Questions

• Reading?
• Quiz?
• Homework?

Exam

• I will replace this exam with the exam on the second exam if that score is higher

Concept: Loops

• Three types of program flow control

  

  • sequential
  • decisions (selection)
  • loops (iteration)
• Two types of looping
  • loop 100 times
  • loop while there is gas in the car
• Keywords for looping
  • while
  • for

Concept: while loop

loop 100 times

initialize a counter to 1
while counter <= 100
    drive around the track again
    add 1 to the counter. (also called incrementing)  //very important and easy to  forget

Activity - Simple while loop

Record your participation in Piazza clicker question Lesson11 Q1

Using Bluej, write a program with a while loop that prints powers of 2 from 1 to 5 (inclusive) all on one line separated by a space. Create a class, put a main method in it, then write your code. (If you are stuck, think about using Math.pow)

2¹ 2² 2³ 2⁴ ... yields output 2 4 8 16 ...

1. I got it, no real problem
2. I got it, but it was hard
3. I couldn't do it

Here is a starter project

*To think about:
What if you want the answer as an int?
What if you only wanted the odd powers 1, 3, 5, ... 9?

Concept: for loop

• Use a for loop when you know exactly how many times you want to execute the loop.
• Any code you write with a for loop can be written with a while loop. But not the reverse
• A for loop is a little less prone to error because it is harder to forget to increment.
• It includes the initialization of the control variable, the test condition, and the increment in one line

  for (int laps = 1; laps <= 100; laps++)
      drive around the track  again
      

 

Activity - for loop

Record your participation in Piazza clicker question Lesson11 Q2

Change the the first problem to use a for loop to print the powers of 2 from 1 to 5 (inclusive). You can put the code in the same class or make a new one.

1. I got it, no real problem
2. I got it, but it was hard
3. I couldn't do it

 

Activity - declaration of variable in a loop

Record your participation in Piazza clicker question Lesson11 Q3

What would this code segment print?

... 
int i = 1;  
while (i <= 5)
{
   double power = Math.pow(2, i);
   System.out.print(power + " ");
   i++;
   ...
}
System.out.println("The last power is " + power) ;

1. 2 4 8 16 32 The last power is 32
2. 2 4 8 16 32 The last power is 64
3. Nothing is printed. The code has a syntax error
4. Nothing is printed. The code throws an exception

Activity - for loop with decrement

Record your participation in Piazza clicker question Lesson11 Q4

Write a program to print the numbers starting at 10 and going backwards to 1 (inclusive). Use decrement operator (i--)

1. I got it, no real problem
2. I got it, but it was hard
3. I couldn't do it

*To think about:
What if you want only the even numbers?

Activity - for loop control variable scope

Record your participation in Piazza clicker question Lesson11 Q5

What is the value of i printed in the last line of the code segment?

public class HelloPrinter
{
   public static void main(String[] args)
   {
      for (int i = 1; i < 5; i++)
      {
          System.out.println("Hello");
      }
      System.out.println(i);
   }
}

1. 5
2. 4
3. Nothing. There is syntax error
4. Nothing. The code throws an exception

*To think about:
If I remove the last statement, how many times will "Hello" be printed

Concept: processing a string with a loop

A for loop is ideal for picking up one character in a string at a time and doing something with it.

Let's write a code segment to print the letters of a string phrase one character per line

• We get a string some how
  String phrase = ...
• We write the for loop header
  for (int i = 0; i < phrase.length(); i++)
• Use substring to get the character at index i
  String ch = phrase.substring(i, i + 1);
• Now print ch
• put it all together

      for (int i = 0; i < phrase.length(); i++)
      {
           String ch = phrase.substring(i,i + 1);
           System.out.println(ch);
      }

 

Activity - processing a string with a loop

Record your participation in Piazza clicker question Lesson11 Q6

Complete the code to count the number of o's in a phrase. For this to work, you have to initialize a variable to count the o's before the loop. That is done for you in this starter(string_processing.zip)

1. I got it, no real problem
2. I got it, but it was hard
3. I couldn't do it

 

Concept: processing a string in reverse with a loop

• What for loop header would you use to process the characters in reverse order?

• for (int i = phrase.length() - 1; i >= 0; i--)

Activity - processing a string in reverse with a loop

Record your participation in Piazza clicker question Lesson11 Q7

Write a program to read a string from the user and print the characters of the string in reverse order one character per line.

1. I got it, no real problem
2. I got it, but it was hard
3. I couldn't do it

Here is input_skeleton2

*To think about:
What if you were writing a method and wanted to return the string in reverse order. What could you do?

Activity - processing a string in reverse with a loop

Record your participation in Piazza clicker question Lesson11 Q8

Complete a class called Words  that has one method reverse which will return the string in reverse order. Test in the Bluej workbench.

Starter project is here (words.zip)

1. I got it, no real problem
2. I got it, but it was hard
3. I couldn't do it