Outline of Class 4: Control Structures

Miscellaneous

• Today we begin our weekly case studies on the design of existing systems (and, at times, their failures). I will give today's report, but you will need to give reports in the following weeks.
• I'd like two volunteers to work together on the next report. If I don't get volunteers, I'll pick some. Note that you can also volunteer for a particular week (first come, first served).
• Today's case study: The Ariane 5.
• Any questions on the second assignment?
• Are you folks looking at the class outlines? If so, any recommendations for changes?

Case Studies

• In many disciplines, practitioners learn and improve their practice by studying strengths and weaknesses of existing systems. Often, this study takes the form of reading reports on the systems.
• In this course, we'll be spending some time each week discussing a system.
• Each of you will be expected to read and report on one such system, usually in a team of two.
• The report will have two components:
  • An oral presentation (given in class)
  • A short written summary, to be emailed to me
• I will provide you with either particular assignments or a list of sources.
• You will all be responsible for understanding all the reports given this semester.
• Why are we doing this?
  • To ground some of our discussions.
  • To get us thinking about various types of tasks that involve computing.
  • To get us thinking about different design and problem-solving strategies.
  • To give you experience presenting and writing.
  • Probably other reasons that don't presently come to mind.

Control Structures

• As do most imperative languages, Java provides a number of control structures for manipulating program flow, including if statements, switch statements, for loops, and while loops.

Basic Conditionals

• Java provides the two basic forms of the if statement.
• if (test) statement executes the statement only when the test holds.
  • Note that the test must return a boolean value.
  • Note that the test must be surrounded by parentheses.
• if (test) statement else statement selects between the two statements. It executes the first if the test evaluates to true and the second if it evalutes to false.
• Java does not provide a special facility for nesting if statements.

Switch Statements

• Often, you need to do more than select between two values, one true and one false.
• While it is possible to do this with a sequence of if statements, such a solution is neither readable nor efficient.
• As an alternative, Java provides the switch statement (like the case statement in Pascal).
• It takes the form

  switch (integer-valued-expression) {
    case value1:
      stuff-to-do;
      break;
    case value2:
      stuff-to-do;
      break;
    ...
    default:
      stuff-to-do;
      break;
  }
  

• If you leave out a break, execution will continue into the next case.
• You can switch on byte, short, int, long, or char valued expressions.
• break has other uses, which you'll see later.

While Loops

• 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) statement
• In executing this loop, Java evalutes the test. If the test fails, the loop terminates. Otherwise, Java executes the statement 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 easeier to use a variant of the while loop called the do loop, and has the form
  do statement while (test).

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.
• For cases like this, Java provides the for loop, which has the form

  for (initialization; test; increment)
    statement;
  

• In effect, this is equivalent to

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

• Note that this is somewhat more general than the for loop of Pascal.
• Any of the four parts of a for loop can be ommitted (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;
  }
  

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

The Block Statement

• In many places in your programs, you will want to include more than one statement, but only one statement is allowed.
• What can you do? It might be possible to create a separate function to handle the multiple statements, but this seems wasteful.
• Java permits you to create a block of statements by surrounding them with curly braces, as in

  {
    statement1;
    statement2;
    ...
    statementn;
  }