Lecture for February 6th

If Statements – Cont.

Inside the parenthesis of the if statement must be a boolean expression. This is an expression that evaluates to either true or false. We can do more complex boolean expressions through logical operators.

Logical Operators

NOT !a this is true when a is false

AND a && b this is true when both operands are true

OR a || b this is true when either a is true OR b is true

Truth Tables

• Show all possible outcomes
• It breaks the expression down into parts

Not

Let’s look at the most simplest case. Not.

AND

Notice here that a && b is only true when both a and b are true.

OR

Notice here that a || b is only false when both a and b are false.

Precedence (Order of Operations)

Playing with Truth Tables Example

a && !b

!a || b

!(a || b && c)

!a || b && c

Distributive Property of Logical Operators

The following statements are equivalent

!(a && b) is equivalent to !a || !b

Notice how when you distribute the ! you have to flip the operand as well. && becomes ||

Same is true for the following example

!(a || b) is equivalent to !a && !b

!(a || b && c) is equivalent to !a && (!b || !c)

Short Circuit Evaluation

In an && (AND) statement, if the left side is false, there is no need to evaluate the right side. Since it’s going to be false anyways!!

false && true; // FALSE no matter what the right side is

In an || (OR) statement, if the left side is `true, there is no need to evaluate the right side. Since it’s going to be true by default!!

true || false; // TRUE no matter what the right side is

Java takes this shortcut by default for efficiency reasons