Lab2d Logic operators

Logic operators
Lab2d

In addition to <, >, <=, >=, ==, != and the .equals( ) method, there are more operators for the booleans.

Operator	Read as	Example	Status
`&&` conjunction	and	`if (n > 3 && n < 7)`	`if n > 3` and `n < 7` are both `true`, then statement is true like when n is 5
`\|\|` disjunction	or	`if (a<5 \|\| a> 8)`	if either `a` `<` `5` or a `> 8` is `true`, then statement is true, in this example it is false when a is 6
`! negation`	not	`if(!(a <= 7)` )	`changes a <= 7 to opposite truth value`

They allow us to make combination statement to look at multiple variables or multiple situations.

Using &&, || and !

For each of the following conditionals determine why they evaluate to true or false.

( 6 > 5 && 4 = = 2 ) false
(3 * 4 >=12 && 7 < 9 ) true
( 9 + 5 != 14 || 6 > 8 ) false
( 8 - 3 ==4 || 0 < 1) true
( !( 4 > 3) ) false

For these assume x = 3, y = 5 and z = 0

( x < y && z == y - 4) false
( x < y || z == y - 4) true
( ! (z == x ) ) true
( y > z || ??? ) true

Sample program

Look at this sample using &&. - Figure out what is happening before you run it. In this example I used only if's.

It would have been better programmed as nested if elses. The code for nested version is in the next box.
Copy, paste and run. there are two flaws in this code - can you find them and fix them?

// boolean operator &&

import javax.swing.*;

public class Lab2d1
{
  public static void main(String[] args)
  {
    String amount = JOptionPane.showInputDialog("How much do you earn each week?");
    double a = Double.parseDouble(amount);

    if ( a <= 0)
     {
      JOptionPane.showMessageDialog(null,amount +" ok - we are serious here!!!");
      }
    if(a > 0 && a < 50) // in algebra this would look like ( 0< a < 50 )
    {
    JOptionPane.showMessageDialog(null,amount +" gives you some spending money");
    }
   if( a > 50 && a < 100 )
   {
    JOptionPane.showMessageDialog(null,amount +" Hey, that is a pretty good job");
    }
    if( a > 100)
    {
     JOptionPane.showMessageDialog(null,amount +" Wow, do you have time to study? ");
     }
    System.exit(0);
  }
}

Surprise! With nesting you did not need the && part. Let's look at this program.

// boolean operator &&

import javax.swing.*;

public class Lab2d2
{
  public static void main(String[] args)
  {
    String amount = JOptionPane.showInputDialog("How much do you earn each week?");
    double a = Double.parseDouble(amount);

    if ( a <= 0)
     {
      JOptionPane.showMessageDialog(null,amount +" ok - we are serious here!!!");
      }
    else
      if( a < 50)
       {
        JOptionPane.showMessageDialog(null,amount +" gives you some spending money");
        }
       else
         if( a < 100 )
          {
          JOptionPane.showMessageDialog(null,amount +" Hey, that is a pretty good job");
          }
         else
         {
          JOptionPane.showMessageDialog(null,amount +" Wow, do you have time to study? ");
         }
    System.exit(0);
  }
}

Notes:
Lab2d1 requires 4 comparisons everytime it runs
Lab2d2 will have 1, 2, or 3 comparisons - but never 4 - therefore more streamlined code

&& and || in a sample

In this example you will see another use of && (and).

Background material: people can tell if a number is even or odd by looking at the last digit. The computer can not look at the last digit so we have to tell the computer some other way to identify an even number from an odd number.

Hint: Think about what happens when you divide an integer by 2.

Copy paste and run it

// sample using && and ||

import javax.swing.*;

public class Lab2d3
{
  public static void main (String[] args)
  {
   String num1 = JOptionPane.showInputDialog("enter an integer ");
    int n1 = Integer.parseInt(num1);

    String num2 = JOptionPane.showInputDialog("enter another integer ");
    int n2 = Integer.parseInt(num2);

   if( n1 % 2 ==0 && n2 % 2 ==0)
      {
     JOptionPane.showMessageDialog(null,num1+" and "+num2+" are both even");
      }
    else
      {
     JOptionPane.showMessageDialog(null,num1+" and "+num2+" are both odd ");
      }
   System.exit(0);
   }
}

Class discussion - This program is not completely correct.
What happens when we enter one even and one odd number?

We will fix it together in class

Using && and || to make a program better

Open Lab2b2 - Bob and the apples. We are going to "fix" to an old program.

Think about what would make this program look bad. Try entering a negative number.
To prevent the user from using negative numbers we are going to add an if...else

Before the first if add this- if your variables are different - change to your variables.

Now try the program with numbers like -6 or -4

if( n < 0 || a < 0) // n is a negative or a is a negative
      {
      // say something about entering bad data
      }
else
   {
   // rest of program goes here

Assignment

Open Lab1c5 and save as Lab2d4 - 50 points
We know that triangles can be classified by the length of sides.

If all 3 sides are the same, it is an equilateral triangle
If 2 of the sides are the equal( congruent) the triangle is isosceles
If all 3 sides are different it is scalene.

This assignment is easy with nested if .. else but lots of if( ) 's work if you are careful.

Here are some sample number for you to check your code

50 points Lab2d5

Science Academy's grades are set up like this
90 - 100 are A's
80 up to 90 are B's
70 up to 80 are C's
less than 70 are failing

We are going to average 3 grades. Here is a template to get you started. There is a trick to get the decimal places for the average. If you can not figure it out, please ask me.

Use conditionals to describe what letter grade it gets and make a comment. You may use if's with && or nested if elses. See Lab2d1 and Lab2d2 for samples.

Warning: be sure to test at the points where comments change - like at 70, 80, 90, and don't forget 100.

// average of 3 grades
import javax.swing.*;

public class Lab2d5
{
   public static void main(String[] args)
   {
    String grade1 = JOptionPane.showInputDialog("Enter your grade 1");
    int g1 = Integer.parseInt(grade1);

System.exit(0);
}
}

My sample runs