Building Blocks of All Programs

Every program ever written — games, apps, websites — is built from just 3 ideas.

1️⃣

Sequence — Do things in order

A computer follows steps one at a time, in order. Like a recipe — you can't frost a cake before you bake it.

2️⃣

Iteration — Repeat an action

Instead of writing the same step 100 times, a loop does it for you. Run one lap, then do it again and again.

3️⃣

Decision — Choose a path

Programs ask questions and take different actions depending on the answer. Is it raining? Bring an umbrella!

Click Next to explore each one interactively.

Sequence

do things in order

A computer follows steps one at a time, in order. You can't bake a cake before you mix the batter!

🎂

Press Run to start cooking!

Crack an egg

→

Mix the batter

→

Bake the cake

Press Run to start cooking!

+ show code (Python)

crack_an_egg()
mix_the_batter()
bake_the_cake()

# Each line runs after the one above it.
# You can't bake before you mix!

Iteration

same thing, over and over

Run one lap — then do it again, and again. The same action repeats 5 times. That's a loop!

1

2

3

4

5

Press "Run loop" to start!

+ show code (Python)

for lap in range(5):
    run_one_lap()

# run_one_lap() runs 5 times.
# Same action, repeated by the loop.
# Change range(5) to range(100) —
# now it runs 100 times. You only wrote it once!

Decision

choose a path

The computer checks a condition and takes a different action depending on the answer. Pick the weather below to see what happens!

Is it raining?

🌂

Bring umbrella

😎

Wear sunglasses

Choose the weather to see what happens

+ show code (Python)

raining = check_weather()

if raining:
    bring_umbrella()
else:
    wear_sunglasses()

# The program checks the condition,
# then runs only ONE of the two paths.