Skip to ContentGo to accessibility pageKeyboard shortcuts menu
OpenStax Logo
Introduction to Python Programming

12.3 Recursion with strings and lists

Introduction to Python Programming12.3 Recursion with strings and lists

Learning objectives

By the end of this section you should be able to

  • Demonstrate the use of recursion to solve a string problem.
  • Demonstrate the use of recursion to solve a list problem.
  • Use the built-in count() list function.

Recursion with strings

A word that is spelled the same forward and backward is called a palindrome. Ex: racecar.

Recursion can be used to identify whether a given word is a palindrome.

Checkpoint

Identifying a palindrome

Concepts in Practice

Recursion with strings

Refer to the animation above.

1.
Which of the following does palindrome() recognize as a palindrome?
  1. madamm
  2. madaM
  3. madame
2.
What would happen if the condition on line 4 of palindrome() is changed to if len(word) == 1:
  1. Only palindromes with an odd number of letters would be recognized correctly.
  2. Nothing. The function would work the same.
  3. The function would not recognize any palindromes.
3.
What would happen if the condition in line 8 palindrome() is changed to if palindrome(word.strip(word[0])).
  1. Nothing. The function would work the same: the first and last letter would be removed.
  2. Some words, such as "madamm", would be incorrectly recognized as a palindrome.

Recursion with lists

The animation below shows a recursive way to check whether two lists contain the same items but in different order.

The count() function returns a count of the number of items in a list that match the given item, and returns 0 otherwise. Ex: For list_num = [1, 3, 3, 4], list_num.count(3) returns 2.

Checkpoint

Checking list permutations

Concepts in Practice

List permutations

Refer to the above animation. What would permu_check() return for each pair of lists below?

4.
list_num = [1, 7, 99, 2]
other_list = [99, 7, 1, 2]
  1. True
  2. False
5.
list_num = [22, 9, 15, 17, 15]
other_list = [17, 9, 22, 22, 15]
  1. True
  2. False
6.
honors_list = ["Bo", "Joe", "Sandy"]
first_names_of_team_1 = ["Sandy", "Bo", "Joe"]
  1. True
  2. False
7.
this_list = [1, 2, [3], [4], 5]
that_list = [[3], 1, 5, [4], 2]
  1. True
  2. False
8.
list_1 = [1, 2, 3]
list_2 = [1, 2, 3]
  1. True
  2. False

Try It

Remove duplicates

Write a recursive rem_dup() function that removes duplicates from a list.

Ex: List [5, 5, 2, 1, 3, 1, 6] should result in an output list [5, 2, 1, 3, 6].

Citation/Attribution

This book may not be used in the training of large language models or otherwise be ingested into large language models or generative AI offerings without OpenStax's permission.

Want to cite, share, or modify this book? This book uses the Creative Commons Attribution License and you must attribute OpenStax.

Attribution information
  • If you are redistributing all or part of this book in a print format, then you must include on every physical page the following attribution:
    Access for free at https://openstax.org/books/introduction-python-programming/pages/1-introduction
  • If you are redistributing all or part of this book in a digital format, then you must include on every digital page view the following attribution:
    Access for free at https://openstax.org/books/introduction-python-programming/pages/1-introduction
Citation information

© Feb 26, 2024 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License . The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.