Conditions

The if statement (a control flow tool), followed by an expression, can be used to execute code conditionally. That means only if that expression is True. For such expressions, logical operators from mathematics can be used.

See also:

• if…elif…else in Python

• Control Flow Tools

Let’s take a look at some expressions first:

a = 3
b = 4

# equal
a == b

False

# not equal
a != b

True

# greater
a > b

False

# less
a < b

True

# greater equal
a >= b

False

# less equal
a <= b

True

The if statement

After using if in combination with an expression, you need to put a colon :. The following code for execution must be indented:

if 3 < 4:
    print("Math is great.")

Math is great.

if 3 > 4:
    print("Math is weird.")

You can also write more sophisticated expressions using and and or:

c = 10

if (c > 4) and (c < 20):
    print("C is between 4 and 20.")

C is between 4 and 20.

If you want to check if an element is in an array, do it like this:

animals = ['cat', 'dog', 'mouse']

if 'cat' in animals:
    print('Our list of animals contains a cat')

Our list of animals contains a cat

You can also analyse strings. For example, check if they start or end with certain characters:

filename = "cells.tif"

if filename.endswith("tif"):
    print("The file is an image!")

The file is an image!

The if-else statement

If you have two different blocks of code that should be executed alternatively, use if-else:

quality_in_percent = 89

if quality_in_percent > 90:    
    print("Our quality is high enough.")
else:
    print("We need to improve our quality.")

We need to improve our quality.

The elif statement

For executing code depending on multiple conditions, use the elif-statement:

# determining the exam grade depending on 
number_of_correct_points = 23
number_of_total_points = 30

# compute percentage
percentage = number_of_correct_points / number_of_total_points * 100

if percentage > 95:
    grade = 1
elif percentage > 80:
    grade = 2
elif percentage > 60:
    grade = 3
elif percentag > 50:
    grade = 4
else:
    grade = 5
    
print("You scored", number_of_correct_points, "out of", number_of_total_points)
print("Your grade is", grade)

You scored 23 out of 30
Your grade is 3

Exercise

Write python code that prints out the daytime, e.g. “morning”, “noon”, “afternoon”, “evening”, and “night” depending on a given time.

# it's 12:15
time_hours = 12
time_minutes = 15