Shortcoming of the FOR-DO loop

1
Shortcoming of the FOR-DO loop

• When you use the FOR-DO loop you must know how
  many times you want to perform an action. What
  if you didnt know how many times an action was
  to be performed?
• For example, suppose you wanted to know how many
  times an action needed to be repeated before a
  certain condition was met.

2
The WHILE loop

• WHILE (some Boolean expression is true) DO
• some particular action
• i 0 initialization of i
• WHILE i lt 100 DO BEGIN
• Write (i, )
• i i 3
• End

3
The WHILE loop (cont)

• The identifiers in the Boolean expression that
  controls the WHILE loop must all be defined. If
  they are not defined by an earlier part of the
  program, you must initialize them (the book calls
  this priming) before the loop begins.

4
The WHILE loop (cont)

• At least one of the statements in the loop must
  change the value of the Boolean expression so it
  will eventually become false. Otherwise, the
  loop will execute forever (this is called an
  infinite loop).
• In Pascal we stop an infinite loop by pressing
  Ctrl-Break
• Poor logic can also cause an infinite loop.

5
Sentinels

• A sentinel is a piece of data that ends a set of
  data but isnt a possible value of the data
• For example, if we write a program that a police
  officer uses to enter the speed of each car (s)he
  gave a speeding ticket to during a week, the end
  of the data can be indicated by a -1.

6
Priming the values

• Initializing values which will be used in a while
  loop is called priming the values. Incorrect
  priming can cause subtle bugs.
• Read(testScore)
• While (testScore ltgt -1) DO BEGIN
• totalScore totalScore testScore
• Read (testScore)
• END

7
Homework due April 4

• Maratecks hw 6
• change program to take upper case and lower case
  letters
• Use a WHILE loop to look for the end of the data.
  The end of the data will be signaled by a period.