If/Else

In real life, we are often faced with a decision: either I do this, or I do that. If it’s less than 15 degrees outside, I need a sweater, otherwise a t-shirt is enough. If I’m sleepy, I go to sleep, otherwise I program :) So we notice 2 branches:

if yes

if not (else)

In programming, we say

if it’s cold (the temperature is less than 15 degrees): take a sweater

else: a t-shirt is enough

temperature = 14
if temperature < 15:
    print("Take a sweater")
else:
    print("A t-shirt is enough")

This is like a story: if the temperature outside is less than 15 degrees Celsius, I need a sweater

Otherwise, a t-shirt is enough

After if and else, we put the colon :

Then everything we write after this : must be indented. Why? So Python knows which lines of code belong to the if block and which do not, and the same for else

temperature = 14
if temperature < 15:
print("Take a sweater")
else:
print("A t-shirt is enough")

Run the code. You can see the error. By the way, try to read the errors because they tell you exactly what you did wrong

The < sign means less than. If the temperature is less than 15: 14, 10, 0, etc

Similarly, we have:

<= means less than or equal to 15: 15, 14, 10, 0, -12

> means greater than 15: 16, 20, 145

>= means greater than or equal to 15: 15, 16, 20, 1000

== means equal. Do not confuse it with = which is used to assign a value to a variable

x = 4
if x = 4:
    print("Boo")

This will generate an error. The correct way is

x = 4
if x == 4:
    print("Boo")

By the way, we don’t always need an else after if. Let’s think about a car. If we turn the key, the engine starts, otherwise nothing happens. Here, we don’t necessarily need an else

However, there are cases where we do need else. If I get at least 50% on an exam, I pass, otherwise I don’t. Here we see that there are two possibilities: either you pass or you don’t. It’s not like you can say 'if I get 50% I pass, if not, nothing happens'. What actually happens is you fail the exam, meaning there is a consequence

username = "Tommy Vercetti"
is_user_online = True

if is_user_online == True:
    print(f"{username} is playing GTA Vice City")
else:
    print(f"{username} is offline")

True and False are pretty straightforward. If the user is online, we enter the if block and will NOT enter the else. We can see that the code in the else block does not execute. Change is_user_online to False and run the code. What do you see?

username = "Tommy Vercetti"
is_user_online = False

if is_user_online == True:
    print(f"{username} is playing GTA Vice City")
else:
    print(f"{username} is offline")

Here we see that the user is offline because is_user_online = False, which means we won’t enter the if block, since we only enter if the condition is true. Since our condition is false, we enter the else block

Mission: Reactor Watch

The reactor reports its temperature in degrees Celsius. Write an if / else that checks it:

• if temperature is greater than 1000 → print Danger: reactor at, the temperature, then degrees - shutting down (for temperature = 1200 that is Danger: reactor at 1200 degrees - shutting down)
• otherwise → print Reactor stable at, the temperature, then degrees (for temperature = 800 that is Reactor stable at 800 degrees)

Create a variable named temperature that stores the temperature, pick any value you like. Descriptive names matter: if someone sees a variable called x, they'll immediately wonder "what is x? Who is x?"

Example

With temperature = 1200, your program should print

Danger: reactor at 1200 degrees - shutting down

Now set temperature = 800 and run again

Reactor stable at 800 degrees

With temperature = 1000, your program should print

Reactor stable at 1000 degrees

Why is the reactor stable at temperature = 1000?