Welcome to Lesson 11!

• What is a counting loop?
• What are the three important components of a counting loop?
• What is a for loop and why was it designed?
• How does a for loop differ from a while loop?
• What are some applications of for loops?
• What is an off-by-one error and how can you avoid this type of error?

1. Lesson 10 Practice Exam Questions

• Label the different parts of the following while loop as: update statement, initialization or test condition.

int count = 1;

while (count <= 10) {

    System.out.println(count);

    count++;

• Correct the loops below. What will happen if you run the code BEFORE making the correction?

Loop 1:

String repeat = "y";

while (repeat.equals("y")) {

    System.out.println("Playing an exciting game!\n");

    System.out.println("Want to play again? (y/n): ");

Loop 2:

    int counter = 1;

  while (counter <= 10) {

      System.out.println(counter);

2. Counting Loops

• Counting is a common task in our lives.
• In programming, counting is also very common.
• For example, we saw an example of counting in an activity, where we used a while loop to count down from 10 to 0.
• In programming we often use loops to count when we know exactly how many times we want a specific piece of code to repeat.
• In fact, counting is such a common programming task, that a special type of loop was designed specifically for counting.
• This loop is called a for loop, and we will be learning more about it in class today.

//counting from 10 to 20 using a while loop

int i = 10; 

while (i <= 20)

{

    System.out.println(i);

    i++;

}

//counting from 10 to 20 using a for loop

for (int i = 10; i <= 20; i++)

{

    System.out.println(i);

}

Image source.

Activity 11.1: Counting Down Part 3 (10 pts)

• Make a new folder named Activity11 in Eclipse.
• Add a new class named Countdown.java into this folder.
  
• Open up your Countdown.java file from last class.
• Copy and paste this code into your new Countdown.java folder.
  
• Locate the while loop in your code.
• Alter this while loop to be a for loop.
• Hint: Make sure that your for loop has 3 parts:

1. initialization (where do you want to start counting?)
2. test condition (when should the loop fail?)
3. update statement (are you counting up or down?)
   

• Compile and run your program and verify that you still get the following output:

NASA Mission Control readying for liftoff.

Initializing countdown from 10...

10

9

8

7

6

5

4

3

2

1

We have liftoff!

• Submit your program to Canvas when you are finished.

image source

3. Applications of For Loops

• We can use for loops in two ways:

1. To repeat a task a specified number of times, where the task performed is not dependent on the counter.
1. For example, making a bar graph (below).
2. To repeat a task a specified number of times, where the task performed is dependent on the value of the counter.
1. For example, summing a sequence of numbers (below).
   

Making Bar Graphs

• We can use a counting loop to make a horizontal bar graph

image source

• However, instead of displaying a rectangle as shown in the image above, we will display a series of horizontal "*" characters for the "bar"
• Thus, all we need to change is the print statement, where we displayed a series of numbers:

System.out.println(i);

• to become:

System.out.println('*');

• We can see this change in the following example:

Example Application That Displays a Bar Chart

public static void main(String[] args) {

int number = 0; Scanner input = new Scanner(System.in); System.out.print("Enter a number and I will show its"              + " bar graph.\nEnter your number: "); number = input.nextInt(); System.out.println("\nBar graph:");         for (int i = 0; i < number; i++) {     System.out.print('*'); } System.out.println(); input.close();

}

Summing Numbers

• One common looping task is to input a list of numbers and calculate their sum

• For example, if we want to add the sum of the first four integers:

  sum = 1 + 2 + 3 + 4 = 10

• As another example, we can add a list of 5 numbers:

  sum = 1 + 2 + 3 + 4 + 5 = 15

• We can generalize our examples to sum from 0 to any number n.

Example Application to Sum a Sequence of Numbers

    public static void main(String[] args) {
        int number = 0;
        int sum = 0;
        Scanner input = new Scanner(System.in);

        System.out.print("Enter the number to sum to: ");
        number = input.nextInt();
               
        for (int i = 1; i <= number; i++) {
            sum = sum + i; //using the counter as a value to add to the sum
        }
               
        System.out.println("The sum is: " + sum);
       
        input.close();
    }

Activity 11.2: Tracing a Loop (10 pts)

• In this exercise, we will trace the execution of the following for loop.
• DO NOT run the below code. Instead, can you figure out what this code would print to the console if you were to run it?
  

for (int i = 10; i <= 15; i++)

{

    if (i == 11 || i == 13 || i== 15)

        System.out.print("*");

    else

        System.out.print("!");

}

• Locate the assignment on Canvas and click submit so that you can see the textbox.
• In the text box, type what the output of the above for loop would be to the console (trace it through by hand!).
• In other words, what sequence of characters will be printed to the console?
  
• Note, you should do this without running the code. 
• Then, change the statement i <=15 to be i < 15.
• Type the output of the for loop with this alteration.
• How is it different?
• The instructor will be going over the examples so you can verify your answers and understanding.
• Don't forget to submit your work!
  

4. More Loopy Errors

• A common problem with counting loops is the off-by-one error
• Finding the correct upper and lower bounds can be confusing
  • Should you start at 0 or 1?
  • Are you counting up to a number (<), or up to and including a number (<=)?
• To understand the counting loop you need to count iterations.
• For instance, the following loop executes b - a times:

  for (int i = a; i < b; i++)

• However, the following loop is executed b - a + 1 times:

  for (int i = a; i <= b; i++)

• The "+1" is the source of many errors
• For example, to sum the numbers from 1 to 5, what is wrong with the following:

  
  public static void main(String[] arg){
      int sum = 0;
      for (int count = 0; count < 5; count++)
      {
          sum += count;
      }
      System.out.println(sum);
  }
  

• One way to visualize the "+1" error is by looking at a fence

Image source.

• How many posts do you need for a fence with three sections?
• It is easy to be "off-by-one" with problems like this
• Forgetting to count the last value is sometimes called a fence-post error

Debugging Technique: Tracing Variables

• One good way to discover loopy errors is to display the variables that are part of the loop
• Tracing variables means watching their values change as the program executes
• You can insert temporary output statements in your program to watch the variables, like:

  System.out.println("count= " + count + ", sum=" + sum);

Activity 11.3: Summing and Averaging (10 pts)

• Let's write a program that uses a for loop to sum a list of numbers and computer their average.
• Open up a new class named Sum.java and add it to your Activity11 folder.
  
• First, declare and initialize 3 variables at the top of main:

• Next, write a print statement to display a welcome message to the user:

Give me a list of numbers and I will compute their sum and average!

• We will need to know the number of user inputs in advance so that we will know the ending condition of our for loop (in other words, how many numbers the user is planning to input).
• Prompt the user to enter the quantity of numbers and read in the value to a variable named quantity:
  

  System.out.print("Enter the quantity of numbers: ");
  quantity = input.nextInt();

• Next, we will write a for loop to read in that number of user inputs.
  

for (int i = 1; i <= quantity; i++) {

    //body of loop goes here

} 

• Inside the loop we ask for each value and add it to the sum

  System.out.print("Enter value #" + i + ": ");
  number = input.nextDouble();
  sum = sum + number; //add number to our running total for the sum

• The for loop will take care of adding up all of the numbers and storing the result in the sum variable.
• Finally, below the for loop, we can print out the sum, and also compute the average.
• Add a print statement to print the sum. Note that we will print both sum and average out to two decimal places.

System.out.printf("The sum of the numbers is: %.2f\n", sum);

• Then, add a second print statement to print out the average.
• Recall that the formula for calculating the average is:

• Therefore, we can computer the average by dividing sum by quantity.

System.out.printf("The average of the numbers is: %.2f\n", (sum / quantity));

• When your program is correctly displaying the sum and average as in the example output below, submit your work to Canvas.

Example Output

Your Program Should Look Identical to the Following:

Activity 11.4: Unicode (10 pts)

• In this exercise we continue exploring some uses of counter-controlled loops.

Background

• Recall that a char data type is stored by the computer as a number using Unicode (Unicode Table). Since a char is stored as an int by the computer, Java lets you cast an int to a char.

int count = 65;
System.out.println((char) count); //What does the (char) do?

• This casting feature lets us construct our own Unicode table.
  

Specifications

• Copy and paste the starter code into a file called Unicode.java
  

public static void main(String[] args) {
        //displaying the characters from 33 to 122
        for (int i = ???; i <=???; i++) {
            System.out.println(i);
        }
        
}

• Alter the for-loop so that it counts from 33 to 122
• Inside the for-loop, alter the System.out.println statement as follows:

• Compile and run your modified program and make sure your output looks like:

  (some output not shown)
  65      A
  66      B
  67      C
  (more output not shown)
  

• When you are finished, play around with the starting and ending values of your for loop.
• What other characters will display?
  
• Submit the final version of your file to Canvas when you are finished.