Robot Decisions

Every day you make decisions. If it is raining, you take an umbrella. If it is sunny, you wear sunglasses. Programs can make decisions too, and that makes them feel clever. Today you will give Python a brain that can choose.

What you need

• Python and IDLE.
• A new file called decisions.py.

The if statement

An if statement runs some lines only when a question has the answer “yes”.

battery = 20
if battery < 30:
    print("Battery low. Time to charge!")
print("The end.")

Battery low. Time to charge!
The end.

The question after if is called a condition. Here it is battery < 30, which asks “is battery less than 30?” The answer is yes, so Python runs the indented line. The last line is not indented, so it runs every time.

Change battery = 20 to battery = 80 and run it again. Now the condition is “no”, so Python skips the indented line and only shows The end.

Here are the comparison signs you can use:

else: the other choice

Add else to say what to do when the answer is “no”:

battery = 80
if battery < 30:
    print("Battery low. Time to charge!")
else:
    print("Plenty of power. Let's play!")

Plenty of power. Let's play!

Here is the same idea as a flowchart:

elif: more than two choices

What if there are three or more choices? Use elif, which is short for “else if”. Python checks the questions from the top and runs only the first one that is “yes”.

battery = 55
if battery > 70:
    print("I feel fantastic!")
elif battery > 30:
    print("I feel okay.")
else:
    print("I feel sleepy.")

I feel okay.

With battery = 55, the first question (battery > 70) is “no”, so Python moves on. The second (battery > 30) is “yes”, so it prints I feel okay. and skips the rest.

Robot mood: ask the user

Now let us combine decisions with input. This program asks for a battery number and decides the robot’s mood. Do not forget int( ) from the last lesson!

battery = int(input("Battery level from 0 to 100: "))
if battery > 70:
    print("Beep boop! I feel fantastic!")
elif battery > 30:
    print("I feel okay.")
else:
    print("Yawn... I feel sleepy.")

Battery level from 0 to 100: 85
Beep boop! I feel fantastic!

Try it again with 50, then with 10. You should see the other two moods.

A tiny quiz

Strings can be compared with == too. Capital letters count, so "Cat" and "cat" are different. Adding .lower() turns the answer into small letters first, so it does not matter how the player types it:

answer = input("What is the capital city of France? ")
if answer.lower() == "paris":
    print("Correct! Well done!")
else:
    print("Not quite. It is Paris.")

What is the capital city of France? Paris
Correct! Well done!

You can also use the words and and or to combine two questions into one:

coins = 5
has_ticket = True
if coins >= 3 and has_ticket:
    print("You can ride the robot train!")
else:
    print("Sorry, no ride today.")

You can ride the robot train!

With and, both parts must be “yes”. With or, only one of them has to be.

Challenges

number = int(input("Pick a number: "))
if number > 10:
    print("Big!")
else:
    print("Small!")

temperature = int(input("What is the temperature? "))
if temperature > 25:
    print("Wear a hat!")
elif temperature > 10:
    print("Wear a T-shirt!")
else:
    print("Wear a coat!")

What you learned

• An if statement runs lines only when its condition is “yes”.
• else runs when the answer is “no”, and elif lets you check more questions in order.
• == asks “are these equal?”, while = stores a value in a variable.
• The lines inside a choice are indented.
• and and or combine conditions, and .lower() makes text comparisons easier.