In order for code to be useful, it is imperative to have the ability to make decisions. In most languages, we use the conditional statement to facilitate decision making.

Before we dig deeper into conditionals, let us first examine the Boolean datatype.

In short, a boolean represents a "yes" or "no" value. In python, booleans are written as:

True # this is a boolean, for "yes"
False # this is a boolean, for "no"

Because booleans are just datatypes, we can store them into variables.

is_it_summer = False
will_it_be_summer_soon = True

Moreover, because booleans are data types, certain operators will evaluate to booleans:

age = 13
is_eligible_to_buy_lotto = age > 13

# ^^ this will evaluate to False and then 
# that value, False, will be stored in variable
# is_eligible_to_buy_lotto

The operator above, > is called a boolean operator. Notice how we stored the evaluation of the > expression into a variable. Remember, booleans are just datatypes, therefore they work the same way we would expect numbers and strings to work - except that the operators look / do different things (but in principle they are one and the same!)

Let's now explore the boolean operators available in python.

my_money = 37.00
total = 35.00

enough_money = my_money > total # True
just_enough_money = my_money >= total # also True

speed_limit = 65
my_speed = 32

under_speed_limit = my_speed < speed_limit # True
at_or_under_speed_limit = my_speed <= speed_limit # also True

Because we use the = symbol for identity (ie: to set a variable), it is not available for comparison operations. Instead, we must use the == and != symbols.

speed_limit = 65
my_speed = 32

are_they_equal = (speed_limit == my_speed) # False
are_they_not_equal = (speed_limit != my_speed) # True

Note that the parens are unnecessary here, but we add them anyways for the sake of clarity.

Also worth noting that the is keyword can be used in lieu of the ==:

pi = 3.14

result = pi is 3.14 # True

x = 2
# a
1 < x < 3 # True

# b
10 < x < 20 # False

# c
3 > x <= 2 # True

# d
2 == x < 4 # True

For a, we check to see if 1 is less than x AND x is less than 3.

For b, we check to see if 10 is less than x (it is not) and stop right there

For c, we check to see if 3 is greater than x AND x is less than or equal to 2.

For d, we check to see if x is equal to 2 AND x is less than 4.

In addition to comparison operators, python also offers support for logical operators - in the form of:

1. not
2. or
3. and

The not operator simply negates. For instance,

is_it_cold = True

result = not is_it_cold # False

Likewise,

is_it_hot = False

result = not is_it_hot # True

The or operator evaluates to True if any one of the operands is true.

is_it_warm = True
is_it_cold = False
is_it_foggy = False

result = is_it_warm or is_it_cold or is_it_foggy # True

Will be true since at least once of the items is True

The and operator evaluates to True is all of the operands are true.

is_it_warm = True
is_it_foggy = True
is_it_humid = True

result = is_it_warm or is_it_humid or is_it_foggy # True

Will be true since at ALL of the items are True

Membership operators are: in and not in. They are used to determine if a value is in a sequence, for instance:

line = 'a b c d e f g'

result = 'a' in line # True
result = 'z' in line # False
result = 'k' not in line # True
result = 'a' not in line # False

A conditional will attempt to evaluate an expression down to a boolean value - either True or False. Based on the boolean evaluation, the program will then execute or skip a block of code.

So for instance:

if True:
    print("this will always run!")

if False:
    print("this will NEVER run!")

However, since we know booleans to be datatypes, any of the operators discussed above can also be used:

temp = 43

if temp < 65:
    print("wear a jacket!")

The code above will only run if temp is less than 65.

We can also do something like:

temp = 43
is_it_raining = True

if is_it_raining and temp < 65:
    print('wear a jacket and bring an umbrella!')

In the example above, we make use of comparison operators and logical operators in a compound statement.

If we have a condition that can only go two ways (ie: it will only be true or false), we can leverage the else statement:

temp = 43

if temp < 65:
    print('wear a coat!')
else:
    print('you will not need a coat!')

But what if we wanted support for multiple possibilities? That's where the elif statement comes in:

temp = 43

if temp < 30:
    print('wear a heavy jacket')
elif temp < 50:
    print('wear a light jacket')
elif temp < 60:
    print('wear a sweater')
else:
    print('you do not need any layers!')

In the example above, we print one of 4 possibilities - the elif allows us to go from 2 potential conditions to N potential conditions.

🚗 PSETS

The problems are reproduced below, but you will want to run on github. First,

$ . ./update

from random import randint

randn = randint(1,3) # generates a random number from 1 to 3
# if 1, print 'red'
# if 2, print 'green',
# if 3, print 'blue'

from random import randint

# generate a random phone number of the form:
# 1-718-786-2825
# This should be a string
# Valid Area Codes are: 646, 718, 212
# if phone number doesn't have this area code, pick
# one of the above at random

p1 = 'r' # or 'p' or 's'
p2 = 'r' # or 'p' or 's'

# Given a p1 and p2
# print 1 if p1 has won
# print 2 if p2 has won
# print 0 if tie
# print -1 if invalid input
# expects both p1 and p2 inputs to be either
# "r", "p", or "s"

from random import randint

p1 = # randomly choose 'r' or 'p' or 's'
p2 = # randomly choose 'r' or 'p' or 's'

# Given a p1 and p2
# print 1 if p1 has won
# print 2 if p2 has won
# print 0 if tie
# print -1 if invalid input
# expects both p1 and p2 inputs to be either
# "r", "p", or "s"

p1 = # from user input
p2 = # from user input

# Given a p1 and p2
# print 1 if p1 has won
# print 2 if p2 has won
# print 0 if tie
# print -1 if invalid input
# expects both p1 and p2 inputs to be either
# "r", "p", or "s"

This is the same as the original RPS problem, except that cannot expect the input to be valid. While we want r or p or s, there is a possibility that input can be anything like...

• ROCK (all caps)
• R (r but capitalized)
• PAPrrRR (incorrectly spelled, upper/lowercased)

Implement conditional statements that will sanitize the user input or let user know that input is invalid.

p1 = # from user input
p2 = # from user input

# Given a p1 and p2
# print 1 if p1 has won
# print 2 if p2 has won
# print 0 if tie
# print -1 if invalid input
# expects both p1 and p2 inputs to be either
# "r", "p", or "s"

p1 = # from user input - we still want validation from above!
p2 = # randomly generated against computer

# Given a p1 and p2
# print 1 if p1 has won
# print 2 if p2 has won
# print 0 if tie
# print -1 if invalid input
# expects both p1 and p2 inputs to be either
# "r", "p", or "s"

grade = 15 # expect this to be a number

# write a program that will print the "letter" 
# equivalent of the grade, for example:
# when grade = 90 # -> expect A
# when grade = 80 # -> expect B
# when grade = 70 # -> expect C
# when grade = 60 # -> expect D
# when grade = 54 # -> expect F
# when grade = -10 # -> expect Error
# when grade = 10000 # -> expect Error
# when grade = "lol skool sucks" # -> expect Error

Challenge: Can you raise an error if unexpected input supplied vs just printing out Error? What's the difference?

Given three numbers, a, b, c, without multiplying, determine the sign of their product.

EXAMPLE: a = -5, b = 6, c = -4, print 1

EXAMPLE: a = 5, b = 6, c = -4, print -1

Given a string str, determine if there are any uppercase values in it. Use only conditional statements and string methods (you may have to look some up!)

EXAMPLE: str = "teSt", print True

Given any empty string, of the form:

''
' '
'  '
# ...
'        ' # etc

determine if the str is empty or not (print True or False)

Given the following inputs:

P = # True or False
Q = # True or False
op = # '^' (logical AND, conjunction)
     # OR, 'v' (logical OR, disjunction)
     # OR, '->' (logical conditional, implication)
     # OR, '<->' (biconditional)

determine the correct outcome.

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