindromes

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

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.

Try a word:

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.

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 punctuation attached to a word. civic. and kayak? should be treated as civic and kayak.
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.

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.
Python can reverse a string with slicing: word[::-1].
Converting both a word and its reverse to the same case, with .lower(), takes care of “Mom” vs. “mom” for you.

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.