best more loops by Evan Korth

1. Introduction to Computers and
Programming
Lecture 8: More Loops
Professor: Evan Korth
New York University

2. 2
Upcoming schedule
• February 21: lecture 8; hw#2 due
• February 26: lecture 9
• February 28: lecture 10; start review; hw#3 due
• March 5: review
• March 7: midterm
• March 13, 15: Spring break (yay!)

3. Road Map
• Sentinels
• Sentinel Controlled Loops
• Case Study: Sentinel Controlled Loop
• Case Study: Nested Control Structures
• do/while Loop
• Summary of looping covered so far
• Reading
– Liang 5:chapter 3: 3.3.1, 3.3.2
– Liang 6: chapter 4: 4.2, 4.3

4. review
• What are three ways to rewrite:
x = x + 1;
• What are the three elements of a while loop?
• What is the difference between pre and post
increment operators?
• Are you guaranteed to execute the body of a while
loop at all?
• What is an infinite loop?
• True or False: You never want to use an infinite
loop?

5. 5
Review
• Given:
char c = 'a';
c++;
What is the value of variable c?
• What method do you use to extract characters
from a String?
– How would you use that method to get the first character
from a String?
• How are characters stored in memory?
• What is the difference between the character '0'
and the integer 0?

6. Sentinels

7. Sentinel-controlled repetition
• Counter Controlled Repetition:
– Simply means that we use a counter to tell you when to stop
repeating a statement or group of statements
– The examples from last class were counter-controlled
repetition
• However, what if we want the user or the input to
decide when to end the program?
– Use a sentinel

8. Understanding Sentinels
• Sentinel: a special value that indicates the “end of
data entry.”
• Also known as signal value, dummy value, or flag
value
• For example:
– -1 means end of data.
– 0 means end of data.
– "END" means ends of data
– Depends on the specific application you are building.
• With a sentinel, we have an indefinite repetition,
because the number of repetitions is unknown at
the time we write the program (or start the loop).
© 2000 Prentice Hall, Inc. All rights reserved. Modified by Evan Korth

9. Using Sentinels
• How are they used?
– Programmer picks a value that would never be encountered
for normal data
– User enters normal data and then when done, enters the
sentinel value
– The loop will stop when seeing the sentinel value

10. Using Sentinels cont’d
• For example, if entering age for people, could pick
a sentinel of –1
• No one would expect to be –1 year old.
• Good practice is to remind the user in each
iteration of the loop what the sentinel value is
– For example,
System.out.println
(" Enter age of current resident or –1 to end" );

11. /* A sentinel controlled loop */
/* A simple census taker */
import javax.swing.JOptionPane;
public class Sentinel
{
public static void main(String [] args)
{
int currentAge = 0 ;
String currentAgeAsString;
/* priming */
currentAgeAsString = JOptionPane.showInputDialog ("Enter age of resident: ") ;
currentAge = Integer.parseInt (currentAgeAsString);
/* testing: keep going until input is sentinel value */
while (currentAge != -1)
{
/* do some calculations with age, e.g. AVERAGE */
/*updating: get the next value from the user */
currentAgeAsString = JOptionPane.showInputDialog ("Enter age of resident: ") ;
currentAge = Integer.parseInt (currentAgeAsString);
}
System.exit (0);
}
}

12. Good Programming tips
• Pick a sentinel value that you are CERTAIN will
never be confused with normal data
• Style: Remind user each iteration what the sentinel
is
• Y2K-like problem
– Programmers often used 9999 as a sentinel to end a loop
– Worry that on September 9, 1999 (sometimes abbreviated
9999) programs would erroneously stop executing before
they were supposed to.

13. Case Study: Using Sentinel
Controlled Loops

14. Formulating Algorithms with Top-Down,
Stepwise Refinement
• Problem becomes:
Develop a class-averaging program that will process an
arbitrary number of grades each time the program is run.
– Unknown number of students
– How will the program know to end?
• Use sentinel value
– Loop ends when user inputs the sentinel value
– Sentinel value chosen so it cannot be confused with a regular
input (such as -1 in this case)
© 2000 Prentice Hall, Inc. All rights reserved.

15. Formulating Algorithms with Top-Down,
Stepwise Refinement
• Top-down, stepwise refinement
– Begin with a pseudocode representation of the top:
Determine the class average for the quiz
– Divide top into smaller tasks and list them in order:
Initialize variables
Input, sum and count the quiz grades
Calculate and print the class average
• Many programs have three phases:
– Initialization: initializes the program variables
– Processing: inputs data values and adjusts program variables
accordingly
– Termination: calculates and prints the final results
© 2000 Prentice Hall, Inc. All rights reserved.

16. Formulating Algorithms with Top-Down,
Stepwise Refinement
• Refine the initialization phase from Initialize
variables to:
Initialize total to zero
Initialize counter to zero
• Refine Input, sum and count the quiz grades to
Input the first grade (possibly the sentinel)
While the user has not as yet entered the sentinel
Add this grade into the running total
Add one to the grade counter
Input the next grade (possibly the sentinel)
© 2000 Prentice Hall, Inc. All rights reserved.

17. Formulating Algorithms with Top-Down,
Stepwise Refinement
• Refine Calculate and print the class average to
If the counter is not equal to zero
Set the average to the total divided by the counter
Print the average
else
Print “No grades were entered”
© 2000 Prentice Hall, Inc. All rights reserved.

18. 18
Initialize total to zero
Initialize counter to zero
Input the first grade (possibly the sentinel)
While the user has not as yet entered the sentinel
Add this grade into the running total
Add one to the grade counter
Input the next grade (possibly the sentinel)
If the counter is not equal to zero
Set the average to the total divided by the counter
Print the average
else
Print “No grades were entered”
Fig. 4.8 Class-average problem pseudocode
algorithm with sentinel-controlled repetition.
© 2003 Prentice Hall, Inc. All rights reserved.

19. © 2003 Prentice Hall, Inc.
All rights reserved.
Outline
19
Average2.java
1 // Fig. 4.9: Average2.java
2 // Class-average program with sentinel-controlled repetition.
3 import java.text.DecimalFormat; // class to format numbers
4 import javax.swing.JOptionPane;
5
6 public class Average2 {
7
8 public static void main( String args[] )
9 {
10 int total; // sum of grades
11 int gradeCounter; // number of grades entered
12 int grade; // grade value
13
14 double average; // number with decimal point for average
15
16 String gradeString; // grade typed by user
17
18 // initialization phase
19 total = 0; // initialize total
20 gradeCounter = 0; // initialize loop counter
21
22 // processing phase
23 // get first grade from user
24 gradeString = JOptionPane.showInputDialog(
25 "Enter Integer Grade or -1 to Quit:" );
26
27 // convert gradeString to int
28 grade = Integer.parseInt( gradeString );
29

20. © 2003 Prentice Hall, Inc.
All rights reserved.
Outline
20
Average2.java
Line 31
Line 45
30 // loop until sentinel value read from user
31 while ( grade != -1 ) {
32 total = total + grade; // add grade to total
33 gradeCounter = gradeCounter + 1; // increment counter
34
35 // get next grade from user
36 gradeString = JOptionPane.showInputDialog(
37 "Enter Integer Grade or -1 to Quit:" );
38
39 // convert gradeString to int
40 grade = Integer.parseInt( gradeString );
41
42 } // end while
43
44 // termination phase
45
46
47 // if user entered at least one grade...
48 if ( gradeCounter != 0 ) {
49
50 // calculate average of all grades entered
51 average = (double) total / gradeCounter;
52
53 // display average with two digits of precision
54 JOptionPane.showMessageDialog( null,
55 "Class average is " + average ,
56 "Class Average", JOptionPane.INFORMATION_MESSAGE );
57
58 } // end if part of if...else
59
loop until gradeCounter
equals sentinel value (-1)

21. © 2003 Prentice Hall, Inc.
All rights reserved.
Outline
21
Average2.java
60 else // if no grades entered, output appropriate message
61 JOptionPane.showMessageDialog( null, "No grades were entered",
62 "Class Average", JOptionPane.INFORMATION_MESSAGE );
63
64 System.exit( 0 ); // terminate application
65
66 } // end main
67
68 } // end class Average2

22. Case Study: Nested Control
Structures

23. Nested control structures
• Problem
– A college has a list of test results (1 = pass, 2 = fail) for 10
students
– Write a program that analyzes the results
• If more than 8 students pass, print "Raise Tuition"
• Notice that
– The program must process 10 test results
• Counter-controlled loop will be used
– Two counters can be used
• One for number of passes, one for number of fails
– Each test result is a number—either a 1 or a 2
• If the number is not a 1, we assume that it is a 2
© 2003 Prentice Hall, Inc. All rights reserved.

24. Nested control structures
• Top level outline
Analyze exam results and decide if tuition should be raised
• First Refinement
Initialize variables
Input the ten quiz grades and count passes and failures
Print a summary of the exam results and decide if tuition should
be raised
• Refine Initialize variables to
Initialize passes to zero
Initialize failures to zero
Initialize student counter to one
© 2003 Prentice Hall, Inc. All rights reserved.

25. Nested control structures
• Refine Input the ten quiz grades and count passes
and failures to
While student counter is less than or equal to ten
Input the next exam result
If the student passed
Add one to passes
else
Add one to failures
Add one to student counter
• Refine Print a summary of the exam results and
decide if tuition should be raised to
Print the number of passes
Print the number of failures
If more than eight students passed
Print “Raise tuition”
© 2003 Prentice Hall, Inc. All rights reserved.

26. 26
Initialize passes to zero
Initialize failures to zero
Initialize student to one
While student counter is less than or equal to ten
Input the next exam result
If the student passed
Add one to passes
else
Add one to failures
Add one to student counter
Print the number of passes
Print the number of failures
If more than eight students passed
Print “Raise tuition”
Fig 4.10 Pseudocode for examination-results problem.
© 2003 Prentice Hall, Inc. All rights reserved.

27. © 2003 Prentice Hall, Inc.
All rights reserved.
Outline
27
Analysis.java
Line 19
Line 29
1 // Fig. 4.11: Analysis.java
2 // Analysis of examination results.
3 import javax.swing.JOptionPane;
4
5 public class Analysis {
6
7 public static void main( String args[] )
8 {
9 // initializing variables in declarations
10 int passes = 0; // number of passes
11 int failures = 0; // number of failures
12 int studentCounter = 1; // student counter
13 int result; // one exam result
14
15 String input; // user-entered value
16 String output; // output string
17
18 // process 10 students using counter-controlled loop
19 while ( studentCounter <= 10 ) {
20
21 // prompt user for input and obtain value from user
22 input = JOptionPane.showInputDialog(
23 "Enter result (1 = pass, 2 = fail)" );
24
25 // convert result to int
26 result = Integer.parseInt( input );
27
28 // if result 1, increment passes; if...else nested in while
29 if ( result == 1 )
30 passes = passes + 1;
Loop until student counter is
greater than 10
Nested control structure

28. © 2003 Prentice Hall, Inc.
All rights reserved.
Outline
28
Analysis.java
31
32 else // if result not 1, increment failures
33 failures = failures + 1;
34
35 // increment studentCounter so loop eventually terminates
36 studentCounter = studentCounter + 1;
37
38 } // end while
39
40 // termination phase; prepare and display results
41 output = "Passed: " + passes + "nFailed: " + failures;
42
43 // determine whether more than 8 students passed
44 if ( passes > 8 )
45 output = output + "nRaise Tuition";
46
47 JOptionPane.showMessageDialog( null, output,
48 "Analysis of Examination Results",
49 JOptionPane.INFORMATION_MESSAGE );
50
51 System.exit( 0 ); // terminate application
52
53 } // end main
54
55 } // end class Analysis

29. do/while Loop

30. The do/while Repetition Structure
• The do/while repetition structure
– Similar to the while structure
– Condition for repetition tested after the body of the loop is
performed
• All actions are performed at least once
– Format:
do {
statement(s);
} while ( condition );

31. 4.8 The do/while Repetition Structure
• Flowchart of the do/while repetition structure
true
false
action(s)
condition

32. 32
public class DoWhileTest
{
public static void main(String args[])
{
int counter;
counter = 1;
do
{
System.out.println ("counter: "+
counter);
counter ++;
} while (counter <= 10);
}
}

33. Summary of Looping so far

34. Summary of Looping
• Two broad types of loops:
– Counter-controlled repetition
• A counter controls the number of repetitions.
• Also known as a definite repetition, because we know in
advance how many times the loop will be executed.
– Sentinel-controlled repetition
• A sentinel controls the number of repetitions
• Also known as indefinite repetition, because we do not know
in advance how many times the loop will be executed.
• In either case, watch out for infinite loops!
• If your program requires some kind of loop, first
determine which kind of loop you want.

35. Summary of Looping
• Once you know which kind of loop you want,
determine which while loop you want:
– While loops
• condition is tested first; then action occurs.
• While loops are much more common than do/while loops.
– Do/while loops
• action is run first; then, condition is tested.
• Use this if you want to make sure that your loop is guaranteed
to be run at least once.