Else-if statements

An else ... if statement allows us to check another condition if the previous if condition was false.

Importantly, the code inside an else if block will only run if the previous if condition was false. Even if the else if condition is true, it won’t run if the previous if condition was true.

We can write this using words like this:

• Check condition1:
  • If condition1 is true, run the code inside the if block.
  • If condition1 is false, check condition2:
    • If condition2 is true, run the code inside the else if block.
    • If condition2 is false, skip the else if block [and move on to the next else if or else block, if any].

Basic syntax

if (condition1) {
    // Code to run if condition1 is true
} else if (condition2) {
    // Code to run if condition1 is false and condition2 is true
}

We can have multiple else if statements to check multiple conditions in sequence:

if (condition1) {
    // Code to run if condition1 is true
} else if (condition2) {
    // Code to run if condition1 is false and condition2 is true
} else if (condition3) {
    // Code to run if condition1 and condition2 are false and condition3 is true
}

Remember that, for example, condition3 will only be checked if both condition1 and condition2 are false.

Example: positive, negative, or zero

Suppose we have an if statement that checks if a number is positive:

int number = 10;
if (number > 0) {
    Console.WriteLine("The number is positive.");
}

But what if we also want to check if the number is negative or zero? We can use else-if statements to handle these additional conditions.

int number = -5;
if (number > 0) {
    Console.WriteLine("The number is positive.");
} else if (number < 0) {
    Console.WriteLine("The number is negative.");
} else if (number == 0) {
    Console.WriteLine("The number is zero.");
}

Output:

The number is negative.

In this example:

• The first if checks if number is greater than 0. Since number is -5, this condition is false.
• The program then checks the else if condition to see if number is less than 0. This condition is true, so it prints “The number is negative.”
• The last else if condition is not checked because one of the previous conditions was true.

You may realise that, if the number is not positive and not negative, it must be zero. So we don’t actually need to check the last condition - and we’ll learn how to do that next!