Booleans and conditionals

Materials:

• Code-along Jupyter notebook

• Filled-in Jupyter notebook

Often in code, we want to take different actions based on the current state of our program (e.g., do we have more or less than 100 samples in our data?). We can ask yes or no questions about this state called boolean expressions. These questions are answered as true or false. We can design the program to perform an action based on the response, which is called a conditional.

Booleans

True and False are keywords in Python. They are a unique data type called booleans.

Capitalization is critical. Booleans in Python have their first letter capitalized and the rest lower-case.

Python

f = False
print(f)

Output

False

Python

t = True
print(t)

Output

True

We can convert other data types to booleans with the function bool(). A number will only convert to False if it is exactly 0 or 0.0. All other numbers convert to True.

Python

print(bool(0)) # False
print(bool(1)) # True

Output

False
True

Similarly, we can convert strings into booleans. Empty strings ('' or "") convert to False and any other string converts to True.

Python

print(bool(''))
print(bool(' '))

Output

False
True

Boolean expressions

Boolean expressions essentially ask questions that evaluate as True or False. These can examine whether two values are equal, if one is larger than another, or similar questions. To ask these questions we need to use special boolean operators that you’ll see below.

Boolean expressions are best used between the same data types. You can easily get unexpected results when comparing strings and ints, for instance.

Equality: ==

The operator == is used to check if two items are equal to each other. Note the distinction between =, which is used for variable assignment, and ==, which checks for equality.

Python

"bad" == "bad"

Output

True

Python

2 == 3

Output

False

Warning

Checking for equality for floats can be tricky given common rounding errors. Try to avoid if possible, and test for inequality (see below).

Not equals: !=

Similar, we can use != to check if two items are not equal to each other. !=` will always return the opposite of == for any given comparison.

Python

"bad" != "BAD" # capitalization matters!

Output

True

Python

"bad" != "bad"

Output

False

Inequalities

There are 4 different boolean operators for comparing inequalities: less than (<), less than or equal to (<=), greater than (>), and greater than or equal to (>=).

Python

1 < 4

Output

True

Python

5.1 > 5.0

Output

True

Python

3 >= 3

Output

True

Python

7 <= 3

Output

False

Inclusivity: in

We can use the keyword in to check if an item is in a data structure (list, dictionary, set, tuple).

Python

my_list = [ 'apple', 'pear', 'grape' ]
'apple' in my_list

Output

True

Because sets are made of unique items, they are perfect for using in.

Python

my_set = { 'orange', 'berry', 'lemon' }
'apple' in my_set

Output

False

You can also use in to check if a smaller string is a part of a larger string.

Python

print('i' in 'team')
print('i' in 'win')

Output

False
True

Python

print('good movie' in 'star wars sequel trilogy')

Output

False

not

Just as adding not in a sentence reverses its meaning (e.g., “The desk is red.” vs “The desk is not red.”), adding the keyword not in front of a boolean expression reverses the value returned -> not 0 == 0 returns False.

Python

not 20 < 40

Output

False

Python

not 'apple' in 'grape'

Output

True

Python

not True

Output

False

Order of Operations

We can chain together boolean comparisons with and and or.

Putting and between two booleans will make the whole statement true only both statements are true.

Python

3 < 4 and "banana" == "banana"

Output

True

On the other hand, or only needs one of the statements to be true.

Python

2 == 3 or 2 == 2

Output

True

Order of operations work with boolean expressions similarly to math. Comparisons run left to right, unless you put parentheses around the comparisons.

Python

print(not 2 == 3 or 2 == 2)
print(not (2 == 3 or 2 == 2)) # parentheses matter!

Output

True
False

Question 1: Boolean expressions

Does the following code evalutate as True or False?

Python

n1 = 45
n2 = -23
n3 = 0
s1 = 'hello'
s2 = 'goodbye'

not (n2 < n3 and s1 == s2 or n1 >= n3)

Solution

• n2 < n3 -> True

• s1 == s2 -> False

• n1 >= n3 -> True

• n2 < n3 and s1 == s2 -> False, because the first part is not True

• That leaves us with n2 < n3 and s1 == s2 or n1 >= n3. Because n2 < n3 and s1 == s2 is False and n1 >= n3 is True, this whole expression is True, as only of the two needs to be True

• The not around the parentheses causes the final value to be False

Question 2: Boolean types

What happens when you compare different data types with ==? What about >, <, <=, or >=?

Conditionals

if statements

The true power of boolean expressions is in making decisions based on whether they are true or false. We do this with if statements. The general syntax follows this format:

Python

if 'a' != 'b':

   print('hello!')

Output

hello!

To break this down:

• if keyword is first word in line

• Boolean expression ('a' != 'b') followed by a colon

• Code below that is tabbed over

• If the expression is true, the code that is below and tabbed is run

• If the expression is false, nothing happens

if 1 == 2:
    print('hello')

Here, nothing was printed, as 2 does not equal 1.

We can also have more complicated boolean expressions, as well.

Python

x = 0
a = 'a'
letters = ['a', 'b', 'c', 'd']

if x > -1 and a in letters:

   print('My number:',x)
   print('My letter:',a)

Output

My number: 0
My letter: a

if-else statement

Often in coding, we want one thing to happen if an expression is true, and another to happen if it is false. To accomplish this, we can add an else statement below the if statement. This will always be evaluated if the expression after if is False, otherwise it will not run: the if and else are mutually exclusive.

Python

x = 23

if x < 20: # if x less than 20; False

   print('Less than 20')

else: # x greater or equal to 20

   print('Greater than 20')

Output

Greater than 20

elif

What if you want to differentiate between more than 2 conditions? We can use the elif keyword, which stands for else if. This goes between the if and the else statements, and must include a new boolean expression.

Again, these options are all mutually exclusive. If the elif code is run, that means the if and else code do not run.

Python

y = 101

if y < 100: # y less than 100

   print('y is less than 100')

elif y < 200: # y is 100-200 (excluding 200)

   print('y is between 100 and 200')

else: # y is 200 or larger

   print('y is a big number')

Output

y is between 100 and 200

If we use elif, an else statement is not required. This will may result in neither the code associated with if nor elif running, however.

Python

y = 400

if y < 100:

   print('y is less than 100')

elif y < 200:

   print('y is between 100 and 200')

# nothing prints here

Regardless of if there is an else statement or not, we can also include as many elif conditions as we want.

Python

favorite_movie = 'Indiana Jones'

if favorite_movie == 'Batman':
   print("I'm Batman.")

elif favorite_movie == 'Lord of the Rings':
   print("And my axe!!")

elif favorite_movie == 'Indiana Jones':
   print('That belongs in a museum!!')

elif favorite_movie == 'The Matrix':
   print('whoa')

else:
   print('No quotes available :(')

Output

That belongs in a museum!!

Using data types in conditionals

We can also use data types as conditions in if statements.

Python

should_be_num = 'banana'

if type(should_be_num)!=int or type(should_be_num)!=float:
   print('need input to be a number')

else:
   print(10**should_be_num)

Output

need input to be a number

Question: Conditionals

Write code that will prints the square root of x if x is larger than 20 and 0 if x is less than 0.

Hint 1: Taking the square root of a number is the same is raising it to the power of 0.5.

Bonus: Print an error message if x is a string or a boolean.

Python

### Your code here:

Solution

Python

x = 22

if type(x) == str:
   print('x needs to be a string.')

elif x >20:
   print(x**1/2)

elif x < 0:
   print(0)

Nested conditionals

We can also put if statements inside of other if statements. Many times, these are unnecesary and can be replaced by elif statements, but not always.

Make sure to add more indentation for the second layer of if statements.

Python

n = 22222 # some integer

if n < 0: # if n is negative

   if n % 2 == 0:
      print("n is a negative even integer")

   elif n % 2 == 1:
      print("n is a negative odd integer")

   else:
      print("n is a negative number")

elif n > 0: #

   if n % 2 == 0:
      print("n is a positive even integer")

   elif n % 2 == 1:
      print("n is a positive odd integer")

   else:
      print("n is a positive number")

else:

   print("n is zero")

Question

Create a list called my_list with four items in it. Create a variable called x with some value.

Create a series of conditionals that check to see if the value in x is in my_list.

If this is the case, print out different text depending on what index the item is at in the list.

0. “From zero to hero”

1. “One is the loneliest number.”

2. “Two’s company.”

3. “Three’s a crowd.”

Resources

• Software Carpentry