# Conditions#

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

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 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 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:
elif percentage > 80:
elif percentage > 60:
elif percentag > 50:
else:

print("You scored", number_of_correct_points, "out of", number_of_total_points)

You scored 23 out of 30

# it's 12:15