Questions

• Reading?
• Quiz?
• Homework?

Concept: do Loop

• Use a do loop when you always want the body of the loop to execute at least one time

  do
  {
      statements
  } while (condition);

• This is especially useful to get valid input

  
      int value;
      do
      {
          System.out.print("Enter an number < 100: ");
          value = scan.nextInt();
      }while (value >= 100); //tells the loop when it can execute again
  
  

Activity - Getting valid input

Record your participation in Piazza clicker question Lesson12 Q1

Supply the code to get a positive integer from the keyboard. Keep prompting until the input is positive. Use input_practice.zip as a starter file.

1. I got it, no real problem
2. I got it, but it was hard
3. I couldn't do it in the time allowed
4. I didn't know where to start

 

Concept: do loop

• Use a do loop when the loop needs to execute at least once
• A do loop is not always appropriate.
• We want to go and lie on the beach as long as the sun is shining.
• Look at this implementation with a do loop

  Go to the beach
  do
      Put on sun screen
      Put down a towel
      Lie in the sun
  while the sun is shining

• What if it is raining when we get to the beach?
• Now look at this one: knocking on a door until until someone answers

  Walk up to the closed door
  do
     knock three times
     wait 30 seconds
  while (no one has answered)
  

 

Activity - reading loop code

Record your participation in Piazza clicker question Lesson 12 Q2

What does the following loop print? Make a trace table

     int n = 1;
     while (n < 30)
     {
         n = 2 * n;
         System.out.print(n + " ");
     }
 

1. 2 4 8 16
2. 2 4 8 16 32
3. 1 2 4 8 16
4. 1 2 4 8 16 32

Activity - Problem Solving: Hand-Tracing

Record your participation in Piazza clicker question Lesson 12 Q3

What value is displayed by this code? Trace the code and make a trace table

Record the values of "n", "sum", and "digit" on a piece of paper or a txt file

int n = 2016;
int sum = 0;
while (n > 0)
{
    int digit = n % 10;
    sum = sum + digit;
    n = n / 10;
}
System.out.println(sum);

Activity - Hand-Tracing

Record your participation in Piazza clicker question Lesson 12 Q4

• Trace

  int i = 0; 
  int sum = 0;
  while (sum < 10)
  {
     i++; 
     sum = sum + i;
  }
  System.out.println( i + " " +  sum);
  

• Trace

  int i = 0; 
  int sum = 0;
  while (sum >= 10)
  {
     i++;
     sum = sum + i;
  }
  System.out.println( i + " " +  sum);
   

• What would be printed if this was a do while loop?

Concept - Sentinel value

• A value used to denote the end of a set of input because it can not be part of the data set.
• What would be a good sentinel value when inputting student grades?
  • What value could not be a student grade?
• Example

          int grade = 0;
  
          while (grade >= 0)
          {
              System.out.print("Enter a grade or -1 to quit: ");     
              grade = scan.nextInt();
              if (grade >= 0)
              {
                  //do something with grade
              }
          }    
      

 

Concept - Sentinel value

• What would be a good sentinel value when inputting length of babies at birth?
  • What value could not be a baby's length?
• Example

          boolean done = false;
          double length = 0;
  
          while (!done)
          {
              System.out.print("Enter the length or 0 to quit: ");     
              length = scan.nextDouble();
              if (length <= 0)
              {
                  done = true;
              }
              else
              {
                  //do something with the length
              }
          }   
      

 

 

Concept - Sentinel value

• What about temperatures in Alaska?
  • What value could not be a temperature
• Have to use a different technique
• Use a non-numeric as sentinel value
• Use Scanner methods hasNextInt(), hasNextDouble
• Example: finding average temperature

          double temperature = 0;
          System.out.print("Enter the temperature, Q to quit: ");
          while (scan.hasNextDouble())
          {
              temperature = scan.nextDouble();
              //do something with the temperature
              System.out.print("Enter the temperature, Q to quit: ");
          }
          

Activity - Using Q as a sentinel value

Record your participation in Piazza clicker question Lesson12 Q5

What would happen is you entered X in the program in the last slide to input temperatures.

1. There would be a syntax error
2. The code will throw an exception
3. The loop will terminate
4. Something else would happen

 

Activity - Using Q as a sentinel value

Record your participation in Piazza clicker question Lesson12 Q6

We are throwing a dart at a number line. (numbers can be positive, negative or zero doubles) Write a program to ask the user for input to represent the result of the throw. Find the sum of the numbers and print it. Use Q as a sentinel value. You can use input_practice.zip again as a starter

1. I got it, no real problem
2. I got it, but it was hard
3. I couldn't do it in the time allowed
4. I didn't know where to start

 

Activity - Using Q as a sentinel value

Record your participation in Piazza clicker question Lesson12 Q7

In the previous activity, you found the sum of inputs using Q as the sentinel value. Now modify the program to also find and print the average.

1. I got it, no real problem
2. I got it, but it was hard
3. I couldn't do it in the time allowed
4. I didn't know where to start