Logical operators

Logical operators are also, maybe more commonly, called boolean operators.

The term “boolean” originates from the book “The Mathemetical Analysis of Logic” written by George Boole in 1847. Boolean logic has been fundamental in the development of computers, and programming, since at their core, computers are all about processing whether or not there is current flow: on vs off (true vs false).

If you are curious, feel encouraged to google and read up on this online, but for now, we can simply look at the 3 fundamental boolean operators and what they do: and, or, and not.

The operator and returns true if, and only if, both values also are true. So, only the expression true and true is also true. All of the expressions true and false, false and true, false and false evaluate to false.

If you think about this, and come up with English sentences, then this will make a lot of sense: At the restaurant I’ll have a tomato soup IF it is vegan AND they still have some.

The operator or on the other hand returns true if at least one of the values is true. So, only if both values are false, the operator returns false.

That’s why it is logically correct to answer the question Would you like tea or coffee for breakfeast? with Yes, please. IF you’d like either tea, or coffee, or both. You’d only say Hell, no! if you’d like an orange juice instead :)

The operator not simply returns the negated, opposite value. not true returns false, and not false returns true. Therefore, the following lines of code are the same:

puts "Always true" if not false
puts "Always true" unless false

Each of these three operators comes in two versions:

• and and &&
• or and ||
• not and !

The difference between them has to do with what is called “operator precedence”.

From math you know that 1 + 2 * 3 evaluates to 7, not 9. This is because the multiplication * operator binds stronger, and precedes the addition operator +. In other words 1 + 2 * 3 is same as 1 + (2 * 3), and not the same as (1 + 2) * 3.

In Ruby, the operators &&, ||, and ! bind stronger than, and thus precede their fellows and, or, and not.