Outline of Class 13: More Computing

Held: Tuesday, March 3, 1998

Administrivia

• Assigned:
  • journal assignment 10
  • computing assignment 6
  • computing assignment 7
  • read chapter 4
• Due:
  • journal assignment 9
  • problem set 5
  • Essay on the convocation speaker

• Grinnell Women in Science are hosting four meetings/lectures over the next two days . There are some posters scattered around the department. I'd recommend attending, even if you aren't in the sciences.
• We'll try to rethink our impressions of Java today and then move on to another topic.
• Please hand in your essays today. If you're not able to get them in today, try to get them in on Thursday. If you weren't able to make it to convocation, I've been told that a videotape will be available in the AV center in the near future.
• I may be getting a contract to write a lab manual in Java for introductory students (scary thought, isn't it?). I'm going to try to arrange for some money for "student testers". If this sounds interesting to you, let me know.
• Today and Thursday we'll begin to plan our class project.

Repairing Our Java Model

• As I reflected on how we've discussed Java over the past few days, I realized that I made the mistake of linking two topics that aren't linked (because Java links them so closely).
• As I noted in the past, Java allows you to modify the "world of Java" in two ways:
  • You can add new classes (ideals)
  • You can write main routines that describe how to build a program from classes.
• You do both by building a class.
• While you can do both at once, it's best to separate them. From now on, we'll build
  • A class that provides functionality (a collection of methods and related things).
  • A class that provides an interface (that reads input, writes output, and uses other classes to do computation).
• We'll do a lab that will help clarify this separation.

Exercise

Work on computing assignment 6.

Arrays

• When writing programs, we often need to refer to groups of things in such a way that we can easily extract an element of the group.
• Java provides a structure called the array that simplifies such references.
• Arrays are collections of elements that you can index by number.
• You can declare arrays of a particular size with
  type[] name = new type[size];
• For example, to create a collection of five numbers named stuff, you would write
  double[] stuff = new double(5);
• You can also fill in arrays when you create them. To do this, you would write
  type[] name = { val1, val2, ..., valn };
• For example, to create an array with the values 3, 2, 6, and 5, I would write
  double[] stuff = {3, 2, 6, 5};
• You can refer to a particular element of an array with name[index]
• For example, I could element 3 of the stuff with
  stuff[3]
• You can also use variables (or even expressions) as indices. For example, if i held the value 3, then we could get element 3 of the stuff with
  stuff[i]
• For an odd reason, Java numbers the elements in an array starting at 0. This means that the initial element in the array is element 0. If the array has N elements (e.g., 5), the last element is therefore at position N-1 (e.g., 4).
• You can always determine the number of elements in an array with name.length
• Once we have values in arrays, it's "easy" to do general computations with them. Here is a method that sums the values in a vector.

  public double sum(double[] stuff)
  {
    double total = 0;	// The total of the numbers in stuff
    int i;		// A counter variable for stepping through
  			// the array
    // Step through the array, adding each subsequent value
    for(i = 0; i < stuff.length; i = i + 1) {
      total = total + stuff[i];
    }
    // Return the total value
    return total;
  } // sum
  

Exercise

Work on computing assignment 7.