CS21 Spring 2005
Homework 4
Due Sunday, Feb. 6, before 11:30pm

Answer the following Review Questions from the C book on your own, but do not turn them in. We will post answers on the CS21 web page next week.

• Chapt. 5 Review Questions 2, 5, 6-10, 14 (page 178)

Please complete the following 3 programs and turn them in. The main task of this week's homework is to create a program that plays the game of Rock-Paper-Scissors. This is a fairly simple game that can be implemented with less than 10 functions. However, to help you get in the habit of testing your functions as you code them in, we want you to write this program in pieces. Program 1 asks you to write a menu function and a GetUserChoice function (and a main function that tests them both). Program 2 asks you to add a DetermineWinner function. Finally, Program 3 asks you to put the whole game together.

We will only grade Program 3. However, you should first implement Program 1, then Program 2, and finally Program 3. By following this approach for future, more complex programs, you will save yourself many hours of debugging time.

• Program 1, Implement a function, DisplayMenu, to display a menu of the following options to a user:
  1. Rock
  2. Paper
  3. Scissors
  Next, implement a function, GetUserChoice, that takes as input low and high menu range values (1 and 3 representing the range of the three possible choices, Rock, Paper and Scissors) and returns the users choice, one of Rock, Paper, or Scissors, that is encoded as an int value 1, 2, or 3 (use constants for these three values). GetUserChoice prompts the user to enter a menu option, reads in the value, and checks to see that the selection is within the range between low and high. If the value is within range, GetUserChoice returns the value to the caller, if it is not in range, then it prints a message stating that the user selected an invalid value and prompts the user to try again (to enter a valid value). GetUserChoice will repeat prompting the user to enter a value until the user enters an in-range value. Your main function should call DisplayMenu to display the menu, then call GetUserChoice to get the user's selection, and finally print out a message stating which value the user has selected: "You have selected (Rock, Paper, or Scissors)"

  Remember to have comments at the top of your program file describing what the program does, to comment every function with what it does, the values it takes in and the value it returns, and to use good function and variable names. Also, think about how you can use constants in your program.

• Program 2 Starting with the code you wrote for Program 1 (copy your program1.c file to a program2.c file using the cp command), add a function, DetermineWinner, that determines who is the winner of the rock, paper, scissors game (either the user or the computer). Your function will take two input values (the user's choice and the computer's choice), and will return a value that indicates who won the game (0 for user, 1 for computer, 2 for tie). To test your DetermineWinner function, after your program reads in the selection from the user, it should call your DetermineWinner function three times from your main function (one time for each possible computer selection). After each call to DetermineWinner, print the value it returns (one of 0, 1, or 2). The rules of the rock, paper, scissors game are the following:
  1. Rock beats Scissors
  2. Scissors beats Paper
  3. Paper beats Rock
  4. Two of the same are a tie

• Program 3 Putting it all together. Start by copying your program2.c file to a program3.c file. You will finish the implementation of a program that plays the rock, paper, scissors game. Start by implementing the a function, GetYesOrNo, that prints a prompt asking the user "do you want to play again (enter 1 for yes/0 for no)", then makes a call to your GetUserChoice function to read in and return the user's choice. Next, add a loop around the code in your main function (print menu, get response, determine winner, print results, see if user wants to play again) that will repeat until the user enters 0 for playing again.

  The computer will play the game based on the Hannah Rule (invented by Jeff Knerr's 4 year old daughter): for the next round of play, the computer will select the value that would beat what the user selected in the current round. For example, if the user selects Rock in round i, then the computer will select Paper in round i+1. You should pick one of Rock, Paper, or Scissors as the computer's selection for the first round of play (e.g. every time you run your program the computer will "pick" Rock as its choice for the first game). You do not need to have the computer choose randomly among the three options.

  To get an idea of what a complete Program3 looks like when run, HERE is sample output from a run of mine. Yours does not need to be identical, but it should have all the required parts described above.

  If, as you implement your solution to this program, you find that there are places where it makes sense to add more functions to your program, feel free to add functions beyond the ones we have specified.

Please use the cs21handin command to turn in your homework.