Revision

1
Revision
Any questions regarding homework-2 or
homework-3? Anyone still facing trouble running
javac compiler in their personal
computers/notebooks?
2
For loops - Revisited

• Make a Hello World program which prints Hello
  World! 5 times on the console.
• Do not repeat System.out.println function 5
  times.
• Instead, use for/while loops.

3
The Calculator problem - Revisited

• Would cover increment/decrement operators and
  for/while loops ( and --)
• Develop an integer calculator which accepts only
  two operands both operands inserted would be
  greater than zero.

4
The Rules

• You have to implement the ADD function.
• Binary operators, and , are not allowed
• You can use unary operator. Usage of lt, gt, gt,
  lt, , and ! operators is allowed

5
The Rules (contd)

• Sample Output
• C\gt java Calc
• Calculator
• 1- Add
• 2- Subtract
• Your choice 1 User input
• Enter integer 1 3 User input
• Enter integer 2 2 User input
• The answer is 5

6
One Possible Solution

• Think of the two operands as buckets containing
  cells.
• Assume var1 has 3 cells while var2 contains 2
  cells

Var2
Var1
7
One Possible Solution

• Think of the two operands as buckets containing
  cells.
• Assume var1 has 3 cells while var2 contains 2
  cells

Var2
Var1
8
One Possible Solution

• Think of the two operands as buckets containing
  cells.
• Assume var1 has 3 cells while var2 contains 2
  cells

Var2
Var1
9
One Possible Solution (contd)

• Algorithm
• Run a loop var2 times
• And in each iteration increment the value of
  var1 by 1

10
One Possible Solution (contd)

• Pseudocode
• for i 1 to var2
• increment var1 by one
• Then finally display the value of var1(final
  answer)

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One Possible Solution (contd)

• Possible transformation to Java code
• for (int i 1 i lt var2 i )
• var1 var1 1 // or var1

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Final Code

• Public static void main (String args)
• int var1 3
• int var2 2
• for ( int i 1 i lt var2 i)
• var1
• System.out.println( The final answer is
  var1 )

13
Related Exercises

• Subtract (change only one line)
• Multiply (nested for loops)
• Divide (nested for loops)

14
Todays Exercise

• Based on your current understanding of the for
  loops, implement the factorial function
• Factorial of a positive integer is the product of
  all the positive integers less than or equal to it

15
Factorial (contd)

• Example
• Factorial( 5 ) 5 4 3 2 1 120
• Factorial( 4 ) 4 3 2 1 24
• Factorial( 1 ) 1
• Important!
• Factorial( 0 ) 1
• Factorial( -ve numbers)?.... invalid input

16
Your Plan

• Dont start coding immediately
• Make your algorithm, and work out your plan on a
  piece of paper first
• Spend 5 minutes thinking over this and come up
  with a pseudocode

17
Pseudocode

• integer input integer from console
• integer output 1 (default value)
• for iterator, i input down to 1
• output output i
• Finally, print output (final answer)

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Pseudocode

• integer input integer from console
• integer output 1 (default value)
• if input 0, then output 1
• else if input lt 0, then print Invalid Input
• else
• for iterator, i input down to 1
• output output i
• Finally, print output (final answer)

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Final Code

• import java.util.Scanner
• public class Factorial
• public static void main( String args )
• int input, output 1
• Scanner keyboard new Scanner( System.in )
• input keyboard.nextInt()
• if ( input 0 ) System.out.println( 1 )
• else if ( input lt 0 ) System.out.println(Invalid
  input)
• else
• for ( int i input i gt 0 i--)
• output output i
• System.out.println( Answer output )