If/Elif/Else

In the previous lesson we saw that we can make decisions in code just like we do in real life

Still, how could I implement the following story?

We have a rocket that launches from the ground. Depending on how many seconds are left until launch, we want to prepare for the big event

If we have 100 seconds left, we start all onboard computers

If we have 60 seconds left, we check the connection with the control tower

If we have 20 seconds left, we start the secondary engines

If we have 10 seconds left, we start the main engines

If we have less than 10 seconds left, we check whether all components are running properly:  If even the smallest error appears anywhere, we cancel the launch

Here we can see that we have 2 conditions inside one another

seconds = 100  # number of seconds
error_detected = False # no error in the beginning

if seconds == 100:
    print("Starting all onboard computers")
elif seconds == 60:
    print("Checking connection with the control tower")
elif seconds == 20:
    print("Starting secondary engines")
elif seconds == 10:
    print("Starting the main engines")
elif seconds < 10:  # the last 10 seconds
    if error_detected == True:
        print("Error detected. Canceling the mission")
    else:
        print("No error detected. Taking off...")
else: # unknown number of seconds
    print(f"{seconds} seconds has no effect")

An explanation of this code would be very complicated. Run the code as it is, then change the two variables. Change seconds to 60, 20, 10, then 9. See how the program’s output changes. Also change error_detected to True and see how the program reacts. The code may look intimidating, but in the end it’s just a story. Feel free to play around

So, we can see how elif is nothing more than another if. Still, why did we use if-elif-elif-elif...else? Why chain them?

If we test the same variable for multiple cases, we chain them with if-elif-else. The program runs from top to bottom. At the first if/elif whose condition is true, it enters and runs the code in that block, and then it exits completely

seconds = 60  # number of seconds
error_detected = False # no error in the beginning

if seconds == 100:
    print("Starting all onboard computers")
elif seconds == 60:
    # the program will enter here and display this print
    print("Checking connection with the control tower")
    # after displaying it, it jumps out of the chain
elif seconds == 20:
    print("Starting secondary engines")
elif seconds == 10:
    print("Starting the main engines")
elif seconds < 10:  # the last 10 seconds
    if error_detected == True:
        print("Error detected. Canceling the mission")
    else:
        print("No error detected. Taking off...")
else: # unknown number of seconds
    print(f"{seconds} seconds has no effect")

print("The program jumps straight here and no longer checks the rest of the elif statements or the else")

We can see the output

Checking connection with the control tower
The program jumps straight here and no longer checks the rest of the elif statements or the else

But why didn't we write the program like this?

seconds = 60  # number of seconds
error_detected = False # no error in the beginning

if seconds == 100:
    print("Starting all onboard computers")
if seconds == 60:
    print("Checking connection with the control tower")
if seconds == 20:
    print("Starting secondary engines")
if seconds == 10:
    print("Starting the main engines")
if seconds < 10:  # the last 10 seconds
    if error_detected == True:
        print("Error detected. Canceling the mission")
    else:
        print("No error detected. Taking off...")
else: # unknown number of seconds
    print(f"{seconds} seconds has no effect")

We see an incorrect output

Checking connection with the control tower
60 seconds has no effect

We see that on the last line it says 60 seconds has no effect, but that’s not true, since Checking connection with the control tower was printed exactly because seconds = 60 :)

The full explanation is complex, but as a general rule: if we test the same variable for multiple cases, we chain if-elif-else

Mission: Oxygen Level Monitor

The station constantly checks its oxygen level (in percent) and whether there is still crew_aboard. Based on the oxygen level, print one status line using a chained if / elif / else:

• oxygen is 80 or more → Oxygen nominal

• oxygen is 50 to 79 → Oxygen low - conserve power

• oxygen is 20 to 49 → Oxygen critical - seal the bay

• below 20 → it's an emergency. Here, check crew_aboard with a nested if:

  • if the crew is still aboard → Emergency - evacuate now
  • otherwise → We have no crew aboard, so venting the bay is safe

• oxygen: oxygen level in percent

• crew_aboard: if the crew is still on the station

Example

With oxygen = 65, your program should print

Oxygen low - conserve power

Now set oxygen = 12 and crew_aboard = True and run again

Emergency - evacuate now

And with oxygen = 12 and crew_aboard = False

We have no crew aboard, so venting the bay is safe