If statements

An if statement runs a block of code if a specific condition is met, we can use conditional operators to create these conditions:

• Equal to: ==
• Not equal to: !=
• Less than: <
• Greater than: >
• Greater than or equal to: >=
• Less than or equal to: <=

height = 4.1

if height < 4.5:
	print("You are not allowed on this ride.")

You have to indent the code, using tab, to make it part of the if statement. Indentation is how python creates blocks of code. Only the block of code in the if statement will run if it’s condition is true.

# All part of the same block
age = 32
name = "John Walker"
rank = "Private"
clearance = False

if rank == "Sergeant":
    # A new block inside the if statement
    clearance = True
    print("Welcome Sergeant")

# Back to the original block
permission = clearance

It’s useful to check for multiple conditions, for that you would use the elif keyword, which checks a condition if the condition above it was false.

anniversary = 25
jubilee = ""

if anniversary == 25:
    jubilee = "silver"
elif anniversary == 40:
    jubilee = "ruby"
elif anniversary == 50:
    jubilee = "golden"
elif anniversary == 75:
    jubilee = "diamond"

print(f"Congratulations on your {jubilee} jubilee")

Note

Pay attention to the difference between `=` and `==`, `=` is for assigning variables while `==` is for checking equality.

If all conditions fail you may run a piece of code using else, which acts as a kind of default.

rank = "General"
clearance = 0

if rank == "General":
    clearance = 4
elif rank == "Colonel":
    clearance = 3
elif rank == "Major":
    clearance = 2
else:
    # If none of the above are true, the else block will run
    clearance = 1

if statements are run sequentially, starting from the top going down, if a condition is met then the if statement will exit. So if two conditions are checking for the same thing then only the first will execute.

    weight = 2000
    toll_fee = 0

    if weight >= 2000:
        # Exits here
        toll_fee = 500
    elif weight >= 2000:
        toll_fee = 1500

    print(toll_fee) # Output: 500

A condition is just an expression that can evaluate to True or False.

Combining conditions

We can combine conditions using and, which runs a block of code if all it’s conditions are true.

age = 10
height = 5.3

if age > 7 and height > 4.5:
	print("You are allowed on this ride")
else:
	print("Sorry you may not enter this ride")

Conversely, you can check if either of the conditions are true using or.

age = 10
height = 5.3

if age > 7 or height > 4.5:
	print("You are allowed on this ride")
else:
	print("Sorry you may not enter this ride")

A condition can be inverted using not.

is_raining = True

if not is_raining:
    print("Let's go for a walk")