Lab1d

Lab1d

You have seen addition, multiplication and subtraction in the c labs. There are 2 more integer operations. Let's look at them now

Division - Copy, paste, compile and run
Use 8 and 4 as the integers first time, use 8 and 3 the second time. What is happening here?????

// integer operations

import javax.swing.*;

public class Lab1d1
{
    public static void main(String[] args)
    {
       String number1 = "";
       String number2 = "";

       number1 = JOptionPane.showInputDialog("Enter the first integer");
       number2= JOptionPane.showInputDialog("Enter the second integer");

      // declare integers
        int n1= 0, n2 = 0;

      // change Strings to integers
      n1 = Integer.parseInt(number1);
      n2 = Integer.parseInt(number2);

int answer1 = n1/n2; // figure out the answer
JOptionPane.showMessageDialog(null,number1+" / "+number2+ " = "+answer1);

System.exit(0);
}
}

This is an example of Integer Division.
RULE: When you divide 2 integers, the answer is always an integer.

It is NOT rounded off. You get the quotient - the answer to a division problem. the remainder is thrown away.

Try it with 9 and 5 - see if you get these result.

There is an integer operation to find the remainder. It is called "mod" for modulus operation.
It is an operation on some calculators.

Add this code before the system exit( 0), compile and run with 9 and 5.

Java uses the % sysmbol to ask for the remainder.

int answer2 = n1 % n2;
JOptionPane.showMessageDialog(null,number1+" mod "+number2+ " = "+answer2);

Mod is used extensively in games with spinners, dice or deck of cards

Arithmetic Operators for Java

These are the arithmetic operations built into Java for integer (int) numbers.
They are + (addition), - (subtraction), * (multiplication), / (division), and % (modulus)

Operator	Use	Description
`+`	`a + b`	Add `a` and `b`
`-`	`a - b`	Subtract `b` from `a`
`*`	`a * b`	Multiply `a` by `b`
`/`	`a / b`	Divide `a` by `b`
`%`	`a % b`	Compute the remainder of dividing `a` by `b`
`-`	`-a`	How to negate `a` (take the opposite)

Decimal number in java

There are 2 primitive data types for decimal numbers. Decimals are called floating point numbers

Floating Point Primitive Data Types

Type	Size	Range
float	32 bits	-3.4E+38 to +3.4E+38
double	64 bits	-1.7E+308 to 1.7E+308

We will use the double for our decimals

Copy, paste, compile and run this program with 9 and 5, 12.4 and 1.3 , 8 and 2.3
It works with integers and doubles. By the way, what happens if you say 5 divided by 0 ?

// doubles

import javax.swing.*;

public class Lab1d2
{
    public static void main(String[] args)
    {
     String number1 = "";
     String number2 = "";

     number1 = JOptionPane.showInputDialog("Enter the first number ");
     number2= JOptionPane.showInputDialog("Enter the second number ");

     // declare doubles
     double n1= 0, n2 = 0;

     // change Strings to doubles
      n1 = Double.parseDouble(number1);
      n2 = Double.parseDouble(number2);

      double answer1 = n1/n2;
      JOptionPane.showMessageDialog(null,number1+" / "+number2+ " = "+answer1);
      System.exit(0);
      }
}

Agreement

Everything must agree:

If you are using decimals, use double and Double.parseDouble
If you are using integers use int and Integer.parseInt

If you mix ints and doubles using the operations ( +, -, *, /, % ) , the answer will always be a double

So, why not just use doubles all the time??? Doubles are tricky.

Tricky doubles

Most of the times we will use integers in CS1. There are times when we need doubles. Doubles are not as easy to use as you will see in this example. Strange things happen.

Look at the code to see if you understand what is happening. then copy, paste and run it.

Run it with the numbers 5 for nickels and 8 for dimes - looks good - Right???
Wrong! - run it with 4 nickels and 6 dimes - it doesn't really look like money with the 0 missing
run it with 5 nickels and 7 dimes YUK!!!! - totally weird!!!!

Doubles will do that to you.

Rule for doubles: When you use doubles, you have to control the number of decimal places

// doubles can cause troubles

import javax.swing.*;

public class Lab1d3
  {
     public static void main (String[] args)
      {
        String num1 = "";
        String num2= "";

        num1 = JOptionPane.showInputDialog("How many nickles do you have?");
        num2= JOptionPane.showInputDialog("How many dimes do you have");

   // declare integers
      int n1= 0, n2 = 0;

   // change Strings to integers
     n1 = Integer.parseInt(num1);
     n2 = Integer.parseInt(num2);

    // formulas
     double total = (n1*0.05) + (n2*0.10); // value of the coins
            // all parentheses use is exactly the same as algebra
     int sum = n1+n2; // for the total number of coins

     JOptionPane.showMessageDialog(null,"You have "+sum+" coins\n"+
                                        "they are worth $"+total);

System.exit(0);
}
}

How to control decimal places (round off)

Copy, paste and run this sample. The new Stuff is in Red

We will discuss what is happening below this block

// controlling number of decimal places in doubles
import javax.swing.*;
import java.text.*; // Decimal Formatting comes from this import

public class Lab1d4
{
  public static void main (String[] args)
  {
    DecimalFormat df2 = new DecimalFormat("0.00");
    DecimalFormat df1 = new DecimalFormat("0.0");

    String num = "";

    num = JOptionPane.showInputDialog("Enter a decimal number ");

    double n;
    n = Double.parseDouble(num);

   String ans = df2.format(n); //takes the decimal number, rounds
                                  //it to 0.00 and saves it as a String

    JOptionPane.showMessageDialog(null,num+" Rounded to two places is "+ ans );
    System.exit(0);
   }
}

Try your program with numbers like:
134.5678 9 2.5 -2.532

1. This line is calling up the decimalformat class with the object DecimalFormat
import java.text.*;

2. These lines are creating 2 instances of the DecimalFormat Object
            DecimalFormat df2 = new DecimalFormat("0.00");
      DecimalFormat df1 = new DecimalFormat("0.0");
                             "0.00" means 2 decimal places
                             "0.0" means 1 decimal place

I use the variable names df1 and df2.
df1 for decimalformat variable for 1 place,
df2 for decimalformat variable for 2 places

3. When you format a decimal to a certain number of decimal places you change it back into a String so you can never, never do math with that number unless you parse it again. That is this line does.
           String ans = df2.format(num);
           We open a String memory location called ans, num must be a double ( or an integer)
            format rounds answer off to 2 places and saves it as a String

4. Add this code before the System.exit(0) to see how the rounding to 1 decimal place works

String ans2 = df1.format(n);
JOptionPane.showMessageDialog(null,num+" Rounded to one place is "+ ans2 );

We are now going to "fix" Lab1d3 so that the decimals look good

1. add the import line.

2. add the DecimalFormat for 2 places at the top of the main method

3. total is the only variable that is a double and needs formatting. After the formula line - add this line
String total2 = df2.format(total);

4. Change total in the showMessageDialog to total2 - try it now with 5 nickles, 7 dimes

New Stuff today:

1. Integer division and integer mod operation

2. When do you need a doubles? How to declare a double double n = 0;

3. Parse a String to get a double n = Double.parseDouble(num);

4. Steps to Round off a double
--- import line
---declare the DecimalFormat object
--- use the object to convert a decimal rounded off to a String

Assignment

Lab1d5 - 40 pts

Lab1d4 would be a good template for this Lab.

On the right you see a sample run for this lab when you enter 6 as the radius.

give answer before rounding and after rounding to 2 decimal places.

Use 3.14159 for pi.

formula
area of a circle =
3.14159 *radius * radius

Lab1d6 - 40 pts

On the right you see a sample run for this lab. Feel free to change the event - ie car wash, etc. You pick the template that you want. None of the samples is perfect.

Enter the amount of money and the number of people.

Show the answer after the number is rounded off for money

Lab1d7 - 20 pts

This lab uses only integers. There is no rounding off. It uses integer operations.

Lab1d1 will help you.

Here is how I got the answer