Conditionals

Applied Review

Boolean Data Type

• Remember that we have four primitive types in Python:
  • int - 0, 1, 2, 3, ...
  • float - 1.23, 2.45, 5.0, ...
  • string - 'hello', 'x', ...
  • boolean - True, False

• Logical tests return boolean values True and False

1 < 0

False

1 < 2

True

General Model

Control Flow

• Flow is to proceed or move

• Control is to exercise restraining or directing influence over

Question: What is control flow?

• Control flow is the order in which individual statements, instructions or function calls of an imperative program are executed or evaluated.

• In other words, it's controlling the flow of the program execution.

• There are two common ways to do this: conditionals and iteration.

Conditionals

• Conditionals are directions on when to do something and when not to do something

• They start with a yes/no (true/false) question (i.e. "Are you tired?")

• And are followed with a direction depending on the result of the question (i.e. "Go to bed.", "Don't go to bed.")

Iteration

• Iteration is the repetition of a process

• The process is generally repeated until a certain condition is met

• Examples:
  • Count to 102
  • Count all of the floors of the Empire State Building
  • Keep walking down this road until you hit 5th Street

Conditionals in a Program

• Conditionals can be used to control the flow of a program

Form

• Within programs, conditionals usually take the form of if/then statements (i.e. "If you are tired, then go to bed.")

• But they can also contain else statements (not statements) (i.e. "If you are tired, then go to bed. If not, stay up and read a book.")

Execution Pattern

Question: Why might we want to use conditionals in Python?

• Data generation (case-when logic, etc.)

• Direct when to run certain code (i.e. "If there are more than 5 flights from Cincinnati to New York, make a data visualization showing average delay time in flights from Cincinnati to New York")

• Benefits include:
  • Error checking
  • Logic checking
  • Automation

Program Control

if Statements

• if statement syntax in Python isn't quite as easy as our examples, but it's pretty close...

• if statements begin with the if keyword and are followed by a logical test and a colon

if tired == 'Yes':

• The then statement directly follows the if statement as an indented block

if tired == 'Yes':
    go_to_bed()

• Note that the actual word then is never used - it's implied by the indent

• Interpret as:

  if condition_holds:
      execute_code()
  

• The implied then-block can have as many lines as necessary

if tired == 'Yes':
    brush_teeth()
    turn_off_lights()
    go_to_bed()

• All of the indented code will execute only if tired == 'Yes' is True

Non-psuedo--code

x = 'tired'

if x == 'tired':
    print('Go to bed!')

Go to bed!

Your Turn

1. Describe what this statement is doing.

   if denominator != 0:
       quotient = numerator / denominator
   

2. Fill in the blanks to print the statement if x is a negative number. Re-run multiple times.

   import random
   
   x = random.randint(-10, 10)
   
   __ x _ __:
       print(f'x = {x}, which is negative')
   

else Statement

• The else statement must follow an if statement and its indented then-block

• Similar to the if statement, it begins with the else keyword and is followed by a colon (:)

if tired == 'Yes':
    go_to_bed()
else:
    read_a_book()

• Similar to if's then-block, there's also an indented block to be run if the else path is taken

Question: When will the else path be taken?

if tired == 'Yes':
    go_to_bed()
else:
    read_a_book()

• The else path is taken in all cases when tired is not equal to 'Yes'

Your Turn

Fill in the blanks to print the relevant statement for x. Re-run multiple times.

import random

x = random.randint(-10, 10)

__ x _ __:
    print('x is negative')
__:
    print('x is positive')

elif Statement

• There's another option available when working with conditionals in Python

• The elif (else-if) statement can be used to add another if statement in the execution path

if tired == 'Yes':
    go_to_bed()
elif tired == 'A little':
    rest_eyes()
else:
    read_a_book()

• You can include as many elif statements as you want
  • if you're familiar with a switch statement in other languages, this is Python's closest relative

Your Turn

Fill in the blanks to print the relevant statement for x.

import random

random.seed(7)
x = random.randint(-10, 10)

__ x _ __:
    print('x is negative')
__ x _ __:
    print('x is positive')
__:
    print('x is zero')

Nesting and Combining Statements

• if-elif-else statements can be nested or combined to account for more complex logic

Nesting Statements

• Assume you want to execute some code if tired == 'Yes' AND the time > 20:00, you can nest if statements

if tired == 'Yes':
    if time > 20:00:
        go_to_bed()

• And you can include a nested else

if tired == 'Yes':
    if time > 20:00:
        go_to_bed()
    else:
        take_a_nap()

Combining Statements

• Multiple statements can also be combined to a single line using logical operators rather than nested

if tired == 'Yes' and time > 20:00:
    go_to_bed()
if tired == 'Yes' and time <= 20:00:
    take_a_nap()

• This will be equally efficient - it's a matter of personal preference and code readability

Variants of Conditional Tests

You will see a few common variants of our conditional test statement

Something has a particular value

All our examples thus far fall into this category:

x = 7

if x > 0:
    print('x is positive')

x is positive

y = 'Ethan'

if y == 'Ethan':
    print('y is not Brad')

y is not Brad

Something exists in a container

Sometimes we just want to know if a particular value exists in an object:

email = ['Ethan', 'Brad', 'spam']

if 'spam' in email:
    print('You have spam in your email!')

You have spam in your email!

Something is a particular type of object

Sometimes we only want to operate on a particular type of object:

isinstance(email, list)

True

if isinstance(email, list):
    print(f'You have {len(email)} email')

You have 3 email

x = 3

if isinstance(x, (int, float)):
    print(x * 4)
else:
    print('x is not a number')

12

All or some of something meets a condition

Sometimes we want to operate on an object if all or any of the values are True:

email_is_spam = [False, False, True]

# use `all()` or `any()`
if all(email_is_spam):
    print('All your emails are spam!')
elif any(email_is_spam):
    print('At least one of your emails is spam!')
else:
    print('No spam!')

At least one of your emails is spam!

There's a lot of flexibility!

Questions

Are there any questions before we move on?