Outline of Class 9: Control Structures

Held: Tuesday, February 17, 1998

Administrivia

• Most of today's stuff can be found in your printed notes from the previous class.
• Today we'll be trying an alternate lab strategy. I'll talk for a few minutes about a subject, then give you about twenty minutes to work on a part of the lab (it's okay if you don't finish), then we'll reflect. We'll do this a number of times so as to cover a variety of topics.
• Much of today will be spent on computing assignment five.
• For Thursday, you'll need to complete this assignment.
• For Thursday, work on journal assignment 7.

The while Loop

• Often, you want to execute a series of statements as long as a condition holds.
• For this case, Java provides the while loop, which has the form

  while (test)
  {
    statements;
  } // while
  

• In executing this loop, Java evaluates the test. If the test fails, the loop terminates. Otherwise, Java executes the statements and then goes back and tries the test again.
• Note that the statement can be empty.
• At times, you will want to have a loop in which the body is executed once before the test. While it is possible to do this by repeating code, it is easier to use a variant of the while loop called the do loop, and has the form
  

  do
  {
    statements
  } while (test);
  

• This is the only time you won't need to comment your ending brace.
• If you only want to execute one (or zero) statements, you don't need the braces. However, I consider it good programming practice to include them.
  • It doesn't change the execution speed of your program.
  • It makes it much easier to extend your program.
  • However, it does increase the length of your program.

See loops in in computing exercise 5 for an example and some exercises.

The for loop

• At times, you want to execute a statement (or block of statements) a fixed number of times.
• For example, to calculate the factorial of N, we might want to multiply 1 by the numbers from one to N.
• Many programming languages provide a structure known as the for loop for situations like this.
• Java provides a more general of the for loop. It has the form

  for (initialization; test; increment)
  {
    statement;
  } // for
  

• In effect, this is equivalent to

  initialization;
  while (test)
  {
    statement;
    increment;
  } // while (test)
  

• Any of the four parts of a for loop can be omitted (in which case it is ignored).
• We can then solve our initial factorial problem as follows

  int i;		// A counter variable
  int factorial;  // The factorial of N
  factorial = 1;
  for (i = 1; i <= N; i = i+1)
  {
    factorial = factorial * i;
  } // for
  

• Most Java programmers increment their counter variables with the postincrement operator, var++.