Programming with Loops

1
Programming with Loops
2
When to Use a Loop

• Whenever you have a repeated set of actions, you
  should consider using a loop.
• For example, if you have to read in the exam
  scores for all students in the class, you should
  write code to read in just one score and repeat
  it for all students in the class.
• If we needed the sum of all the scores we could
  add each new score as it was entered to a
  variable containing the sum.

3
Initializing Statements

• An initializing statement just sets a beginning
  or initial value for a variable to be used later.
• If we dont initialize a variable before it is
  used, we cant be sure what value it contains.
• For example, in order to be able to sum values
  into a variable, that variable must be
  initialized to zero before we begin.

4
Initializing Statements (contd)

• You dont always initialize variables to zero.
• For example,
• for (count 1, count lt n count)
• Read a number into the variable next.
• product product next

5
Ending a Loop

• If you know the number of iterations required
  before entering the loop you can use a for loop.
  Such loops are called count-controlled loops.
• One way to end the loop is to ask the user if
  its time to end the loop. This is called
  ask-before-iterating.
• A third way to end a loop is by using a sentinel
  value.

6
Ending a Loop Using a Sentinel Value

• A sentinel value is a special input value,
  different from valid input values, that is used
  to signal the end of the loop.
• For example, if you are reading in exam scores
  and you dont know how many you have, if you know
  that no score can be less that zero, you can use
  a negative value as the sentinel.
• Your condition can just say to keep reading
  scores until you read a negative score. Then
  youre done.

7
Example of Using a Sentinel Value to End a Loop

• System.out.println(Enter scores for all
  students.)
• System.out.println(Enter a negative number
  after)
• System.out.println(you have entered the
  scores.)
• Scanner keyboard new Scanner(System.in)
• double max keyboard.nextDouble()
• double min max//The max and min so far are the
  first score.
• double next keyboard.nextDouble()
• while (next gt 0)
• if (next gt max)
• max next
• if (next lt min)
• min next
• next keyboard.nextDouble()
• System.out.println(The highest score is
  max)
• System.out.println(The lowest score is min)

8
Nested Loop Example

• do
• System.out.println()
• System.out.println(Enter all the scores to be
  averaged.)
• System.out.println(Enter a negative number
  after)
• System.out.println(you have entered all the
  scores.)
• sum 0
• numberOfStudents 0
• next keyboard.nextDouble()
• while (next gt 0)
• sum sum next
• numberOfStudents
• next keyboard.nextDouble()
• if (numberOfStudents gt 0)
• System.out.println(The average is
  (sum/numberOfStudents))
• else
• System.out.println(No scores to average.)

9
Loop Bugs

• Programs with loops are more likely to have bugs
  than simpler programs.
• The two most common bugs associated with loops
  are
• Unintended infinite loops
• Off-by-one errors

10
Unintended Infinite Loops

• If the condition tested to end the loop is
  incorrectly written, e.g. (a lt 0) instead of (a gt
  0), the loop may never end if a starts off with a
  positive value and during each iteration a is
  increased.
• Be aware that a loop may work correctly for some
  input values, but become an infinite loop for
  others. Running one test case may not be enough
  to insure correctness.

11
Off-By-One Bugs

• These errors are the ones where your loop
  iterates one to many or one to few times. You
  may have written the condition as (number lt 0)
  when you really meant (number lt 0).
• To reduce the possibility of these errors you
  should always test at and near the values used in
  the condition controlling the loop.

12
Tracing Variables

• If your program does not work correctly and it is
  not clear what the problem is, it is probably a
  good idea to trace some key variables. That is
  to watch what happens to them as they are changed
  when the program executes.
• Many systems have built in utilities for tracing
  variables. If not you can trace them by printing
  them out from the program, or you can trace your
  algorithm by hand using pencil and paper.