CS110_L4_Notes.py

Python3 Editor

# boolean values can be set to True or False
isRaining = True
isSunny = False

# Spacing is really important in Python. If a line ends with a colon, it is a "block" of code.
# All code that goes inside that block, must start with 1 tab (or 4 spaces) from the start of the line
# defining that block. In this case, it would need 4 spaces from the start of the if-statement.
# This spacing is given to you automatically, when you press Enter after typing a colon in Python3 Editor.

# The format for a condition statement in Python is:
# if condition:
#     code that runs when condition is True

# you can check if a variable is True by just writing its name
if isRaining:
    print("It's raining")
    
# elif means "otherwise if". It must follow an if-statement and cannot stand on its own.
# An elif statement is only checked if all if and elif-statements before it are evaluated as False.
elif isSunny:
    print("It's sunny")
    
# else means "otherwise". It will run the code within only when all if and elif-statements that come before it in the chain
# are evaluated as False.
else:
    print("It's cloudy out.")
    
# 'and' and 'or' are written using the full word in Python. The conditions on both sides must be FULL conditions.
# These keywords also have "short-circuiting". 
# Since A and B is True if and only if both are True, then if the first condition is False, it will NOT check the second condition.
# Since A or B is False if and only if both are False, then if the first condition is True, it will NOT check the second condition.
if isRaining and isSunny:
    print("Where's the rainbow?")
else:
    print("Aww ... no rainbows ...")
    
# The keyword 'not' is used to check if the condition that follows is False.
if not isRaining and not isSunny:
    print("It MUST be cloudy")
    
# The condition below is logically equivalent to the one on line 38.
if not (isRaining or isSunny):
    print("It MUST be cloudy")
    
print()
#-----------------------------------------------------
#    Comparing Numbers
#-----------------------------------------------------

# > greater than
# < less than
# >= greater than OR equal to
# <= less than OR equal to
# == equals
# != not equals

x = 5
y = 10
if x > 5:
    print("x > 5")
elif x <= 5 and y > 8:
    print("x <= 5 and y > 8")
elif y == 10 and x == y/2:
    print("y == 10 and x == y/2")
    
# The cool thing about Python and comparing numbers is that, if you are checking if a value is in a specific range,
# you can use one line without an 'and' unlike other programming langauges. Remember: this does NOT work in Java.
if 5 <= x <= 8:    # checks if x >= 5 AND x <= 8
    print('5 <= x <= 8')
print()

#------------------------------------------------------
#    Comparing Strings
#------------------------------------------------------

# == equals
# != not equals
# not s == also not equals

s = "Hello"
if s == "Hello":
    print(s)
elif not s == "Hello":        # could also write it as: elif s != 'Hello'
    print("Goodbye!")

print()
#---------------------------------------------------------
#    Be careful with 'and' and 'or'
#---------------------------------------------------------

# When you use 'and' and 'or', you need to have a FULL condition on both sides of the keyword.
s = 'Cube'

# BAD EXAMPLE: This will always trigger the if-statement to be true, since the first condition is checking if
# s == 'Rock', but the second condition just checks if 'Paper' is not None. It's a string literal so it's not None.
# Since there is one true condition in the statement combined with 'or', the whole statement is true, so it reads as VALID.
if s == 'Rock' or 'Paper' or 'Scissors':
    print("Valid")
else:
    print("Invalid")
    
s = 'Rock'

# CORRECT EXAMPLE:
if s == 'Rock' or s == 'Paper' or s == 'Scissors':
    print('Valid')
else:
    print("Invalid")
    
# Alternatively, if you are using or to check a value, you can check if the value is in a list like this:
if s in ['Rock', 'Paper', 'Scissors']:
    print("Valid input")
else:
    print("Invalid input")

print()

#--------------------------------------------------------
#    None and the is keyword
#--------------------------------------------------------

# The keyword 'is' should only be used to check EXISTENCE, or to check if two values are pointing to the same location in memory.
# It SHOULD NOT be used to check if two values are equal
a = ['Bob', 5, 'Hello']
b = ['Bob', 5, 'Hello']
c = a

# Since c = a, they are pointing to the same spot in memory, so the if-statement is false, but the elif is true,
# and it will print out that 'a is c'.
if a is b:
    print('a is b')
elif a is c:
    print('a is c')
    
d = None
# Since d is None, it will go to the else statement.
# You could switch these two conditions by changing the if-statement to check if d is None, 
# then swap the print statements.
if d:
    print("d has a value")
else:
    print("d does not have a value")
    
print()

#---------------------------------------------------------------------------
#    REVIEW
#---------------------------------------------------------------------------

# Any of the functions we went over in previous classes that return booleans (True or False) values,
# can be used in an if-statement.
s = input("Enter something: ")
if s.isalpha():
    print("All letters.")
if s.isdigit():
    print("All digits.")
if s.isalnum():
    print("Has letters and/or digits.")
if s.isspace():
    print("All spaces.")
if s.islower():
    print("All lowercase.")
if s.isupper():
    print("All uppercase.")

# The same goes with collections of data and the use of the keyword 'in'.
d = {
    'x': 200,
    'y': 150,
    'color': 'blue'
}
if 'color' in d:
    print(d['color'])
    
# the following if-statement prints nothing since 'name' is not in the dictionary d
if 'name' in d: 
    print(d['name'])

( around 146 lines Python3 code )

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Created on 2020-11-02T14:19:28Z

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