CSCI E-50a, Harvard Extension School Monday Section Home Page

TF: David Roazen (droazen at fas dot harvard dot edu)
Section meeting time/place: Mondays 8:00-9:00 PM, Science Center Room B-10
Office hours: Immediately following section, or by appointment

On this page I'll be posting the exercises we do in the Monday section for CSCI E-50a. Please check this page regularly, as it will be updated with new exercises after each section meeting. Please feel free to email me if you have questions about any of the exercises we do in section.

To download an exercise to your local machine, simply right click (or control click, on a Mac) on it and select "save link as..." in your web browser. To download an exercise to your Harvard FAS account, use the wget command while logged in to nice.harvard.edu, and give it the full URL of the file you want to download. The syntax is:

wget 'url_of_file_to_download'

(note the single quotes around the URL). For example, to download the Hello World exercise, you would type the following at the Unix prompt:

wget 'http://www.people.fas.harvard.edu/~droazen/e50a/unit2/HelloWorld.java'

Contents:

• Announcements
• Helpful Documents
• Unit 1 Section Exercises
• Unit 2 Section Exercises
• Unit 3 Section Exercises
• Unit 4 Section Exercises

• All unit 2 grades have been returned by email.
• I've posted guidelines for Java coding style in the documents section below.

• Coding Style: How to format your code with readability in mind
• Common Unix Commands
• Homework Submission Instructions (NOTE that you do NOT need to turn in hardcopy paper printouts of your work -- an electronic submission alone is sufficient)
• Java 6 API

• dancerInterrupted.sb, a program that produces a simple animation of a man dancing, and allows the user to stop and start the dance by pressing the spacebar. Demonstrates how to use a variable to keep track of the state a program is in, and provides practice with loops, if statements, and responding to keypresses.
• taxBreak.sb, a program that prompts the user for the number of pools and yachts he/she owns, and then uses that information to determine whether the user is eligible for a tax break. Practice with user input, variables, boolean expressions, and if statements.
• multTable.sb, a program that causes a sprite to recite the multiplication table from "1 times 1" up to "5 times 5". Practice with variables, loops, and boolean expressions, and in particular the concept of a nested loop (a loop inside of another loop).
• sumRange.sb, a program that prompts the user for two integers, and then computes the sum of the integers in that range, inclusive. More practice with user input, variables, and loops.
• dogYears.sb, a program that converts dog years to human years using the following formula: the first 2 years of a dog's life count as 10.5 human years, and all subsequent years count as 4 human years. Practice writing complex arithmetic expressions in Scratch, and error-checking user input to make sure that it makes sense before using it in a calculation.
• nestedTriangles.sb, a program that draws a series of nested right triangles. Practice drawing plane shapes in Scratch using the "pen" feature.

• HelloWorld.java, the classic Hello World program! Prints the message "Hello, world!" to the screen.
• PrintPuzzles.java, practice with print()/println() statements, escape sequences, String concatenation, and printing the values inside of variables.
• CheckerBoard.java, a class that prints a checkerboard pattern to the screen. Demonstrates how to use stepwise refinement to break a large task up into smaller subtasks, and then break those subtasks up into still smaller subtasks, until we reach the smallest possible subtasks that can't be broken up any further. Each subtask we identify becomes a method in our class.
• CheckerBoardWithFinalVariables.java, an improved version of the CheckerBoard class that uses named constants (ie., final variables) to store the values that don't change during program execution. This makes it easy to modify these values in the future, and improves the readability of the code by replacing literal values like 4 and 6 with descriptive names like SQUARE_SIZE and BOARD_HEIGHT.
• IntegerDivision.java, a class that demonstrates the difference between integer division, in which the remainder is discarded, and floating-point division, in which the remainder is retained.
• OperatorPractice.java, a class that provides practice with operator precedence and associativity.
• PerfectSquares.java, a program that prints the first 10 perfect squares. Practice writing simple for loops.
• Powers.java, a program that uses nested for loops to produce output in which the 1st line contains the integers 1 through 10 raised to the 1st power, the 2nd line contains the integers 1 through 10 raised to the 2nd power, the 3rd line contains the integers 1 through 10 raised to the 3rd power, and so on. Practice with for loops, named constants, and using methods from the Math class.
• Summation.java, a program that uses nested for loops to print the sum of the integers from 1 to 1, then from 1 to 2, then from 1 to 3, and so on up to the sum of the integers from 1 to 10. Practice with a type of nested for loop in which the number of times the inner loop loops depends on the current value of the index variable of the outer loop.
• OddEven.java, a program that prompts the user for an integer, then prints out whether that int is even or odd. Practice with if/else statements and user input using Scanner objects.
• NerdSurvey.java, a program that asks the user how many computers he/she owns, and then prints an appropriate reaction message. Practice with if/else if/else statements and user input using Scanner.
• Prob6.java, a corrected version of the class from problem 6 on the practice midterm.
• Prime.java, a class that prints out all of the prime numbers between 101 and 201, inclusive. Solution to problem 7 on the practice midterm. See PrimeWithBooleanVariable.java below for an alternate version of this class.
• SmallestEven.java, a class that reads in 10 integers from the keyboard and prints out the smallest even integer entered, or an error message if no even integers were entered at all. More practice with user input using Scanner and if statements. See SmallestEvenWithBooleanVariable.java below for an alternate version of this class.

• PrimeWithBooleanVariable.java, an alternate version of Prime.java from the unit 2 section above that uses the unit 3 concept of a boolean variable to keep track of whether or not the current number is prime.
• SmallestEvenWithBooleanVariable.java, an alternate version of SmallestEven.java from the unit 2 section above that uses the unit 3 concept of a boolean variable to keep track of whether or not an even number has been encountered.
• NaturalizationSurvey.java, a program that asks the user a series of true/false questions, and then determines whether the user is eligible to become a U.S. citizen based on his/her answers. The questions, bizarre though they may be, are mostly taken verbatim from the U.S. Citizenship and Immigration Services "N-400 Application for Naturalization" form. Provides practice with boolean expressions and boolean variables.
• WhileVSDoWhile.java, a class designed to highlight the difference between a while loop, which performs its boolean test before executing the loop body and which may potentially execute zero times, and a do-while loop, which performs its boolean test after executing the loop body and which will always execute at least one time.
• ForToWhile.java, a class that shows how to re-write a simple for loop using a while loop, so that the while loop behaves identically to the original for loop in all cases.
• DiceGame.java, a program that plays a simple game involving a pair of six-sided dice. The object of the game is to roll doubles before rolling a 7. Practice with random number generation using Math.random(), do-while loops, and boolean variables.
• BiasedCoinToss.java, a program that simulates tosses of a biased coin -- that is, a coin in which the probability of getting heads is not the same as the probability of getting tails. Shows how to use Math.random() to cause something to happen with a particular probability. Also shows how to use a do-while loop to repeatedly prompt a user until he/she enters input that is legal.
• GlobalVariable.java, a class that shows how to declare a global variable (a variable that can be accessed and modified by any method in a class). Global variables are usually not the preferred choice for sharing data between multiple methods, but until we learn about parameters and return values it's the only method we've got!
• RandomInRangeBoneheadedVersion.java, a silly, overly repetitious program written all in main() that generates a series of random numbers within different ranges. Shows the need for reusable methods to avoid this sort of boneheaded repetition!
• RandomInRangeWithGlobalVariables.java, a second version of the RandomInRange program that creates a separate method to handle the random number generation, and uses global variables to pass information back and forth between that method and main(). An improvement over the original, but still not ideal...
• RandomInRangeWithParametersAndReturns.java, a third version of the RandomInRange program that also creates a separate method to handle the random number generation, but uses parameter-passing and return values to pass information back and forth between that method and main(). Definitely the best of the three versions of the program from the standpoint of conciseness and modularity!
• PrintVertical.java, a program that prompts the user for a sentence, and then calls upon a method named printVertical() to print that sentence "vertically", with one character per line. The program continues until the user types "quit". Practice with Strings, parameter passing, and while/do-while loops.
• VoidVSNonVoid.java, a class that highlights the differences between the ways void and non-void methods can legally be called.
• PredicateMethods.java, a class containing a number of examples of predicate methods -- that is, methods that return a boolean value of true or false: is20thCenturyYear(), isInFirstHalfOfAlphabet(), defeatsInRockPaperScissors(), and charIsInString()