Decision structures: if, if-else, and switch

IF syntax

There are 4 ways of writing if statements

if (condition) {
   statement;
   statement;
   ...
}

if (ball.y < 0) {
   xspeed +=5;
   yspeed = -yspeed;
}

if (condition) statement;

if (x > 5) gameOver = true;

if (condition) {
   statement;
   statement;
   ...
} else {
   statement; 
   ...
}

if (ball.y < 0) {
   xspeed +=5;
   yspeed = -yspeed;
} else {
   ...
}

if (condition) {
   statement;
   ...
} else if (condition) {
   statement; 
   ...
} else {
   statement;
}

if (answer.equals("Yes") {
   food +=5;
} else if (answer.equals("No")) {
   //do nothing;
} else {
   //invalid answer
}

• The most normal way of writing IF statements is using code blocks
• If you write a one line if statment, don't break it into two lines
• If you write if statements as 1 line, and then try and make them into code blocks, you'll often get logic errors because you'll forget to put in { and } .
• Elseifs are very useful on occasision, but it is very easy to introduce complex logic bugs when using else-if. So please avoid them when possible.

Errors with IF (and other control statements)

This common error will change how your program is working and you may not notice why. It can happen to if, for, and while.

This is what we want to have happen: you press the red button , then a message is sent out, the computer sleeps for 60 seconds, then selfdestructs.

if (redButtonPressed) {
    System.out.println("Self destruct activated. T-60 seconds.");
    Timer.sleep(60);	//this actually won't work
    selfDestruct();
}

This is the screwed up version. Can you find the error?

if (redButtonPressed); {
    System.out.println("Self destruct activated. T-60 seconds.");
    Timer.sleep(60);
    selfDestruct();
}

Note the ; after the () in the IF.
This makes the code equivalent to this:

if (redButtonPressed); 
{
    System.out.println("Self destruct activated. T-60 seconds.");
    Timer.sleep(60);
    selfDestruct();
}

which is the same as this

if (redButtonPressed); // do nothing

System.out.println("Self destruct activated. T-60 seconds.");
Timer.sleep(60);
selfDestruct();

What this does is (i) nothing happens if the redbutton is pressed.
(ii) No matter what happens, the selfDestruct code will ALWAYS be executed -- EVEN if you did not press the button.

Logic and IF statements.

An AND condition can be written as two nested IF statements (demonstrate)

An OR condition can be written as two successive IF statements (demonstrate)

When thinking about an ELSE statement, you need to consider how many possible options there are to your condition and if they are mutually exclusive or not.
example: if your mark is > 50, print pass, if > 75 print good, if > 85 print excellent.

An Elseif can always be written as a nested if.

if (n > 0) {
   ...   
} elseif (n < 0) {
   ...
} else {
   ...
}

if (n > 0) {
   ...
} else {
   if (n < 0) {
      ...
   } else {
      ...
   }
}

But an elseif can only be written as three separate if statements if the statements are mutually exclusive. IF they are not mutually exclusive, then the order of the conditions is important.

Switch

This is an alternative to IF-ELSEIF-ELSE or a series of independant IF statements

Pros:
Much easier to read,
Fewer logic errors since you can only use one variable (this is the key!)

Cons:
You can only use one variable, so it can't be used for everything
The test conditions are a lot more limited. (I think you can only test if the variable is equal to a literal)
Switch statments can only be used on primitive data types or Strings (Java 7 onwards)

Example: You're playing an adventure game and get input. You split the input up into words and the first word is a verb, the second word is a noun - eg. take lamp, shoot gun, etc.

switch (word1) {
   case "inventory":
      displayInventory();
      break;
   case "take":
   case "pickup":
      getObject(word2);
      break;
   case "fire":
   case "shoot":
      fireObject(word2);
      break;
   case "n": 
   case "s":
   case "e":
   case "w":
      move(word1);
      break;
   default:
      message("invalid command");
}

if (word1 == "inventory") displayInventory();
if (word1 == "take") getObject(word2);
if (word1 == "pickup") getObject(word2);
... etc ...

if (word1 == "take" || word1 == "pickup") getObject(word2);

case "take":
case "pickup":
   getObject(word2);
   break;

Make sure you know:

• syntax of first line: switch ( variable ) {
• syntax of case lines: case "string" :