Class 07: Loops

Held Wednesday, February 3

Summary

• Repetition
• for loops
• while loops
• Assignment 1 due (optional).
• Assignment 2 due.
• Lab: J5: Control Structures for Repetition

Contents

Notes

• For today's lab, try to do experiment J5.1 during class and J5.3 over the next few days. J5.1 covers while loops and J5.3 covers for loops.
• The schedule has been rearranged somewhat. This Friday, we'll be playing with graphics.
• Pop quiz!

Control Structures for Repetition

While loops

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

  while (test) {
    statements;
  } // while
  

• In executing this loop, Java evaluates the test. If the test fails, the loop terminates and Java moves on to the next statement. Otherwise, Java executes the statements in the middle of the loop and then goes back and tries the test again.
• 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.
  • It does, however, increase the physical length of the code for your program.

For loops

• 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) {
    statements;
  } // for
  

• In effect, this is equivalent to

  initialization;
  while (test) {
    statements;
    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

    /**
     * Compute n!.
     */
    public int factorial(int n) {
      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
      return factorial;
    } // factorial(int)
  

• We could also use while loops or recursive procedure calls.
• Most Java programmers increment their counter variables with the preincrement or postincrement operator, var++.

      for (i = 1; i <= n; ++i)
  

• It is also possible (and even encouraged) to declare the counter variable within the for loop, as in

      for (int i = 1; i <= n; ++i)
  

Lab

Do Lab J5: Control Structures for Repetition, Experiment J5.1.

Reflect

• What looping mechanisms did you use in Scheme?
• What does your count method look like?
• What is wrong with readPass. How did you fix that problem?

History

• Created Monday, January 11, 1999.
• Added short summary on Friday, January 22, 1999.
• Filled in the details on Wednesday, February 3, 1999. (Many details taken from last year's class.)