Transcript 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.

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 didn’t 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.
The WHILE loop
WHILE (some Boolean expression is true) DO
some particular action
i := 0;
{initialization of i}
WHILE i < 100 DO BEGIN
Write (i, ‘ ‘);
i = i + 3;
End;
The WHILE loop (con’t)
• 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.
The WHILE loop (con’t)
• 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.
Sentinels
• A sentinel is a piece of data that ends a set
of data but isn’t 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.
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 <> -1) DO BEGIN
totalScore := totalScore + testScore;
Read (testScore);
END;
Homework due April 4
• Marateck’s 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.