CISC 181 Introduction to Computer Science, Fall, 1998

Lab 2 - program using conditional and repetition statements and
an applet using basic graphics

1. Set up your java and course environment.
   • enter this unix command:
     ln -s ~saunders/181 181
     This creates a "link" to the course directory in your directory. After you have done this it will be as if the course directory is a subdirectory of your own directory, and you can access course files such as Hello.java by referencing 181/Hello.java. Do this only once, there will be no need to repeat the linking in future sessions.
   • Include the commands in 181/.localenv in your own .localenv file in your home directory.
2. This exercise is to do some modifications to the program developed in class. This is to be done and handed in at the end of lab on Monday, Sept 21. Modify either the file 181/Evens.java developed by section 81 or the file 181/ex3_16.java developed by section 80.
   • Change the userInteger method so that it prints no prompt, including no ": ", when the prompt string contains only whitespace. This is an exercise in examining the characters in a String.
   • Overload the userInteger method so that it can be used with no string argument at all, i.e. called like " n = userInteger(); ". The form with no prompt string given should use "Integer please: " as the prompt.
   • The output given is unsatisfactory, it prints things like
     The sum 2 + 4 + ... + 2 is 2
     The sum 2 + 4 + ... + 3 is 2
     The sum 2 + 4 + ... + 4 is 6
     Modify it so that
     1. if 10 or fewer even numbers are added, it prints all of them. Remark: write a loop. You could actually write out all the cases in your code. Don't do that, write the loop.
     2. if more than 10 items are summed, have it print as it does now, except that the last number should be the actual last number added (not the input, if the input is odd)
   • Modify the program so that it repeats everything until the user enters the value 2.
3. This exercise is to be completed by the following lab, on Monday, Sept 28. Write an applet which illustrates a MonteCarlo approximation of Pi.

   Since the area of a circle of radius r is pi*r^2 and the area of a square exactly containing that circle is (2r)^2, the ratio of the two areas is pi/4. If a point is picked at random (random x and y coordinates) within the square, the probability that it is in the circle is pi/4.

   The Monte Carlo method is to pick a large succession of random points and count how many are in the circle. The expectation is that (number of points in circle)/(total number of points) approximates pi/4.

   The project is to write an applet that displays a circle in a square, then adds random points to the picture one by one. The display should also contain the current total number of random points that have been put in the picture and the current approximation to pi. Use color appropriately. In particular, use a different color for points within the circle and for points outside (but in the square).