Conditions

So far the program ran every line in turn. A condition lets it choose: do something, or skip it.

If

if checks a condition. The lines below it, indented by four spaces, run only when the condition is true.

temperature = 25
if temperature > 20:
    say("It is warm today")

It is warm today

The colon at the end of the line opens a block, and the indent shows what belongs to it. You can compare with > greater, < less, >= greater or equal, <= less or equal, == equal and != not equal.

One = puts a value into a name; two == compare. They are different things.

Else, else if

When the condition is false, the block under else runs. Between them you may put as many else if blocks as you like: Yu takes the first one whose condition is true.

score = 7
if score >= 10:
    say("Excellent")
else if score >= 6:
    say("Good")
else:
    say("Almost there")

Good

And, or, not

Conditions can be combined. and is true when both parts are true, or when at least one is, and not turns the answer around.

age = 12
if age > 6 and age < 18:
    say("Goes to school")
say("Not grown up yet:", not (age >= 18))

Goes to school
Not grown up yet: true

True, false, nothing

A comparison gives one of two values: true or false. There is also nothing, which is how Yu says that there is no value.

rain = false
say("Rain:", rain)
found = nothing
say("Found:", found)

Rain: false
Found: nothing

Try it yourself

1. Check whether a number is even. A hint: an even number divides by 2 with no remainder.

number_to_check = 10
if number_to_check % 2 == 0:
    say("Even")
else:
    say("Odd")

Even

2. A ticket costs 50, and children under 12 go free. Print the price for the age 10.

age = 10
if age < 12:
    say("Price: 0")
else:
    say("Price: 50")

Price: 0