A commonly used problem solving technique

  • A commonly used programming technique to solve problems is: 

      Start with some initial (incorrect) solution

  as long as ( solution is not correct )
  {
      improve the solution
  }

  • Example: make a tasty lemonade: 

      Start with lemonade with no sugar added

  as long as (lemonade is not tasty enough)      
  {
     add 1 teaspoon of sugar // ≡ improve the solution
     taste lemonade
  }

  • The while-loop helps you construct this problem solving technique in Java

The while loop

  • Syntax of the while-statement in Java: 

       while ( loop-contuation-condition )
       one-statement;   // Loop body

or: 

   while ( loop-contuation-condition )
   { 
      statement1;        // Loop body
      statement2;
      ...
   }

  • Effect of the while-statement:

    1. Check the loop-contuation-condition

    2. If true: execute the loop body and repeat
      If false: end the loop and continue with the next statement in the program

The while loop    --    Example

  • Example of the while-statement in Java: 

     int i;

 i = 0;
 while ( i < 4 )
 { // Loop body
   System.out.println("Hello World");
   i++;
 }

  • Effect of this while-statement:

      • As long as i < 4 is true, the statements in the loop body will be executed (repeatedly)

    Result:

    • The loop body is executed when i = 0,1,2,3 but when i=4, the loop exits

DEMO: demo/05-loops/01-intro/While.java     (DEMO in BlueJ)

The while loop    Flow chart 

     i = 0;
 while ( i < 4 )
 { // Loop body
   System.out.println("Hello World");
   i++;
 }

  • The flow chart of this while-statement is:

Exercise

Write a program that computes the sum of 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 

  Program to compute  1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10

   Variables:

       We need to store these information:  

           (1) the sum 
           (2) the current number to add to sum

   Algorithm:
      
       sum = 0              // Set initial value when no number has been added
       current number = 1   // First number to add to sum

       as long as ( current number <= 10 )
       {
            add current number to sum
	    increase the current number to the next number
       }

Write program in BlueJ

Exercise

Write a program that computes the sum of 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 

public class Summation 
{
   public static void main(String[] args) 
   {
      int currNum, sum;
      
      sum = 0;
      currNum = 1;  // Important: we must give currNum the correct initial value !
 
      while ( currNum <= 10 )  // All numbers from 1 to 10
      {
        sum = sum + currNum;
        currNum++;
      }
      
      System.out.println("Sum = " + sum);
   } 
}

DEMO: demo/05-loops/02-while/Summation.java      Trace execution in BlueJ

A common error that results in a infinite loop

If you forget the braces, you will create an infinitely loop: 

public class Summation
{
   public static void main(String[] args) 
   {
      int currNum, sum;
      
      sum = 0;
      currNum = 1;
 
      while ( currNum <= 10 )
                                 // Forgot braces !
        sum = sum + currNum;     
        currNum++;               // Not executed inside loop !!
     
      
      System.out.println("Sum = " + sum);
   } 
}

DEMO in BlueJ

Programming exercise

  • Write a number guessing game in Java:

      • Write a program that randomly generates an integer between 0 and 100, inclusive.

      • The program then prompts the user to enter a number continuously until the number entered is equal to the randomly generated number.

      • For each user input, the program tells the user whether the input is too low or too high, so the user can make the next guess intelligently.

Exercise

  • Start with a plan: (use a mix of English and Java to write the plan) 

      
 int x = generate a random integer between [0..100]

 while ( user's guses is not equal to x )
 {
    guess = read next number from keyboard

    if ( guess < x )
       print "too low"
    else if ( guess > x )
       print "too high"
    else
       print "correct"
 }

     

Exercise

  • Write the steps of the plan out in Java: 

            Scanner input = new Scanner(System.in);
      
      int x = (int) (101*Math.random());
      int guess;
      
      while ( guess != x )
      {
         guess = input.nextInt();
         
         if ( guess < x )
            System.out.println("too low");
         else if ( guess > x )
            System.out.println("too high");
         else
            System.out.println("correct");
      }

    There is one small error in the program...

Exercise

  • You cannot use the variable guess without initializing it first: 

            Scanner input = new Scanner(System.in);
      
      int x = (int) (101*Math.random());
      int guess = -1;  // Need to start with an incorrect guess... 
      
      while ( guess != x )
      {
         guess = input.nextInt();
         
         if ( guess < x )
            System.out.println("too low");
         else if ( guess > x )
            System.out.println("too high");
         else
            System.out.println("correct");
      }

    Run program in BlueJ

DEMO: demo/05-loops/02-while/GuessNumber.java      (You can step through to program)

Controlling a while loop using a sentinel

  • Sentinel:

    • Sentinel = a special value that indicate the end of the input values

    Example: a list of positive numbers can use the sentinel number  0  to indicate the end 

     2   3   4   7   0

  • Program exercise:

    • Write a program that read a list of (positive) integer numbers until 0 and print their sum

Controlling a while loop using a sentinel

  • Start with a plan: (use a mix of English and Java to write the plan) 

      
 int sum = 0;
 int x;

 while ( x != SENTINEL )
 {
    print "Enter an integer (the input ends if it is 0): "
    x = read next number 

    if ( x != SENTINEL )
       Add x to sum;
 }

 print sum;

     

Controlling a while loop using a sentinel

  • Write the steps of the plan out in Java: 

            Scanner input = new Scanner(System.in);
      
      final int SENTINEL = 0;
      int sum = 0;
      int x;
      
      while ( x != SENTINEL )
      {
         System.out.print("Enter an integer (ends if it is 0): ");
         x = input.nextInt();

         if ( x != SENTINEL )
            sum = sum + x;  // Add a legitimate input
      }
      
      System.out.print(sum);

    There is one small error in the program... Can you fix it ?

Controlling a while loop using a sentinel

  • Like the previous program, we must initialize x before using it in the while-condition: 

            Scanner input = new Scanner(System.in);
      
      final int SENTINEL = 0;
      int sum = 0;
      int x = -1;
      
      while ( x != SENTINEL )
      {
         System.out.print("Enter an integer (ends if it is 0): ");
         x = input.nextInt();

         if ( x != SENTINEL )
            sum = sum + x;  // Add a legitimate input
      }
      
      System.out.print(sum);

    You can pick any initial value except the sentinel 0 !

DEMO: demo/05-loops/02-while/ReadAndSum.java