Decision-Structures

Slide 3 Flow of Control ● Flow of control is the order that a program performs actions, or executes statements ● By default, statements are executed in sequential order, i.e., statement are executed one after another, in the same order they appear in your program ● Most programming languages use 2 kinds of structures to control the flow of execution – Decision structures choose among several possible actions; also called conditional or branching structures – Repetition structures repeat an action until some stop condition is met; also called iteration or loop structures “Would you tell me, please, which way I ought to go from here?” “That depends a good deal on where you want to get to,” said the Cat. --- Lewis Carroll, Alice in Wonderland

Slide 6 if Statement Examples Gross pay calculation for regular (vs. overtime) hours hours_worked = float(input('Enter hours worked: ')) if hours_worked <= 40: gross_pay = hours_worked * 15.50 print('Gross pay:', gross_pay) Sales price calculation where discounts only apply to $10+ items original_price = float(input("Enter the item's original price: ")) if original_price > 10.00: sale_price = original_price - (original_price * 0.2) total_price = sale_price * 1.0475 print('Total price is', total_price) Time conversion only applies to a positive number of seconds total_seconds = int(input('Enter number of seconds: ')) if total_seconds > 0: hours = total_seconds // 3600 remaining_seconds = total_seconds % 3600 minutes = remaining_seconds // 60 remaining_seconds = remaining_seconds % 60 print('Hours:', hours, '\tMinutes:', minutes, '\tSeconds:', remaining_seconds)

Slide 9 Using if-else to Compare Strings # Compare 2 strings with the == operator password = input ( 'Enter a password:' ) if password == 'somethingcryptic' : print ( 'Password accepted.' ) else : print ( 'Sorry, that is the wrong password' ) Source: Starting Out with Python by Tony Gaddis password_test.py Checking to see if 2 string variables have the same value, or in other words, are the string variables equal?

Slide 12 Nested Decision Structures Example # This program determines whether a customer qualifies for a loan min_salary = 30000.0 min_years = 2 salary = float ( input ( "Enter your annual salary: " )) years_on_job = int ( input ( "Enter number of years employed: " )) if salary >= min_salary : 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' ) else: print ( 'You must earn at least $' ,min_salary, ' per year to qualify.' , sep = '' ) print ( 'Loan qualification completed.' ) loan_qualification.py

Slide 15 Temperature Conversion with nested decision structures ● Suppose you want to ensure that the temperature value is numeric before trying to convert it? ● You can use the Python string function: temp . isnumeric() – temp is a string variable with the value the user entered – Returns True if all characters in the string are numeric (0-9) – Returns False if the string variable contains any characters that are not numeric temp = input ( 'Enter a temperature value: ' ) if temp. isnumeric() : temp = float (temp) conversion = input ( 'Enter type of conversion (f2c or c2f): ' ) # Code to convert the temp goes here else : print (temp, "is not a valid temperature value" )

Slide 18 What if the temperature conversion program needed to handle Kelvin temperatures, too? temp = input ( 'Enter a temperature value: ' ) if temp.isnumeric(): temp = float (temp) conversion = input ( 'Enter type of conversion (f2c, c2f, k2f, f2k, k2c, or c2k): ' ) if conversion == ' f2c ' : # Fahrenheit to Celsius converted_temp = (temp - 32 ) * 5.0 / 9 print (temp, 'degrees Fahrenheit is ' , format (converted_temp, '.2f' ), ' degrees Celsius' ) else : if conversion == ' c2f ' : # Celsius to Fahrenheit converted_temp = (temp * 9 / 5.0 ) + 32 print (temp, 'degrees Celsius is ' , format (converted_temp, '.2f' ), ' degrees Fahrenheit' ) else : if conversion == ' k2f ' : # Kelvin to Fahrenheit converted_temp = (temp - 273.15 ) * 1.8 + 32 print (temp, 'degrees Kelvin is ' , format (converted_temp, '.2f' ), ' degrees Fahrenheit' ) else : if conversion == ' f2k ' : # Fahrenheit to Kelvin converted_temp = (temp - 32 )* 5 / 9 + 273.15 print (temp, 'degrees Fahrenheit is ' , format (converted_temp, '.2f' ), ' degrees Kelvin' ) else : if conversion == ' k2c ' : # Kelvin to Celsius converted_temp = temp - 273.15 print (temp, 'degrees Kelvin is ' , format (converted_temp, '.2f' ), ' degrees Celsius' ) else : if conversion == ' c2k ' : # Celsius to Kelvin converted_temp = temp + 273.15 print (temp, 'degrees Celsius is ' , format (converted_temp, '.2f' ), ' degrees Kelvin' ) else : print (conversion, 'is not a valid conversion type' ) else : print (temp, "is not a valid temperature value" ) 6 possible conversion types: f2c , c2f , k2f , f2k , k2c , or c2k

Slide 21 if-elif-else example What is the ticket price for each seat location? if location == 'suite' : price = 100.00 elif location == 'box' : price = 75.00 elif location == 'orchestra' : price = 50.00 elif location == 'mezzanine' : price = 40.00 elif location == 'balcony' : price = 30.00 else : print ( 'Enter a valid seat location' ) seat_location_if_elif.py

Slide 22 When to use: nested if-else vs. if-elif ● if-else is generally used most often because it can represent any decision structure ● if-elif is typically used when testing the value of a single variable (e.g., seat location, temp conversion type) – Use of if-elif may avoid deeply nested indentation – Code may be easier to understand than nested if-else 's ● if-elif is never required: it can always be re-written as a nested if-else statement ● But , a nested if-else statement cannot always be re-written as if-elif. For example, recall the loan qualification program where we were checking for the salary and years on the job – these are different variables, so using if-elif is not a good choice

Slide 23 Part 1 ● if and if-else statements Part 2 ● Nested Decision Structures with if-else ● if-elif-else statement Part 3 ● Logical operators ● Boolean variables

Slide 24 When one Boolean expression is not enough... Logical Operators and ● Connects two Boolean expressions into one compound expression ● Both sub-expressions must be true for the compound expression to be true or ● Connects two Boolean expressions into one compound expression ● One or both sub-expressions must be true for the compound expression to be true not ● Reverses the Boolean value of its operand Usage example: Google Advanced Search Expression Meaning (x > y) and (a < b) Is x greater than y AND is a less than b ? (x == y) or (x == z) Is x equal to y OR is x equal to z ? not (x > y) Is the expression x > y NOT true?

Slide 25 Truth Tables for Logical Operators Pseudocode examples: If (userid is valid) and (password is valid) Log user into app If (bank balance > loan amount) or (credit is good) Loan the money Value of x Value of y x and y x or y not(x) true true true true false true false false true false false true false true true false false false false true

Slide 26 Loan Qualifier Program, revised, to use a Logical Operator salary = float ( input ( "Enter your annual salary: " )) years_on_job = int ( input ( "Enter number of years employed: " )) Original version if salary >= min_salary: 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' ) else : print ( 'You must earn at least $' ,min_salary, ' per year to qualify.' , sep = '' ) Revised version if salary >= min_salary and years_on_job >= min_years: print ( 'You qualify for the loan.' ) else : print ( 'You do not qualify for the loan' )

Slide 27 Boolean Variables ● Data types so far: int , float , and str (string) ● bool data type allows you to create Boolean variables that have the values True or False ● Example: today_is_Thursday = True To check the value of a Boolean variable: if today_is_Thursday: print('Then it must be Thursday') This is the same as: if today_is_Thursday == True: print('Then it must be Thursday') Note: checking equality with == is not necessary, and a little redundant

Slide 28 Loan Qualifier with Boolean Variables if salary >= min_salary: customer_meets_salary_requirement = True else: customer_meets_salary_requirement = False if years_on_job >= min_years: customer_meets_years_on_job_requirement = True else: customer_meets_years_on_job_requirement = False Boolean variables may make your code a little verbose, but they can also make your code easier to read and understand. if customer_meets_salary_requirement and customer_meets_years_on_job_requirement: print('You qualify for the loan.') else: print('You do not qualify for the loan')

Slide 29 Summary: Why Use Decision Structures? ● All programs solve some type of problem that requires data – that data may be provided by a user and/or generated by your program. ● When you design a program, you have to decide what data you need to solve the problem, and what conditions that data must satisfy. ● Decision structures, i.e., if-else and if-elif statements, provide the mechanism for specifying the conditions that must be met. ● Conditions are often specified to validate input data and to process data

Slide 30 Summary: Using Decision Structures 4 conditions → 4 if-else statements 2 conditions to validate input 2 conditions to process input loan_qualification_v2.py min_salary = 30000.0 min_years = 2 salary = input ( "Enter your annual salary: " ) if salary.isnumeric(): salary = float (salary) years_on_job = input ( "Enter number of years employed: " ) if years_on_job.isnumeric(): years_on_job = int (years_on_job) if salary >= min_salary: 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' ) else : print ( 'You must earn at least $' , min_salary, ' per year to qualify.' , sep = '' ) else : print (years_on_job, "is not a valid value for years employed" ) else : print (salary, "is not a valid value for salary" ) print ( 'Loan qualification completed.' )