Branching

Dr Andy Evans

Branching

Processing can rarely be done by a single list of instructions.
We usually need to have different sets of code running, depending on conditions or user interaction.
For this, we need to branch our code down one or more execution paths.

If

The simplest form is the if-statement:

	
if (some condition) { 
    do something; 
}

For example:

	
if (earthRadius == 6378) {	
    System.out.print("radius correct");
}

Block only done if the whole condition is true.
Note that if there's only one line, it can be written without brackets, but avoid this.

if... else...

The if-else statement lets us branch down two mutually exclusive routes:

	
if (condition) {do something;} 
else {do something else;}

For example:

	
if (earthRadius == 6378) {	
    System.out.println("radius correct");
} else {
    earthRadius = 6378;
    System.out.println("radius corrected");
}

Ternary kung fu

The ?: Ternary operator
- A replacement for the if / else statement in assignment.
- The most nifty move you can pull. Use it and other coders will give you a knowing nod in the street. You will be the Jackie Chan Kung Fu King/Queen of Code.
How do I pull this amazing move?
```
	
variable = condition?expression1:expression2;
```
If the condition is true, expression1 is used, otherwise expression2 is used.

Ternary example

	
name = (fileFound==true)?(filename):(defaultName);

...is the same as...

	
if (fileFound==true) {
    name = filename;
} else {
    name = defaultName;
}

Why use it?
Because we can... just because we can.

if / else / if ladder

For more complex statements with multiple choices.

	
if (condition) 
    statement or {statements;}
else if (condition) 
    statement or {statements;}
else if (condition) 
    statement or {statements;}
else
    statement or {statements;}

You need to put the elements with the most chance of being true at the top, otherwise it is very inefficient.

Switch

	
switch (variable) {
    case value1:
		statements1;
		break;
    case value2:
		statements2;
		break;
    case value3:
		statements3;
		break;
    default:
		statements4;
}

Slightly slower, but not as slow as a bad if-else-if ladder.
The break's are optional, if you leave one out, the computer will just run on into the next values case statements.

	
int day = 7;

switch (day) {
    case 6:
		System.out.println("Saturday");
		break;
    case 7: 
		System.out.println("Sunday");
		break;
    default: 
		System.out.println("Meh, whatever");
}

Note that unlike the 'if / else if / else' ladder the conditions generally have to be fixed; they can't be variables (this changed with Java 7, but many people have JRE 6).

Review

	
if (some condition) { 
    do something; 
} else {
    do something else; 
}