Booleans and Conditional logic¶

Booleans¶

A Boolean is a datatype that can only take two values: True or False.

Comparison Operators¶

Comparison operators tell how two Python values relate and they always result in a boolean (True or False). In other words, they answer yes/no questions.

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2 == 3
2 == 3

Out[28]:

False

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2 < 3
2 < 3

Out[29]:

True

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2 > 3
2 > 3

Out[30]:

False

We will often use comparison operators between two variables, or between a variable and a value:

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#assigning a variable
a = 2

#Applying the comparison operator: Is a equal to 2? Well yes, we just defined it!
print(a == 2)
#assigning a variable a = 2 #Applying the comparison operator: Is a equal to 2? Well yes, we just defined it! print(a == 2)

True

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b = 3

#Applying a comparison between two variables:
print(a == b)
b = 3 #Applying a comparison between two variables: print(a == b)

False

The following comparison operators exist between two integer variables: Assume a = 2 and b = 3 as above.

operator	Description	Result	Example (`a, b = 2, 3`)
`==`	a equal to b	True if a has the same value as b	`a == b # False`
`!=`	a not equal to b	True if a does NOT have the same value as b	`a != b # True`
`>`	a greater than b	True if a is greater than b	`a > b # False`
`<`	a less than b	True if a is less than be b	`a < b # True`
`>=`	a greater than or equal to b	True if a is greater than or equal to b	`a >= b # False`
`<=`	a less than or equal to b	True if a is less than or equal to b	`a <= b # True`

Note: The result of a comparison is defined by the type of a and b, and the operator used.

All conditional checks must always resolve to True or False, or an error if the two object types are not comparable.

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#try it out!
print(a > b)
print(a < b)
print(a != b)
#try it out! print(a > b) print(a < b) print(a != b)

False
True
True

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#Just like a variable, we can also store the result of the comparison. It will be a boolean

result = a > b
print(result)
print(type(result))
#Just like a variable, we can also store the result of the comparison. It will be a boolean result = a > b print(result) print(type(result))

False
<class 'bool'>

The same operators can also be applied to two string variables.

Greater/smaller than comparisons are based on the alphanumeric sort:

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print("a" == "b")
print("a" != "b")
print("a" == "b") print("a" != "b")

False
True

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print("a" < "b")
print("a" > "b")

#also works on multi character strings:
print("carl" < "cbrl")
print("a" < "b") print("a" > "b") #also works on multi character strings: print("carl" < "cbrl")

True
False
True

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#Why is "cdk10" smaller than "cdk8"? 
print("cdk10" < "cdk8")
#Why is "cdk10" smaller than "cdk8"? print("cdk10" < "cdk8")

True

Equality¶

More generally, we can test the equality of two variables or objects with the equality operator ==.

Operator	Description	Example (`a, b = 2, 3`)
`==`	True if both sides evaluate to the same	`a == 2 # True`
`==`	True if both sides evaluate to the same	`a == b # False`
`!=`	True if both side do not evaluate to the same	`a != b # True`
`!=`	True if both side do not evaluate to the same	`a != 3 # True`

There is also the identity operator is, but it does something different than == and the explanation goes beyound the scope of this course. You can read about it here if you're curious. For now, in 99% of case you actually want ==.

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a = 2
b = 3
print(a == b)
print(a != b)
a = 2 b = 3 print(a == b) print(a != b)

False
True

This works between all types of objects:

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#comparing the equality of lists
alist = [1, 2, 3]
blist = [1, 2, 3]
print(alist == blist)

#comparing dictionaries
food_dict = {
    'fruit': ['banana', 'apple', 'strawberry'],
    'veggies': ['eggplant', 'carrot']
}

city_population = {
    'Tokyo': 13350000, # a key-value pair
    'Los Angeles': 18550000,
    'New York City': 8400000,
    'San Francisco': 1837442,
}

print(food_dict == city_population)

# we already saw == on integers and strings above
#comparing the equality of lists alist = [1, 2, 3] blist = [1, 2, 3] print(alist == blist) #comparing dictionaries food_dict = { 'fruit': ['banana', 'apple', 'strawberry'], 'veggies': ['eggplant', 'carrot'] } city_population = { 'Tokyo': 13350000, # a key-value pair 'Los Angeles': 18550000, 'New York City': 8400000, 'San Francisco': 1837442, } print(food_dict == city_population) # we already saw == on integers and strings above

True
False

Exercise 1¶

~ 5 minutes

a. What will be the result of this comparison?

x = 2
    y = "Anthony"
    x < y

True
False
Error

b. What about this comparison?

x = 12.99
    y = 12
    x >= y

True
False
Error

c. And this comparison?

x = 5
    y = "Hanna"
    x == y

True
False
Error

Truthiness¶

In addition to comparing objects to each other, it is important to note that all Python objects are by themselves either True or False. We call this concept Truthiness.

x = 1
y = 0
bool(x) # True
bool(y) # False

Empty objects such as empty lists/tuples/arrays/strings, the None object, and the int 0 and float 0.0 are by definition False.

All non-empty objects are by definition True.

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# empty list
a = []
print(bool(a))
# empty list a = [] print(bool(a))

False

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# 0, '' and None are False
a = 0
b = None
c = ''

print(bool(a))
print(bool(b))
print(bool(c))
# 0, '' and None are False a = 0 b = None c = '' print(bool(a)) print(bool(b)) print(bool(c))

False
False
False

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# non-empty objects are True
a = [1,2,3]
b = 5
c = 'Hi!'

print(bool(a))
print(bool(b))
print(bool(c))
# non-empty objects are True a = [1,2,3] b = 5 c = 'Hi!' print(bool(a)) print(bool(b)) print(bool(c))

Logical Operators¶

Logical operators are used in conditional statements.

and, True if both a AND b are true (logical conjunction)
or, True if either a OR b are true (logical disjunction)
not, True if the opposite of a is true (logical negation)

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# logical operator: and
# will only return true if both conditions are true!
a = True
b = True

res1 = a and b
print(res1)
# logical operator: and # will only return true if both conditions are true! a = True b = True res1 = a and b print(res1)

True

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c = False
res2 = a and c
print(res2)
c = False res2 = a and c print(res2)

False

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# logical operator: or
# will return true if at least one of the conditions is true

#remember a = True and c = False as defined above
res3 = a or c
print(res3)
# logical operator: or # will return true if at least one of the conditions is true #remember a = True and c = False as defined above res3 = a or c print(res3)

True

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# logical operator: not
# negates: True becomes False and False becomes True

#remember a = True and c = False as defined above
res4 = not a
print(res4)
res5 = not c
print(res5)
# logical operator: not # negates: True becomes False and False becomes True #remember a = True and c = False as defined above res4 = not a print(res4) res5 = not c print(res5)

False
True

Membership operators¶

Membership operators test whether a certain element is part of a container.

Operator	Description	Example (`a = [1, 2, 3]`)
`in`	True if value/variable is found in the sequence	`2 in a # True`
`not in`	True if value/variable is not found in the sequence	`5 not in a # False`

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aa = ['alanine', 'glycine', 'tyrosine']
'alanine' in aa
aa = ['alanine', 'glycine', 'tyrosine'] 'alanine' in aa

Out[45]:

True

Remember that strings are containers of characters, so in works on them as well!

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print(aa[1])
'gly' in aa[1]
print(aa[1]) 'gly' in aa[1]

glycine

Out[47]:

True

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my_s = "Python is great!"
'great' in my_s
my_s = "Python is great!" 'great' in my_s

Exercise 2¶

~10 minutes

a. What is the result of the following expression?

a = True
b = False
a or b

What do you need to change to make this expression evaluate to False?

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# your code here
# your code here

b. What about the result of this expression? Why is this the result?

A hint: Check bool(y) and bool(x) individually.

x = 15
y = 0
bool(x or y)

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# your code here
# your code here 

c. What is the result of the following expression?

x = 0
y = None
x < y

Can you compare x and y in a way that does not produce an error?

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# your code here
# your code here

d. How can you find out if the following list contains 'Copenhagen'?

capitals = ['Berlin', 'London', 'Copenhagen', 'Madrid']

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# your code here
# your code here

Flow of code¶

So far we have made code that flows linearly. The statements are executed one after each other like here:

my_var = 1

my_var += 10

print(my_var)

print(my_var == 11)

You can see, all lines start at the same left edge of the page. We say they are on the same indentation level.

But often when we program we want different things to happen depending on a condition:

The way we control flow in python is with indentation. It defines where a line of code belongs. Lines that are on the same level of indentation will be executed linearly, one after another. Lines on a different indentation level means there is some sort of flow control happening.

if some condition is True:
    do something
    everything I write here is part of the if block
    it will only be executed if the condition is true 
    more commands

#when we return to the outermost indentation level python knows the if block is over    
elif some other condition is True:
    do something else

else:
    do something

Conditional Statements¶

Conditional statements use the keywords if, elif and else, and they let you control which pieces of code are run based on the value of some Boolean condition.

The basic syntax of an if block is:

if some condition is True:
    do something
elif some other condition is True:
    do something
else:
    do something

The lines containing if, elif and else always need to end with a : (colon).
Only if and elif are followed by conditions, whereas else is never followed by anything else than a : (colon).
if and elif are exclusive. They cannot simultaneously be true! In fact, the elif condition will only be checked if the if condition is False.
Code to be executed if the condition is True always needs to be indented.

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# if/else

age = 23

if age >= 18:
    print("Here's your beer")
else:
    print("Come back when you're 18!")
# if/else age = 23 if age >= 18: print("Here's your beer") else: print("Come back when you're 18!") 

Here's your beer

The condition to be checked can also be a boolean itself! For example:

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# if/else

is_weekend = True

if not is_weekend:
    print("It's Monday.")
    print("Go to work.")
else:
    print("Sleep in and enjoy the beach.")
# if/else is_weekend = True if not is_weekend: print("It's Monday.") print("Go to work.") else: print("Sleep in and enjoy the beach.") 

Sleep in and enjoy the beach.

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# if/elif/else
# the elif condition will only be checked if the if condition is False

birth_year = 1950

if birth_year < 1964:
    print('Ok, boomer.')
elif birth_year < 1990:
    print("Sorry, the 90's was not 15 years ago. That's 2008.")
else:
    print("Yeah yeah, talk to me when you're a grown-up, zoomer")
# if/elif/else # the elif condition will only be checked if the if condition is False birth_year = 1950 if birth_year < 1964: print('Ok, boomer.') elif birth_year < 1990: print("Sorry, the 90's was not 15 years ago. That's 2008.") else: print("Yeah yeah, talk to me when you're a grown-up, zoomer")

Ok, boomer.

Exercise¶

~ 5 minutes

If you set the name variable to "Gandalf" and run the script below, what will the output be? How do you get the output 'Move on then'?

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name = ""
if name == "Gandalf":
    print("Run, you fools!")
elif name == "Aragorn":
    print("There is always hope.")
else:
    print("Move on then!")
name = "" if name == "Gandalf": print("Run, you fools!") elif name == "Aragorn": print("There is always hope.") else: print("Move on then!")

Exercise 3¶

~ 10-15 minutes

Create a variable and assign an integer as value, then build a conditional to test it:

If the value is below 0, print "The value is negative".
If the value is between 0 and 20 (including 0 and 20), print the value.
Otherwise, print "Out of scope".

Test it by changing the value of the variable.

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