How do we write code that tells us whether a user is logged in to our program? Booleans are the solution to these questions.

What are booleans?

A boolean refers to a value that is true or false. Those are the only values of a boolean expression, and these are useful if we want to check if something is true or false.

Meet George Boole

Let's meet the fellow behind the name, "booleans," George Boole. Mr. Boole was an English-born mathematician, philosopher, and logician. He introduced boolean algebra in The Mathematical Analysis of Logic (1847) and Investigation of the Laws of Thought (1854). Many consider him to be one of the founders of Computer Science.

George Boole

Example with booleans

How about an example? Let's create a variable and set it equal to true. Then, we'll print the variable.

We first want to declare the variable, loggedIn, and set it to true or false. The box below shows the value of loggedIn.

var loggedIn = true;


Now, we can go ahead and print it out. We can similarly set the variable to false instead:

var loggedIn = false;

Notice that we do not need to have quotations around true or false.

The full example looks like:

// In this program we declare a boolean
// value to represent whether the user
// is logged in, and then print it out.
function start(){
    var loggedIn = false;
    println("User logged in?: " + loggedIn);

Remember, use booleans if you want to set a variable to true or false!