4.0 - Table of Contents

img

4.1 - Example

  • some code on runs on certain conditions
  • e.g. check if person is over 18

Example: if_intro.py

4.1.1 - Combine with Traversal

  • this can be combined with list traversal to increase efficiancy of code Example: for_if.py

4.2 - Conditions

  • the core of an if statement is in its conditional statement
  • Code in if will only run when the condition is True
  • Boolean value
  • if the condition is False, the code in else will run
  • else is optional

4.2.1 - Check if Equal

  • to check if two values are the same, use == and palce the two values to check on each side.
  • if the two values are identical then it returns True else False
  • = is define while == is comapre Example: double_equal.py

4.2.2 - To Check Letter Cases or Not

  • when comapring two strings, they have to be identical in order to return True
  • even letter cases
  • to ignore letter cases, unify them with .lower(), .upper(), or .title() Example: letter_cases.py

4.2.3 - Check if Unequal

  • use != to check if they are unequal
  • ! means not Example: not_equal.py

4.2.4 - Compare Numbers

  • numbers follows the same rules
  • True if the two numbers are the same and False if not
  • >, >=, <, <= Example: compare_numbers.py

4.2.5 - Multiple Conditions

  • sometimes you may want to check multiple conditions
  • and and or is useful in connecting these conditions
  1. and
    • To check if both conditions are True
    • only True if both are True
    • if one is False then it is False
    • Truth Table | A | B | A and B | | :—–: | :—–: | :—–: | | True | True | True | | True | False | False | | False | True | False | | False | False | False | Example: and.py
  2. or
    • To check if any conditions are True
    • returns True if either one is True
    • if both is False then it is False
    • Truth Table | A | B | A and B | | :—–: | :—–: | :—–: | | True | True | True | | True | False | True | | False | True | True | | False | False | False | Example: or.py

4.2.6 - Check if Value is in List

  • before certain actions the values of the list need to be checked
  • e.g. before register a user, check if the username already exists
  • use keyword in to check if value is in list Example: in.py

4.2.7 - Check if Value is not in List

  • it is also important to check if values are not in a list
  • e.g. ban list for a website
  • key not in is used Example: not_in.py

4.3 - if Statements

  • there are many types of if statements
  • if
  • if-else
  • if-elif-else
  • …

4.3.1 - Simplest if strcture

  • one condition and one action
  • first line includes the condition
  • next indented line includes the action
  • the indented line only runs when the if statement is True Example: if.py

4.3.2 - if-else strcture

  • two routes depending on the condition
  • True runs the code under the if
  • False runs the code under the else
  • “Do A if condition is true, else do B” Example: if-else.py

4.3.3 - if-elif-else strcture

  • multiple possible actions depending on the cases Example: if-elif-else.py

4.4 - Process List with if

  • combining for loop and if statement can process special elements in a list
  • making sure the code will work in any conditions

4.4.1 - Special Element in List

  • sometimes certain values requires special treatment Example: toppings.py

4.4.2 - Check if List is empty

  • there was an assumption that there is at least one element in list
  • sometimes something special need to be done when list is empty Example: empty.py

4.5 - Formatting Guide

  • to make the code easier to read, a space (` ) is usually added to either side of the condition
  • does not affect how the code runs
  • asthetic choices
  • a == b

  • a==b

4.6 - Sample Code

if_intro.py

age = 30
if age >= 18:
    print("adult")
else:
    print("child")

Output:

adult

for_if.py

ages = [17, 18, 32, 68, 2, 10]
for age in ages:
    if age >= 18:
        print("adult")
    else:
        print("child")

Output:

child
adult
adult
adult
child
child

double_equal.py

name = "Ava":
print(age == "Ava")
print(age == "Ethan")

Output:

True
False

letter_cases.py

name = "Ethan Hu"
print(name == 'ethan hu')
print(name.lower() == 'ethan hu')

Output:

False
True

not_equal.py

name = "Ethan Hu"
print(name != 'John')
print("John" != 'John')

Output:

True
False

compare_numbers.py

age = 18
if age < 18:
    print("not yet adult")
if age == 18:
    print("just turned adult")
if age >= 18:
    print("adult")

Output:

just turned adult
adult

and.py

age1 = 18
age2 = 17
print(age1 >= 18 and age2 >= 18)

Output:

False

or.py

age1 = 18
age2 = 17
print(age1 >= 18 or age2 >= 18)

Output:

True

in.py

name = "Ethan"
invitations = ["Ethan", "Ava", "John", "Bob"]
print(name in invitations)

Output:

True

not_in.py

name = "Dillon"
banned = ["Ethan", "Ava", "John", "Bob"]
print(name not in invitations)

Output:

True

if.py

if 10 > 9:
    print("10 is larger than 9")

Output:

10 is larger than 9

if-else.py

if 9 > 9:
    print("9 is larger than 9")
else:
    print("9 is not larger than 9")

Output:

9 is not larger than 9

if-elif-else.py

age = 12
if age < 4:
    print("baby")
elif age < 12:
    print("kid")
elif age < 18:
    print("teenager")
else:
    print("adult")

Output:

teenager

toppings.py

requested_toppings = ["mushrooms","green pepper", "olive","extra cheese"]
stock = ["olive", "extra cheese", "pepperoni"]
for topping in requested_toppings:
    if topping in stock:
        print("Adding" + topping)
    else:
        print("We are out of " + topping)
print("Pizza is finished")

Output:

We are out of mushrooms
We are out of green pepper
Adding olive
adding extra cheese
Pizza is finished

empty.py

requested_toppings = []
stock = ["olive", "extra cheese", "pepperoni"]
if requested_toppings:
    for topping in requested_toppings:
        if topping in stock:
            print("Adding" + topping)
        else:
            print("We are out of " + topping)
    print("Pizza is finished")
else:
    print("Just a plain pizza?")

Output:

Just a plain pizza?