1. Intro to Computer Science CS1510 Dr. Sarah Diesburg
Repetition
Intro to Computer Science
CS1510
Dr. Sarah Diesburg

2. Today’s Agenda From last week Repetition Selection statements
Conditional statements
Compound conditional statements
Repetition
Reasons for repetition
Tools

3. if, elif, else, the Process
Evaluate boolean expressions until:
The boolean expression returns True
None of the boolean expressions return True
If a boolean returns True, run the corresponding suite. Skip the rest of the if
If no boolean returns True, run the else suite, the default suite

4. Compound Expressions
Logical Operators (lower case)
and or
not

5. Compound Expressions Examples
if (a==b) and (c<d):
if (a==b) or (c<d):
if not (a==b):
Remember, evaluate each statement first before looking at and, or, not

6. Truth Tables

7. Truth Tables

8. Truth Tables

9. Truth Tables

10. Chained Comparisons You are going to be tempted to write:
0 <= myInt <= 5
But you will need to be very careful

11. Compound Evaluation
Logically 0 < X < 3 is actually (0 < X) and (X < 3)
Evaluate using X with a value of 2: (0< X) and (X< 3)
Parenthesis first: (True) and (True)
Final value: True
(Note: parentheses are not necessary in this case.)

12. Compound Evaluation BUT, I’ve seen students write: 3 < X < 0
Meaning they want to know if x is outside of that range.
But this is actually (3 < X) and (X < 0)
Evaluate using X with a value of 5: (3< X) and (X< 0)
Parenthesis first: (True) and (False)
Final value: False

13. How can we introduce error checking?
Suppose that students are entering numbers into your miles per gallon program that aren’t valid

14. How can we introduce error checking?
Suppose that students are entering numbers into your miles per gallon program that aren’t valid
Values not above 0
Ending value that is “smaller” than the starting value
Maybe even values above some upper limit
How can we correct them and ask them to try again?

15. Repeating Statements
Besides selecting which statements to execute, a fundamental need in a program is repetition
repeat a set of statements under some conditions
Between selection and repetition, we have the two most necessary programming statements

16. While and For Statements
The while statement is the more general repetition construct. It repeats a set of statements while some condition is True.
Often called a sentinel controlled loop
The for statement is useful for iteration, moving through all the elements of data structure, one at a time.
Often called a count controlled loop

17. while Loop Top-tested loop (pretest) while boolean expression:
test the boolean before running
Run the program suite
test the boolean before each iteration of the loop
while boolean expression:
statementSuite

19. Repeat While the Boolean is True
while loop will repeat the statements in the suite while the boolean is True (or its Python equivalent)
If the boolean expression never changes during the course of the loop, the loop will continue forever.

20. While Loop Example x_int = 0 while x_int < 10: print (x_int)
x_int = x_int + 1
print()
print( "Final value of x_int: ", x_int)
What is the Final Value printed by this code?

21. General Approach to a While
outside the loop, initialize the boolean
somewhere inside the loop you perform some operation which changes the state of the program,
eventually leading to a False boolean and exiting the loop
Have to have both!

22. For and Iteration
One of Python’s strength’s is it’s rich set of built-in data structures
The for statement is a common statement for manipulation of a data structure
for each element in the datastructure
perform some operation on that element

24. For Loop Example numbers = [0,1,2,3,4,5,6,7,8,9] for xInt in numbers:
print (xInt)
print()
print ("Final value of xInt: " + str(xInt) )

25. How can we introduce error checking?
Suppose that students are entering numbers into your miles per gallon program that aren’t valid
Values not above 0
Ending value that is “smaller” than the starting value
Maybe even values above some upper limit
How can we correct them and ask them to try again?