Copyright © 1997, 2013
Robert W. Hasker

Exam 1

• Material through Ch. 4 (depending on how far we get today)
• Tomorrow: review session
  • But what does that mean? I should reteach everything?
  • Studying 1011:
    • Know facts: rules for identifiers, if statements, types
    • Know how to do things: write code which ...
    • Know concepts: why a program has a main, why formatting code is a good idea, why having named constants (final) improves code
  • Bring questions
    • Or send them by email in advance!

Loops

• Ch. 4

Intro

• Decision: a fork in the road
  • Decide which of two paths [draw horizontally]
• Loop: going back to the start
  • Why: keep forgotting things, running a race and just need to make several laps
• Pseudocode for the forgetful traveller:

  	 while I have more stuff to get,
  	    go back in house and get another item
  

• General form:

  	 while condition,
  	    actions
  

• Written in Java:

  	while ( test )
  	{
  	   statements
  	}
  

• Suppose we want to read the distance to some point
  • pseudocode:

    	prompt for distance
    	while distance < 0, do
    	   prompt for distance again
    

  • in Java:

            System.out.print("Enter the distance: ");
    	double distance = in.nextDouble();
    	while ( number < 0 )
    	{
    	   System.out.print("Cannot have a negative distance; please re-enter: ");
               distance = in.nextDouble();
    	}
    

Count-controlled While Loops

• Suppose we want to write 10 stars on the screen:
  • pseudocode:

    	stars_printed <- 0
    	while stars_printed < 10, do
    	   print '*'
    	   increment stars_printed
    

  • Java:

    	int stars_printed;
    	stars_printed = 0;
    	while ( stars_printed < 10 )
    	{
    	   System.out.print('*');
    	   stars_printed++;
    	}
    

    • [trace this loop]
    • note: body of while loop indented 3 spaces
    • What would happen if I wrote

      	int stars_printed;
      	stars_printed = 0;
      	while ( stars_printed < 10 );
      	{
      	   System.out.print('*');
      	   stars_printed++;
      	}
      

      • never put a semicolon right after the test!
• What do you get from

  	int counter;
  	counter = 1;
  	while ( counter <= 10 )
  	{
  	   System.out.print(counter);
  	   counter++;
  	}
  

  • How to change to print backwards from 100 to 1?
• Note the structure of these loops:

  	initialize loop-control variable
  	while not at limit, do
  	   process data
  	   update loop-control variable
  

• EXAMPLE: compute total cost of order with numItems entries
• EXAMPLE: How to change to also count number of entries over $10?

Sentinel-controlled While Loops

• Suppose we wanted to average arbitrary list of numbers.
  • examples: 1 5 6; -2 5 3 -23
• how would we know when to stop reading numbers?
• solution: use a sentinel
• structure of loop:

  	read first item ("priming read")
  	while item not sentinel
  	   process
  	   read next item
  

• Example: compute average until read 9999
  • add check for zero value
• pseudocode:

  	sum <- 0
  	count <- 0
  	read num
  	while num != 9999
  	   sum = sum + num
  	   increment count
  	   read num
  	print sum / count
  

• in Java:

          final int SENTINEL = 9999;
  	int sum, count, num;
  	sum = 0;
  	count = 0;
          num = in.nextInt();
  	while ( num != SENTINEL )
  	{
  	   sum = sum + num;
  	   count++;
  	   num = in.nextInt();
  	}
  	System.out.println((double)sum / (double)count);
  

• Note structure: read before loop and at end of loop.
  • what if move read to top of loop?
• Other examples:
  • average fleet of cars, stop at negative mpg b
  • count dice rolls until roll a 7
  • compute pay for employees at $6.50/hour until read negative hours

Loop-de-loop

• EXAMPLE: work in pairs to complete following, put answers on board:

          // print specified number of lines of stars with width stars on
          //    each line
          public static void int main(String[] args)
          {
             final int WIDTH = ??;
             final int LINES = ??;
          
  
          }
  

  • note especially the initialization of the inner loop's counter
  • [Ask: how to change program to prompt user for the number of rows.]
    • This is where it's important that the condition be tested before entering the loop.
• More examples to try:
  • print the positive values between two numbers entered by the user
  • prompt user for given number of exam scores, then prompt for that number of scores and compute an average
  • prompt user for given number of dice rolls, then count all the times 7 turns up
  • compute average mpg for a fleet of cars

Review

• while loops
• count-controlled, sentinel-controlled loops