Welcome to Lesson 10!

• How do you test multiple conditions using logical operators?
• How do && and || operate when:
• both test conditions evaluate to true?
• both test conditions evaluate to false?
• one test condition evaluates to true and one to false?
• What are some pitfalls you might encounter when using logical operators and how can you avoid them

Announcements

• Midterm after a short lesson and the break
  
• No Lab this Friday due to Presidents' Day Holiday
  
• Assignment 9 due Thursday - don't forget!
• Come to De Anza Women in CS Club meeting tomorrow at 12:30 in the ATC 203 computer lab!
  

Logical Operators

• In certain situations, we may wish to use more than one test condition inside of an if statement.
• To do so, we will need to chain the test conditions together using logical operators (either && or ||)
  
• For example, recall our guessing game program:

Review of Boolean Variables

• Sometimes we need to evaluate a logical condition in one part of a program and use it elsewhere
• To store a condition that can only be true or false, we use a Boolean variable
• Boolean variables are named after George Boole (1815-1864), a pioneer in the study of logic
• We specify a Boolean variable using the boolean type, which can hold just one of two values: true or false

  boolean isCool = true;
  boolean lies = false; 
  

Test Conditions and Boolean Values

• Remember that test conditions always evaluate to true or false

  if (num > 0)
  

• Thus we can use a boolean variable as a test condition

  boolean isPositive = (num >= 0);
  if (isPositive)
  

• Note that we do not need to add a relational expression to a boolean variable, like:

  if (isPositive == true) // avoid!

• Since the boolean variable already evaluates to true or false, adding the == true is redundant
• Likewise, we do not need to use:

  if (isPositive != false) // avoid!

• If we want to reverse the test condition, we can use the not (!) operator

  if (!isPositive)

• We can see the use of a Boolean variable in the following example

Example Application Using a Boolean Variable

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public static void main(String[] args)
{

    int num;
    Scanner input = new Scanner(System.in);
    System.out.print("Enter a number: ");
    num = input.nextInt();

    boolean isPositive = (num >= 0);
    System.out.println("The test evaluated to: " + isPositive);
    if (isPositive) {
        System.out.println("The number was 0 or positive");
    } else {
        System.out.println("The number was negative");
    }
}

Introducing &&, || and !

Logical operators and search engines video

Combining Test Conditions with Logical Operators

• A logical operator, or Boolean operator, is an operator that treats operands as Boolean values (true or false)
• Java has several logical operators, but we only need to use three to create any possible test condition
• These three operators are and, or and not, which are discussed below
• These logical operators are traditionally written as && (and), || (or) and ! (not)

Truth Tables for AND, OR and NOT

&& (AND) Operator Truth Table

expr1	expr2	expr1 && expr2	Example	Result
true	true	true	5 < 10 && 5 > 2	true
true	false	false	5 < 10 && 5 < 2	false
false	true	false	5 > 10 && 5 > 2	false
false	false	false	5 > 10 && 5 < 2	false

|| (OR) Operator Truth Table

expr1	expr2	expr1 || expr2	Example	Result
true	true	true	5 < 10 || 5 > 2	true
true	false	true	5 < 10 || 5 < 2	true
false	true	true	5 > 10 || 5 > 2	true
false	false	false	5 > 10 || 5 < 2	false

not (!) Operator Truth Table

If expr is...	Then ! expr is...	Example	Result
true	false	!true	false
false	true	!(5 < 2)	true

Testing Multiple Conditions Using Logical Operators

• Sometimes we need to test for multiple conditions using a single if statement
  
• In this case, we will need to chain together the two conditions using a logical operator, either && or ||
• Note that one either side of the logical operator there must be a stand alone test condition that resolves to true or false
  

if (age >= 18 && age <= 25) //Correct!!!

if (age >= 18 && <= 25) //No! Incorrect!!

• For example, we want to test if an age is between 18 and 65
• We need to test that both parts of are true: is the age >= 18 AND is the age <= 65
  

int age = 0;
System.out.print("Enter your age: ");
age = input.nextInt();
if (age >= 18 && age <= 65)
{
    System.out.println("Adult!");
} else {
    System.out.println("Child, Teen or Older Adult!");
}

• Another way to use logical operators to test the age is:

  int age = 0;
  System.out.print("Enter your age: ");
  age = input.nextInt();
  if (age < 18 || age > 65)
  {
      System.out.println("Child, Teen or Older Adult!");
  } else {
      System.out.println("Adult!");
  }
  

• Many people confuse && and || conditions, especially when learning about logical operators
• A value lies between 18 and 65 if the value is at least 18 and at most 65
  
• A value is outside that range if it is less than (but not equal to) 18 or greater than (but not equal to) 65
  
• There is no golden rule; we have to think carefully and test our conditions

Parenthesis

• Remember that Boolean expressions in an if statement can be enclosed in parenthesis
• Thus, an if statement with && might look like:

  if ((guess != GUESS1) && (guess != GUESS2))

• However, relational operators have a higher precedence than logical operators, so the extra parenthesis are not necessary
  
• As such, we can remove the inner parenthesis without affecting the logical meaning:

  if (guess != GUESS1 && guess != GUESS2)

• However, if using parenthesis is easier to understand then use the extra parenthesis

More Information on Logical Operators

• Domino Addition - Numberphile: Logical operator circuits using dominoes