Conditional statements
Contents
Conditional statements¶
Most of the time when we are programming we are asking the computer to perform certain tasks if certain conditions are met. For example:
If a person in the dataset is older than 30, then print out their name
If a tweet contains a specific phrase, then automatically retweet it
If then say “Congratulations!”
In the second part of this week we will learn about conditional statements (if, elif, else), comparison operators (>, <, ==), and Boolean values (True, False).
A simple conditional statment¶
Here is an example. A messenger arrives at a crossroads and has to make a decision about which way to go. We would like them to turn left. This is what the code could look like:
direction = 'left'
if direction == 'left':
print("Turns left")
Turns left
There are two important Python elements presented in the code above: a comparison and a conditional statement. We compared whether the variable direction is equal to the value left. Then we printed Turns left if this condition was True.
Conditional statements are written in a similar way to for loops. The if statement checks to see whether the variable value is True or False. If a True is returned, then we execute the indented command below.
Alternatively, the messenger might decide to turn right.
direction = 'right'
if direction == 'left':
print("Turns left")
This time the first statement (if direction == 'left':) was ignored because the value returned was False. Nothing was returned.
elif¶
We can add more complexity in a conditional statement by using an elif statement. If we wanted to add an option for turning right we could write:
direction = 'right'
if direction == 'left':
print("Turns left")
elif direction == 'right':
print("Turns right")
Turns right
Note
It is good practice to use elif statements beneath if statements to explicitly state that the cases are mutually exclusive (i.e. if we turn left, we cannot turn right).
What happens if our messenger wanted to go straight?
direction = 'straight'
if direction == 'left':
print("Turns left")
elif direction == 'right':
print("Turns right")
Both statements return a False value, no commands are executed, and nothing is returned.
else¶
To catch these alternative options, we can add an else statement.
direction = 'straight'
if direction == 'left':
print("Turns left")
elif direction == 'right':
print("Turns right")
else:
print("Stay straight")
Stay straight
The command beneath the else statement will be executed if all if and elif statements return False.
Note
The else clause is optional. If it is present, there can be only one and it must be specified last.
Tip
Along with the == operator, there are a few more options to know about:
>: greater than
<: less than
==: equal to
!=: does not equal
>=: greater than or equal to
<=: less than or equal to
Boolean alegbra¶
Our conditional statements are not limited to strings, we can also use numbers. In addition, we might want to include logical operators such as and, or, and not into our conditions. We can do this as follows:
temperature = 60
if (temperature > 90) or (temperature < 30):
print("Stay at home")
else:
print("Cross the pass and deliver mail!")
Cross the pass and deliver mail!
Here we combined two conditional statements using the or operator to determine whether our messenger should go and deliver mail.
Note
An alternative for and is &. Likewise, or can be replaced with |
Keeping track¶
We can use boolean conditional statements and += operator to keep track of values. += adds another value to the current value of the variable. Let’s say our messenger wanted to buy an apple and an orange but did not know what fruit the shops had in stock.
fruit_stand = ['lemons', 'pears']
food = 0
if fruit_stand.count('apples') > 0:
food += 1
if fruit_stand.count('oranges') > 0:
food += 1
else:
pass
food
0
Note
This time our statements are not mutually exclusive (i.e. it is possible that we can find both apples and pears) so it is OK to use two if statements.
Unfortunately, there were no apples and oranges for sale so our messenger still has no fruit to eat.
corner_shop = ['apples', 'oranges', 'pears', 'melons', 'grapefruits']
items = 0
if corner_shop.count('apples') > 0:
items += 1
if corner_shop.count('oranges') > 0:
items += 1
else:
pass
items
2
Luckily, the corner shop had apples and oranges so our messenger now has two pieces of fruit to eat.
Combining for loops and conditional statements¶
Conditional statements are often combined with for loops. Let’s say we wanted to calculate how many days the messenger could cross the pass and deliver mail over a 50-day period.
# Import package
import numpy as np
# Make a new array of random temperatures between 10 and 100 for 50 days
temperatures = np.random.randint(10, 100, 50)
# Set days equal to zero
days = 0
# Loop over every day
for i in temperatures:
if (i < 90) and (i > 30):
days += 1
else:
pass
print(f"The messenger delivered mail on {days} days")
The messenger delivered mail on 30 days