Conditions#

The if statement can be used to execute code conditionally. That means only if an expression is true.

See also:

Let’s take a look at some expressions first

3 > 4

False

a = 3
b = 4

a > b

False

a < b

True

# not equal
a != b

True

# equal
a == b

False

# Note: Do not mix it up with this:
a = b

The if statement#

After using if in combination with an expression, you need to put a colon : and following code 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 comparisons:

c = 10

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

C is between 4 and 20.

Or combine expressions using and and or:

if c > 4 and c < 10:
    print("C is between 4 and 10.")

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!

if "cells" in filename:
    print("There are cells in the file!")

There are cells in the file!

The if-else statement#

If you have two different pieces 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 95 >= percentage > 80:
    grade = 2
elif 80 >= percentage > 60:
    grade = 3
elif 60 >= percentage > 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