Course Info

Welcome to CS21. This course will introduce fundamental ideas in computer science while also teaching you how to write computer programs. We will study algorithms for solving problems and implement solutions in the Python programming language. Python is an interpreted language that is known for its ease of use. We also introduce object-oriented programming and data structures. A deeper coverage of these topics will be presented in CS 35.

This course is appropriate for all students who want to learn how to write computer programs and think like computer scientists. It is the usual first course for computer science majors and minors. Students with advanced placement credit or extensive programming experience should place out of this course and instead begin with CS31 or CS35.

Piazza: CS21 Q&A forum

Meeting Times:

Section Days Time Room Instructor




Sci 256

Lisa Meeden




Sci 256

Aline Normoyle




Sci 256

Jeff Knerr

Lab Day Time Room Instructor




Sci 256

Charlie Kazer




Sci 256

Charlie Kazer




Sci 256

Aline Normoyle




Sci 256

Aline Normoyle




Clothier 16

Scout Sinclair




Clothier 16

Scout Sinclair

Support Staff & Office Hours

Course and Lab Instructors:

Name Office Office Hours

Lisa Meeden

Sci 243

Wed and Thu 1:30 PM - 3:30 PM (or by appt)

Jeff Knerr

Sci 238A

Fri 12:30 PM - 2:00 PM (or by appt)

Aline Normoyle

Sci 252C

Fri 3:30 PM - 5:00 PM, or by appt

Scout Sinclair

Sci 262A

Mon Noon-2:00 PM, Thu 11am-12:30, or by appt

Charlie Kazer

Sci 252B

Mon 2-4pm, Fri 10:30am-12:30, or by appt

Lauri Courtenay

Sci 257

Tue 10:00 AM - 11:00 AM, Thu 10:00 AM - 11:00 AM

Academic Support Coordinator: Lauri Courtenay

Student assistants/Ninjas: Alex Fischmann, Emma Jin, Lyla Kiratiwudhikul, Syndey Levy, Rebecca Lin, Jasiel Lopez, Saul Lopez-Valdez, Sumi Onoe, Otis Peterson, Bilal Soukouna, Riley Thompson, Catherine Wang


Book Cover

We will primarily be using the online book How to think like a computer scientist: Learning with Python by Elkner, Downey and Meyers.

See the Schedule for each week’s reading assignment.

Here are a few other useful online resources:

Course Goals

By the end of the course, we hope that you will have developed the following skills:

  • Given a program, be able to simulate on paper how a computer would execute the program, and show the results that would be produced.

  • Given a problem, be able to design a clear, concise, and correct pseudocode algorithm to solve it.

  • Given a pseudocode algorithm, be able to successfully implement it in Python.

  • Be able to use top-down design to sub-divide a large problem into reasonably-sized modular sub-problems.

  • Given several algorithms for solving the same problem, be able to analyze which algorithm would be more efficient in terms of running time.

  • Develop debugging and unit testing skills. Consistently use these skills while implementing programs.



Jan 20


Course Introduction

Lecture 0
Lab 0: UNIX, editing

Jan 22


Jan 24


Jan 27


Numbers, Strings, and Loops

Lab 1: First Programs

Jan 29


Jan 31


Feb 03


Conditions and Boolean Logic

Lab 2: Numbers and Strings

Feb 05

Quiz 1 Study Guide

Feb 07

Quiz 1


Feb 10


First Functions, while Loops

Lab 3: if statements, for loops

Feb 12


Feb 14


Feb 17


Fruitful Functions

Lab 4: while loops, functions

Feb 19

Quiz 2 Study Guide

Feb 21

Quiz 2


Feb 24


Graphics, Using Objects

Lab 5: more functions

Feb 26


Feb 28


Mar 02


Top-Down Design

Lab 6: Graphics

Mar 04

Quiz 3 Study Guide

Mar 06

Quiz 3


Mar 09

Spring Break (extended for COVID-19)

Mar 11

Mar 13

Mar 16

Mar 18

Mar 20


Mar 23


More Top-Down Design

Lab 7: TDD Memory Game

Mar 25


Mar 27


Mar 30



Lab 7: TDD Memory Game

Apr 01


Apr 03

Quiz 4 Study Guide


Apr 06

Quiz 4


Lab 8: Searching restaurants

Apr 08


Apr 10


Apr 13

Quiz 5 Study Guide


Lab 9: Sorting restaurants

Apr 15


Apr 17


Apr 20

Quiz 5

Classes and Objects

Lab 10: recursion

Apr 22


Apr 24


Apr 27


More Classes and Objects

Lab 11: cats!

Apr 29


May 01

Quiz 6 Study Guide

Quiz 6 (May 07)

Grading Policies

Grades will be weighted as follows:

The final exam was canceled when the College changed its grading policy for this semester to CR/NC.


Lab assignments




Class Participation

Exam and Quizzes

Quizzes will be given at the beginning of class on the days posted on the schedule. Please look over these dates carefully and contact the professor in advance if you cannot be in class for a quiz.

If you are not present on the day of a quiz, and do not let us know ahead of time that you are missing class, you will receive a zero for that quiz.

There will be one final exam for the semester. Details and dates will be released during the semester. Please read the section on accommodations if you are in need of extra time. You must inform us of accommodations or conflicts at least 2 weeks in advance of the exam.

Lab Policy

This course features weekly lab assignments which are the largest component of your course grade. Lab attendance is required by all students, unless you have already completed and submitted the lab assignment for the week. You must attend the lab session for which you are enrolled.

Lab assignments will typically be assigned in class at the beginning of the week and will be due before midnight on Saturdays. You are strongly encouraged to start early and to attend the study sessions for extra practice.

You will submit you assignments electronically using the handin21 program. You may submit your assignment multiple times, and a history of previous submission will be saved. You are encouraged to submit your work regularly.

Late Policy

Labs will typically be due Saturdays before midnight. Late submissions will not be accepted. Even if you do not fully complete a lab assignment you should submit what you have done to receive partial credit.

The CS labs are open 24 hours a day, 7 days a week for you to use for CS21 lab assignments.

If you feel that you need an extension on an assignment or that you are unable to attend class for two or more meetings due to a medical condition (e.g., extended illness, concussion, hospitalization) or other emergency, you must contact the dean’s office and your instructors. Faculty will coordinate with the deans to determine and provide the appropriate accommodations. Note that for illnesses, the College’s medical excuse policy states that you must be seen and diagnosed by the Worth Health Center if you would like them to contact your class dean with corroborating medical information.

Academic Integrity

Academic honesty is required in all your work. Under no circumstances may you hand in work done with (or by) someone else under your own name. Your code should never be shared with anyone; you may not examine or use code belonging to someone else, nor may you let anyone else look at or make a copy of your code. This includes, but is not limited to, obtaining solutions from students who previously took the course or code that can be found online. You may not share solutions after the due date of the assignment.

Discussing ideas and approaches to problems with others on a general level is fine (in fact, we encourage you to discuss general strategies with each other), but you should never read anyone else’s code or let anyone else read your code. All code you submit must be your own with the following permissible exceptions: code distributed in class, and code found in the course text book. In these cases, you should always include detailed comments that indicates on which parts of the assignment you received help, and what your sources were.

Failure to abide by these rules constitutes academic dishonesty and will lead to a hearing of the College Judiciary Committee. According to the Faculty Handbook: "Because plagiarism is considered to be so serious a transgression, it is the opinion of the faculty that for the first offense, failure in the course and, as appropriate, suspension for a semester or deprivation of the degree in that year is suitable; for a second offense, the penalty should normally be expulsion." Be aware that we will be routinely running plagiarism detection software on the lab solutions of students from all sections of CS21

The spirit of this policy applies to all course work, including code, homework solutions (e.g., proofs, analysis, written reports), and exams. Please contact us if you have any questions about what is permissible in this course.

Exam Integrity

Students must strictly adhere to the following policy, which applies to all exams taken in a Computer Science course at Swarthmore:

Exam takers must place all non-essential items at the front of the room (or other designated area). Unless otherwise permitted, students may not have any electronic devices or course materials in their possession during the entirety of the exam. This includes cell phones, tablets, laptops, smart watches, course notes, articles and books, among others. These items should be placed at the front of the room near the proctor. If you need to leave the room during the exam, you must obtain permission from an instructor first. Any non-permitted discussion or aide in regards to exam material will result in immediate forfeiture of the exam and a report to the College Judiciary Committee. Please discuss any concerns or accommodations with your instructor prior to starting the exam.

Student Support

Lauri Courtenay is the CS Department’s Academic Support Coordinator. She will be working closely with our student mentors, also known as Ninjas, to help you learn how to program and think like a computer scientist. The CS21 Ninjas will assist us in class and run evening study sessions in the birds' nest (Sci 256). The CS21 Ninjas (student mentors) are: Alex Fischmann, Emma Jin, Lyla Kiratiwudhikul, Syndey Levy, Rebecca Lin, Jasiel Lopez, Saul Lopez-Valdez, Sumi Onoe, Otis Peterson, Bilal Soukouna, Riley Thompson, Catherine Wang.

Study Sessions

You are invited — and encouraged — to participate in Ninja evening study sessions to prepare for quizzes, to discuss programming concepts, and to get friendly assistance in working on lab assignments. Our CS mentoring team is dedicated to helping students, who have no prior knowledge of computer science, learn to program in Python while keeping their senses of humor intact. As an added bonus, free snacks will be provided at the sessions. The sessions are held:

Weekly Evening Ninja Sessions



Sci Center 256



Sci Center 256

Accessing the CS labs after hours

You can use your ID to gain access to the computer labs at nights and on the weekends. Just wave your ID over the onecard reader next to the lab doors. When the green light goes on, just push on the door handle to get in (the door knob will not turn). If you have issues with the door locks, send an email to If the building is locked, you can use your ID to enter the door between Martin and Cornell library. For this class, your ID will give you access to the labs in rooms SCI 238, 240, 256, and the Clothier basement.

Academic Accommodations

If you believe you need accommodations for a disability or a chronic medical condition, please contact Student Disability Services (Parrish 113W, 123W) via e-mail at studentdisabilityservices at to arrange an appointment to discuss your needs. As appropriate, the office will issue students with documented disabilities or medical conditions a formal Accommodations Letter. Since accommodations require early planning and are not retroactive, please contact Student Disability Services as soon as possible. For details about the accommodations process, visit the Student Disability Services website.

To receive an accommodation for a course activity, you must have an Accommodation Letter from the Office of Student Disability Services and you need to meet with us to work out the details of your accommodation at least two weeks prior to the activity.

You are also welcome to contact us privately to discuss your academic needs. However, all disability-related accommodations must be arranged, in advance, through Student Disability Services.

How to Succeed in CS21

  • Attend class.

    The primary introduction to course material is through class lecture. Additionally, we often do lab exercises during class. This is an important part of the learning process, as these exercises give you immediate experience with the material we are covering.

  • Read the material before you come to class.

    Don’t worry about total comprehension, but at least get a feel for what we will be covering that week. If you have some understanding of the material coming into class, it will be easier for you to ask questions during class, rather than later when help may not be available.

  • Ask questions if you don’t understand.

    This means both during class and while doing lab assignments. This class continually builds on previous material, so if you don’t understand something one week, it will continue to be a problem the next week, and the week after that, and so on. If you need help, ask your classmates (make sure you have read the "Academic Integrity" section below first), attend study sessions, come talk with us during our office hours, post on piazza, or email us your question.

  • Start the lab assignments early.

    We realize this one is not always easy to do, but if you can get in the habit of doing this, you will be much better off. If you get stuck early (i.e., not two hours before it is due), there will be time to look for help. In addition, if you start early enough, you can take a break, go do something else, and come back later. We find we always have at least a few new ideas when coming back to a problem after a break. If you wait until the last minute, you can’t do this.

  • Practice, practice, practice.

    The only effective way to learn the material and pass the quizzes and exams is to consistently do the labs. Finish all of the assigned programs (and do some extras, for fun!). Even if you don’t get them done on time, they will still help you learn the material.

  • Seek help early and often.

    Because course material builds on previous material, it is essential to your success in this class that you keep up with the course material. If you are having difficulty with a programming assignment, if you didn’t follow something covered in lecture, if you feel you need some extra help understanding or practicing some of the course material, or if you just want to discuss something from a lecture, an assignment, or the text, please come to my office hours.

  • Attend CS21 Labs and Study Sessions.

    Lab attendance is required. The student Ninjas hold evening study sessions each week. You are encouraged to participate in these sessions. The Ninjas will help you prepare for quizzes, will provide additional instruction in programming concepts, and will provide friendly assistance on your lab assignments.

Programming Style

Programming is not a dry mechanical process, but a form of art. Well written code has an aesthetic appeal while poor form can make other programmers and instructors cringe. Programming assignments will be graded based on style and correctness. Good programming practices usually include many of the following principles:

  • A comment at the top of the program that includes

    • Program authors

    • Date or Dates

    • A brief description of what the program does

  • Concise comments that summarize major sections of your code

  • Meaningful variable and function names

  • Function comments that include:

    1. description of what function does

    2. description of input value(s) (parameter values)

    3. description of return value

  • Well organized code

  • White space or comments to improve legibility

  • Avoidance of large blocks of copy-pasted code


This semester we’ll be using Piazza, an online Q&A forum for class discussion, help with labs, clarifications, and announcements that pertain to all sections of CS21. You should have received an email invitation to join CS21 on Piazza. If you didn’t, please let us know.

Piazza is meant for questions outside of regular meeting times such as office hours, ninja sessions, class, and lab. Please do not hesitate to ask and answer questions on Piazza, but keep in mind the following guidelines:

  1. Piazza should be used for ALL content and logistics questions outside of class, lab, office hours, and ninja sessions. Please do not email instructors or ninjas with your code or questions about the assignments.

  2. If there is a personal issue that relates only to you, please email your instructor.

  3. We encourage non-anonymous posts (which will count towards participation), but you may post anonymously.

  4. Do NOT post long blocks of code on Piazza - if you can distill the problem to 1-2 lines of code and an error message, that’s fine, but try to avoid giving out key components of your work.

  5. By the same token, when answering a question, try to give some guiding help but do not post code fixes or explicit solutions to the problem.

  6. Posting on Piazza counts toward your participation grade, both asking and answering!

Links that are related to the course may be posted here. If you have suggestions for links, let us know.