1. COMP 110 Loops, loops, loops, loops, loops, loops…
Michele Weigle - COMP 14 - Spr 04
COMP 110 Loops, loops, loops, loops, loops, loops…
Luv Kohli
September 22, 2008
MWF 2-2:50 pm
Sitterson 014

2. Michele Weigle - COMP 14 - Spr 04
Announcements
Program 2 due date extended to Friday, Sep 26, 2pm
Lab 4 due date extended to Wednesday, Oct 1, 2pm
Print out your programs and hand the in
Interesting talk today at 4pm, SN011
William Swartout, “Toward the Holodeck: Integrating Graphics, Artificial Intelligence, Entertainment and Learning”

3. Michele Weigle - COMP 14 - Spr 04
Questions?
How is Program 2 going?
Any other questions?

4. Michele Weigle - COMP 14 - Spr 04
Today in COMP 110
More loop examples
If time: indentation

5. Designing a loop
Initializing statements
Loop body
Ending a loop

6. Designing a loop body
Example: calculate the sum of numbers entered by the user

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

8. 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 the sum
Initializing statements
How do we end the loop? How many iterations?

9. 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.

10. What should sum’s initial value be?
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 the sum

11. Initialize sum 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;

12. What should sum’s initial value be?
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 the sum
Initializing statements

13. Initializing statements
Michele Weigle - COMP 14 - Spr 04
Initializing statements
Always set initial value to 0?
What if we calculate the product of all the input values?
product = product * next;
After the first iteration, we expect
product == next (the first input value)
Initial value of product?
product = 1;

14. Ending a loop How do we end the loop? How many iterations?
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 the sum
How do we end the loop? How many iterations?

15. Ending a loop Count-controlled loops User-controlled loops
If you know the number of loop iterations
for (count = 0; count < iterations; count++)
User-controlled loops
Ask-before-iterating
Sentinel value

16. Count-controlled loops
You know how many iterations in advance.
Ask every student in the class for his/her age and calculate the average age
sum = 0;
for (student 1 to student n) {
Ask his/her age
sum = sum + age; }
average = sum / n;

17. Count-controlled loops
Use a for loop in most cases
37 students in the class: 37 iterations
numStudents = 37;
sum = 0;
for (count = 1; count <= numStudents; count++) {
Ask student #count for his/her age
sum = sum + age; }
average = sum / numStudents;

18. Count-controlled loops
Can also use a while loop, but for loop is easiest way to implement count-controlled loops
numStudents = 37;
count = 1;
sum = 0;
while (count <= numStudents) {
Ask student #count for his/her age
sum = sum + age;
count = count + 1; }
average = sum / numStudents;

19. User-controlled loops
Also called ask-before-iterating
Ask the user if it is time to end the loop
Example: add a bunch of numbers
Lab 4: when the user enters a negative number (a sentinel value), it’s time to end the loop

20. User-controlled loops
Use while and do-while in most cases
do-while
If you know that the user wants at least one iteration
Ask the user whether the loop should end at the end of the loop body

21. User-controlled loops: do-while
do { // do stuff in your code here Prompt user for input Read a number into variable next sum = sum + next; // ask if we should do another iteration System.out.print(“Continue (yes/no)? ”); answer = keyboard.nextLine(); } while (answer.equalsIgnoreCase(“yes”));

22. User-controlled loops: while
Use while if there could be zero iterations
Example, summing numbers:
When the user inputs a negative value:
End the loop
The negative value is not used in calculating the sum
If the first input value is negative, no iterations

23. User-controlled loops: while
sum = 0; while (next >= 0) { Prompt user for input Read a number into variable next sum = sum + next; }
Is this code correct?
What is the initial value of next?

24. Solution
sum = 0; Prompt user for input Read a number into variable next while (next >= 0) { sum = sum + next; }
Is this code correct?

25. Solution, another try
sum = 0; Prompt user for input Read a number into variable next while (next >= 0) { sum = sum + next; }
Is this code correct?

26. Example problem Find the lowest and highest ages in the class
Use a loop?
Yes
What goes in the loop body?

27. Loop body
Ask student 1 for age Update min/max ages Ask student 2 for age ... Ask student 37 for age End loop Output min and max ages

28. Min/max ages
Which kind of loop?
Let’s use a for loop

29. Min/max ages
int min = 2000; // initialize to large value int max = 0; // initialize to small value for (int count = 1; count <= 37; count++) { Ask student #count for age if (age > max) max = age; if (age < min) min = age; } Output min and max ages

30. Min/max ages Ages max min 20 23 18 25 12 94 36 … 20 23 25 94 2000 2020 23 25 94
2000 20 18 12
if (age > max)
max = age;
if (age < min)
min = age;

31. Michele Weigle - COMP 14 - Spr 04
Wednesday
Nested loops
Loop bugs