1. Announcements Lab 3 was due today Assignment 2 due next Wednesday
Quiz at end of class

2. Questions?
Last time we covered loops:
for
while
do-while

3. Today in COMP 110
More Programming with Loops
review
nested loops

4. Review The while statement can be used to construct a loop in Java
while(Boolean_Expression) {
Statement_1
Statement_2 …
Statement_N }
As long as Boolean_Expression is true, the statements in the loop body are executed

5. Review Do-While Syntax Execute Statements_1…N
Statement_N
} while (Boolean_Expression); //note the semicolon!
Execute Statements_1…N
If Boolean_Expression is true, repeat

6. Review
For Loop Syntax
for(Initializing_Action; Boolean_Expression; Update_Action) {
Statement_1
Statement_2 …
Statement_N }
Initializing_Action is used to initialize a counter variable
Boolean_Expression is the stopping condition for the loop
Update_Action is used to update a counter variable after the loop body is executed

7. Review Choosing a loop Do-While For While
When you want the body to be executed at least once
Useful for checking user input
For
More convenient/readable when the number of iterations is known beforehand, e.g. stored in some counter variable
While
Safest choice, can be used to create any kind of loop
When it might be necessary for the loop to iterate zero times

8. Nested Loops
It’s possible, and sometimes necessary to use loops inside other loops
This is called nesting
Just like with nested if-statements

9. Nested Loops Example int sum = 0; int i = 0; while(i < 10) {
for(int j = 0; j < 20; j++)
sum = sum + j;
i++; }

10. Commas in For Statements
We can perform multiple initializations or updates using the comma operator
int n,c;
int product;
for (n = 1, c = 1; n <= 5; c = c + 2, n++) {
product = product * c * n; }
System.out.println(product);

11. Designing Loops Creating a loop involves designing three things
Initialization of Variables
Loop Body
Stopping Condition

12. Designing a Loop Body
How to determine what statements should be inside the body of a loop?
Example
Calculate the sum of numbers entered by the user

13. Designing a Loop Body Repeated statements become your loop body
Output instructions to the user
Initialize variables
Prompt user for input
Read a number into variable next
sum = sum + next;
...
Output sum
Repeated statements become your loop body
Statements that are only done once are not part of your loop body

14. Pseudocode with Loop Body
Output instructions to the user Initialize variables Do the following for the appropriate number of times: { Prompt user for input Read a number into variable next sum = sum + next; } Output sum
Initializing statements
How do we end the loop? How many iterations?

15. Initializing Statements
Variables used in your loop need to be initialized (set to a value) before the loop
next
Read a number into variable next
We read a new value for next before using it during each iteration of the loop so we do not need to initialize it
sum
sum = sum + next;
sum is on the right side of an assignment statement. sum MUST have a valid value before the loop starts.

16. Initialize Sum What should sum be initialized to?
Consider the first iteration
After executing the first iteration, the expected value of sum should be
sum == next (the first input value)
The assignment statement is
sum = sum + next;
Therefore, initial value of sum is
sum = 0;

17. Loop with Initializations
Output instructions to the user sum = 0; Do the following for the appropriate number of times: { Prompt user for input Read a number into variable next sum = sum + next; } Output sum
Initializing statements

18. Ending Loops How to choose a stopping condition for your loop?
Count-controlled loops
When you know number of loop iterations
for(count = 0; count < iterations; count++)
User-controlled loops
Ask-before-iterating
Sentinel value
Boolean
The value of a boolean indicates whether the loop should stop

19. Count-Controlled Loops
A program to average exam scores input by the user
We know the size of the class is "numStudents"
double next, average, sum=0;
int i;
for(i = 0; i < numStudents; i++) { //# iterations is fixed
next = keyboard.nextDouble();
sum = sum+next; }
average = sum/numStudents;

20. Ask-Before-Iterating
Sometimes it is useful to have the user decide when a loop should end
String answer;
do {
//perform some computations
System.out.print("Continue? yes/no");
answer = keyboard.nextLine();
} while (answer.equalsIgnoreCase("yes"));
Do this on board

21. Sentinel Value Signal end of input
System.out.print("Enter a negative number to end the loop");
int next = keyboard.nextInt();
int sum = 0;
while (next >= 0) {
sum = sum + next;
System.out.print("Enter a number: ");
next = keyboard.nextInt(); }

22. Booleans int next, sum = 0; boolean moreNumbers = true;
Scanner keyboard = new Scanner(System.in);
while(moreNumbers ) {
next = keyboard.nextInt();
if (next < 0)
moreNumbers = false; //this will be the last iteration
else
sum = sum + next; }
System.out.print("The sum is " + sum);

23. Off-by-One errors Loop repeats one too many or one too few times
for (count = 1; count < 10; count++);
Loop for count = [1,9] (9 iterations)
for (count = 1; count <= 10; count++);
Loop for count = [1,10] (10 iterations)
for (count = 0; count <= 10; count++);
Loop for count = [0,10] (11 iterations)
Do this on board

24. Exercise What does the following display? int product = 1;
int max = 4;
for(int i = 0; i <= max; i++)
product = product * i;
System.out.println("The product is " + product);
Output
The product is 0

25. Example Problem
Find the lowest and highest ages in the class

26. Loop Body Get age of student 1 Update min/max ages
...
Get age of student 38
End loop
Output min and max ages

27. Min/Max Ages
int min = 2000; // initialize to large value int max = 0; // initialize to small value for (int count = 1; count <= 38; count++) { //Ask student #count for age ... //Compute min and max if (age > max) max = age; if (age < min) min = age; } //Output min and max ages

28. Min/Max Ages if (age > max) max = age; if (age < min) min = age;
2000 20 23 18 25 12 94 36 …
if (age > max)
max = age;
if (age < min)
min = age;

29. Debugging
What to do if there’s something wrong with your program, but you’re not sure what?
Trace the variables
Watch the variables change as the program executes

30. Tracing Variables Two ways to trace variables Manually Automatically
Using print statements
Automatically
Using the debugger

31. Manual Tracing
Insert print statements to output the values of variables
System.out.print("Enter a negative number to end the loop");
int next = keyboard.nextInt();
int sum = 0;
while (next >= 0) {
sum = sum + next;
System.out.println(sum);
System.out.print("Enter a number: ");
next = keyboard.nextInt(); }

32. Using a DEBUG flag
Use a DEBUG flag to disable the tracing of variables when no longer needed
final boolean DEBUG = true;
if(DEBUG) {
//print the values of variables }

33. Automatic Tracing Use a debugger
jGRASP provides an integrated debugger
Allows you to “step” through your program to see how variables change with each line of code

34. Programming Demo Hand-shaking problem
Given a group of n people, everyone shakes hands with everyone else
How many handshakes?

35. Programming Demo Example, n = 4 6 handshakes
Person 1 shakes hands with Person 2
Person 1 shakes hands with Person 3
Person 1 shakes hands with Person 4
Person 2 shakes hands with Person 3
Person 2 shakes hands with Person 4
Person 3 shakes hands with Person 4
6 handshakes

36. Programming Demo Example, n = 4 6 handshakes
Person 1 shakes hands with Person 2
shakes hands with Person 3
shakes hands with Person 4
Person 2 shakes hands with Person 3
Person 3 shakes hands with Person 4
6 handshakes

37. Handshake Problem
n = 6 1 2 3 4 5 6 X

38. Programming Demo
Implementation using nested loops

39. Friday Recitation Bring Laptops (fully charged)
Questions about Program 2