LECTURE NOTES
OCTOBER 11 2004

ASSIGNMENTS
Fri 10/15 CodeLab #6
Mon 10/18 Assignment 2

SHORTHAND ASSIGNMENT OPERATORS

Why?
- common to do an operation to a variable, and assign it back to itself
What?
- shorthand assignment operators
How?
= +=, -=, *=, /=, %=

What?
- shorthand assignment precedence
How?
- lower than arithmetic operators

THE WHILE STATEMENT

Why?
- repeatedly execute a block of code until some condition is true
What?
- type of control statement
- *repetition/loop statement*
- while statement
How?
- syntax
  while ( <boolean expression> )

      <statement>

What?
- using statements inside a loop
How?
- <statement> can be single or compound (requires braces)
- *loop body*

What?
- control diagram of while statement
How?
- diamond for loop condition
- rectangle for loop body
- arrow into diamond
- true arrow from diamond to rectangle
- arrow from rectangle to above diamond
- false arrow out of diamond

TYPES OF WHILE LOOPS

Why?
- execute a loop a specific number of times
What?
- *count-controlled loop*

Why?
- execute a loop until some special value is encountered
What?
- *sentinel-controlled loop*

USING WHILE LOOPS

Why?
- improving the user interface
How?
- handle invalid input
- let the user input a value until it is correct

Why?
- have to get some initial input to start with
What?
- *priming read*

COMMON LOOP ERRORS

What?
- *infinite loop*

Why?
- usually arithmetic performed during an infinite loop
What?
- *overflow error*

Why?
- overflow error
What?
- particular way that Java handles
How?
- reals: assigns a reserved value representing infinity
- integers: value wraps around

What?
- using real numbers for testing and increment
Why?
- real numbers are infinite
- the computer can only approximate this
- comparisons and operations can appear non-deterministic

What?
- handling real number imprecision
How?
- avoid using them when require test for equality

Why?
- think too quickly
What?
- *off-by-one error*

What?
- make a loop execute N times
How?
- initialize the counter to 0 and use < N
- initialize the counter to 1 and use <= N

PRACTICE PROBLEMS

1. Identify errors:
   a. while (x < 1 && x > 10) {
          a = b;
      }
   b. while (a == b) ;
      {
          a = b;
          x = y;
      }
2. Write a while statement to compute the following sum.
   1 + 2 + 3 + ... + 100
   int count = 0, sum = 0;
   while (count < 100) {
       count++;
       sum += count;
   }