Conditional Statements

Introduction
If Statements
- Else If Statements
- Else Statements
Switch Statements
The Ternary Operator

Introduction

Conditional statements allow the computer to execute some code only if a certain condition is true. This is incredibly important for programming, as it combines with loops to allow the program to make decisions. The main types of conditional statements are the family of if, else if, and else, and the switch statement.

If statements

If statements execute a piece of code if a certain condition is true (conditions are anything which can resolve to a boolean value). Otherwise, they skip it. They are written:

if (condition) {
    // code you want to run
}

If condition is true, the code will run, if not, it won't.

Else If statements

An else if statement is tacked onto the end of an if statement, written as follows:

if (condition1) {
    // some code
} else if (condition2) {
    // some other code
} else if (condition 3) {
    // yet more code
}

The code in an else if statement will execute if condition 1 is NOT true, but condition 2 IS. You can put as many else if statements in a row as you like, and each will only execute if all the conditions above them are false, but they are true.

Else statements

An else statement is put after an if or else if statement, and will automatically run if none of the conditions above it are true. For example:

if (condition1) {
    // code
} else if (condition 2) {
    // more code
} else if (condition 3) {
    // different code
} else {
    // yet more code
}

If condition1 is true, // code will execute. If condition1 is false but condition2 is true, // more code will execute. If condition1 and condition2 are false but condition3 is true, // different code. Finally, if none of the conditions are true, // yet more code will execute.

Switch statements

A switch statement is used when the alternative would be making a really long series of if, else if, else if, else if etc statements. It works by looking at a variable, usually an int, and checking its value against different expected values. It is written like this:

switch (variable) {
    case 1:
        // some code
        break;
    case 2:
        // other code
        break;
    case 3:
        // more code
        break;
    case 4:
        // extra code
        break;
    default:
        // possibly code?
        break;
}

If variable is equal to one, // some code will execute. If variable is equal to two, // other code will execute, and so on. If variable is not equal to any of the cases, the default case will execute.

note: remember the break;s! If you leave them out the code will continue to execute and you'll end up running all the code in the statement.

note: variable can be a string too, in which case you'll want to write stuff like case "*":, case "/":, case "+": and case "-":

The Ternary Operator

The ternary operator is a special operator (like +, /, *, -) except that instead of taking two operators, it takes three (this is where it gets the name ternary, meaning three). It works differently from the other conditional statements in that it "resolves" to a value, so you can only use it where you would otherwise be able to use a variable or a mathematical expression. In Java it is written as follows:

condition ? expression1 : expression2;

It may look a little confusing, but basically:

If the condition is true, the ternary operator is equal to expression1.
If the condition is false, the ternary operator is equal to expression2.

For example, let's say I want to find out which of two variables is larger.

int x = 5;
int y = 15;
String comparison;
comparison = x > y ? "x is larger than y" : "y is larger than x";

In this case, the ternary operator resolves as follows:

comparison = x > y ? "x is larger than y" : "y is larger than x";
comparison = 5 > 15 ? "x is larger than y" : "y is larger than x";
// conditional is false so the second expression is chosen
comparison = false ? "x is larger than y" : "y is larger than x";
comparison = "y is larger than x";

if x was larger than y, comparison would be made equal to "x is larger than y".

note: the condition MUST be able to resolve to true/false, and each expression must be something you would be able to put in an assignment statement (i.e. variable = expression)

Conditionals