2. Topics The if Statement The if-else Statement Comparing Strings
Nested Decision Structures and the if-elif-else Statement

3. Boolean Values Boolean Values can only be one of only two values True
False

4. If/Else Decision Structure
All previous instructions have been consecutively executed
One
After
The
Other
If/Else checks a Boolean condition then makes a decision
If condition is
True: one set of instructions are executed
False: a different set of instructions is executed

5. The if Statement
All previous instructions have been consecutively executed
One
After
The
Other
If statement makes a decision
If a Boolean expression or condition is
True: one set of instructions are executed
False: a different set of instructions is executed

6. Python Structures
Control structure: logical design that controls order in which set of statements execute
Sequence structure: set of statements that execute in the order they appear
Decision structure: specific action(s) performed only if a condition exists
Also known as selection structure

7. The if Statement Flowchart
Diamond represents true/false condition that must be tested
Actions can be conditionally executed
Performed only when a condition is true
Single alternative decision structure: provides only one alternative path of execution
If condition is not true, exit the structure

8. The if Statement Flowchart
Single alternative decision structure
Provides only one alternative path of execution
If condition is not true, exit the structure

9. The if Statement Syntax
Python syntax:
if condition:
Statement
if statement
Includes the keyword if followed by a Boolean condition
The condition can be true or false
Condition has to be followed by a colon like above
When the if statement executes, the condition is tested
If it is true the subsequent indented statements (called block statements) are executed
Indentation is required to tell Python when block is finished
Otherwise, block statements are skipped

10. Relational Operators Operator Meaning = = Equal to ! = Not equal to
>
Greater than
> =
Greater than or equal to
<
Less than
< =
Less than or equal to
Used to compare values and determine whether relationships exist
Greater than
Less than
Equal to
Compare two values and determine how they relate to each other

11. Boolean Expressions
Boolean expression: an expression tested by if statement to determine if it is true or false
Example: a > b
true if a is greater than b; false otherwise
> is the relationship
a > b is an expression

12. Boolean Expressions
== operator determines whether the two operands are equal to one another
Do not confuse with assignment operator (=)

13. Boolean Expression in Flow Chart
Using a Boolean expression with the > relational operator

14. Example Science teacher gives three tests
Wants to write a program that averages test scores
Wants to congratulate students if average is great than 95

15. Example Pseudocode/algorithm
Get the first test score
Get the second test score
Get the third test score
Calculate the average
Display the average
If average is > 95
Congratulate the user

16. Program
# This program gets three test scores and displays their average. # It congratulates the user if the # average is a high score. high_score = 95 # Get the three test scores. test1 = int(input('Enter the score for test 1: ')) test2 = int(input('Enter the score for test 2: ')) test3 = int(input('Enter the score for test 3: ')) # Calculate the average test score. average = (test1 + test2 + test3) / 3 # Print the average. print('The average score is', average) # If the average is a high score, congratulate the user. if average >= high_score: print('Congratulations!') print('That is a great average!')

17. The if-else Statement
Dual alternative decision structure: two possible paths of execution
One is taken if the condition is true, and the other if the condition is false
Syntax: if condition:
statements
else:
other statements
if clause and else clause must be aligned
Statements must be consistently indented

18. if-else Statement Flow Chart

19. The if-else Statement Flow

20. Example
Auto repair business wants a payroll program to pay overtime when an employee works > 40 hours
Pseudocode/algorithm
Get the number of hours worked
Get the hourly pay rate
If employee worked more than 40 hours
Calculate and display pay with overtime
Else
Calculate and display pay as usual

21. Program
# Variables to represent the base hours and the overtime multiplier. base_hours = 40 # Base hours per week ot_multiplier = 1.5 # Overtime multiplier # Get the hours worked and the hourly pay rate. hours = float(input('Enter the number of hours worked: ')) pay_rate = float(input('Enter the hourly pay rate: ')) # Calculate and display the gross pay. if hours > base_hours: overtime_hours = hours - base_hours overtime_pay = overtime_hours * pay_rate * ot_multiplier gross_pay = base_hours * pay_rate + overtime_pay else: gross_pay = hours * pay_rate # Display the gross pay. print('The gross pay is $', format(gross_pay, ',.2f'), sep='')

22. Comparing Strings
Strings can be compared using the == and != operators
String comparisons are case sensitive
Strings can be compared using >, <, >=, and <=
Compared character by character based on the ASCII values for each character
If shorter word is substring of longer word, longer word is greater than shorter word

23. Example
name1 = 'Mary' name2 = 'Mark' if name1 > name2: print(name1, ">", name2) else: print(name2, '>', name1) Output Mary > Mark

24. Comparing Strings

25. Example
# This program compares two strings. # Get a password from the user. password = input('Enter the password: ') # Determine whether the correct password # was entered. if password == 'prospero': print('Password accepted.') else: print('Sorry, that is the wrong password.')

26. Nested if-else Statement
Sometimes a decision needs to be made after another decision was made
Called nesting
Commonly needed in programs
Example:
Determine if someone qualifies for a loan, they must meet two conditions:
Must earn at least $30,000/year
Must have been employed for at least two years
Check first condition, and if it is true, check second condition

27. Nesting Flow Chart

28. Program
# This program determines whether a bank customer qualifies for a loan. MIN_SALARY = # The minimum annual salary MIN_YEARS = 2 # The minimum years on the job salary = float(input('Enter your annual salary: ')) years_on_job = int(input('Enter the number of ' + 'years employed: ')) if salary >= MIN_SALARY: # Determine whether the customer qualifies. if years_on_job >= MIN_YEARS: print('You qualify for the loan.') else: print('You must have been employed', \ 'for at least', MIN_YEARS, 'years to qualify.') print('You must earn at least $', \ format(MIN_SALARY, ',.2f'), ' per year to qualify.', sep='')

29. Nested if-else Statement
Important to use proper indentation in a nested decision structure
Important for Python interpreter
Makes code more readable for programmer
Rules for writing nested if statements
else clause should align with matching if clause
Statements in each block must be consistently indented

30. The if-elif-else Statement
Special version of a decision structure
Uses elif instead of multiple nested elses
Makes logic of nested decision structures simpler to write
Can include multiple elif statements
Syntax:
if condition_1:
statement(s)
elif condition_2:
elif condition_3:
else
Insert as many elif clauses
as necessary.

31. The if-elif-else Statement
Alignment used with if-elif-else statement:
if, elif, and else clauses are all aligned
Conditionally executed blocks are consistently indented
if-elif-else statement is never required, but logic easier to follow
Can be accomplished by nested if-else
Code can become complex, and indentation can cause problematic long lines