03. The three building blocks

The previous chapter showed you flowcharts with actions, questions, and loops. This chapter zooms out and gives those three patterns their proper names. Once you can see them, every program in the rest of the book will be made of them, and only them.

Yes — every program. There is no fourth one. People have studied this for sixty years and the answer always comes back the same: every program is built from three patterns.

The three patterns

The names are:

1. Sequence — do this, then this, then this.
2. Decision — if something is true, do A; otherwise do B.
3. Repetition — do something over and over until a condition says stop.

That is the whole list. Reread it. The rest of the chapter shows what each one looks like, with an everyday example.

1. Sequence

A sequence is a list of actions in order. The flowchart is a straight line down the page; each box happens once.

You used sequence in the brush-your-teeth flowchart in Chapter 02. It is the simplest pattern and the easiest to write. It is also the only pattern in this book's first project — the ASCII Name Banner — which is nothing but a long sequence of print calls.

You will write this in code in Chapter 7. A sequence in Lua is just one line after another:

print("Put bread in toaster")
print("Push the lever down")
print("Wait two minutes")
print("Take the bread out")

That is literally a sequence flowchart, written down.

2. Decision

A decision asks a yes/no question and runs different actions depending on the answer. The flowchart uses a diamond with two arrows leaving it.

A decision is what makes a program react. Without decisions, a program runs the same way every time, no matter what happens. Add one decision and it can take different paths on different days.

You used decisions in the what-to-watch and what-to-wear homework. Real programs use them for every kind of "if". If the player's score is more than 100, give them a bonus. If the file does not exist, create it. If the touch came from a Player, give them a coin.

You will write this in code in Chapter 16. A decision in Lua looks like this:

if door_is_locked then
    find_the_key()
    unlock_it()
else
    open_it()
end

Same shape: question on top, two paths, joined at the end.

3. Repetition

Repetition is doing the same kind of thing over and over until a condition says stop. The flowchart uses an arrow pointing back to a step you have already passed.

Every loop has the same parts:

• a starting condition (count is 0),
• a question (is count 10?),
• the work done each round (do a push-up),
• something that moves toward the stop (add 1 to count).

If a loop is missing the moves toward the stop part, it never ends. That is a bug called an infinite loop, and it is the most common bug in early programs. If your program seems frozen, you probably wrote one. (Use Ctrl + C in the terminal to stop it.)

You used repetition in the press-ups challenge in Chapter 02. Real programs use loops to go through every item in a list, draw every frame of an animation, ask the user again until they type a valid answer.

You will write this in code in Chapter 18. A loop in Lua looks like this:

for i = 1, 10 do
    print("Push-up " .. i)
end

Ten lines of output from three lines of code. Loops are how programs do work too big to write out by hand.

Mixing the three

A real program is the three patterns nested inside each other: sequences holding decisions, decisions holding loops, loops holding other loops. The Number Guessing Game from Part 3 has all three:

• The whole program is a sequence: pick a number, then play, then announce the winner.
• The play part is a loop: ask for guesses until one is correct.
• Each round of the loop has a decision: was the guess too high, too low, or correct?

Once you can spot the three patterns, you can read any program by asking, at each line, which one is this?. Most lines are sequence. A line with if is the top of a decision. A line with while or for is the top of a loop. That is most of programming.

Homework

Still paper — the last paper homework before you install Lua.

Stuck or finished? Open the homework solutions page.