← Starting Henceforth

Episode 8

Loops

Teach the dog to repeat — count up, count down, compound, and ask a question every step.

2:42 · Waltz of the Flowers (Nutcracker, Op. 71)

Watch here · code in the app

Henceforth is the real terminal — one purchase ($9.99) for iPhone, iPad and Mac. Type the commands below into the app as you watch.

App Store ↗Mac App Store ↗

Code along

Tap to copy, then type it into Henceforth.

What you’ll learn

  • ✓ do … loop counts between two numbers, and they go on the stack first: 11 1 do runs i from 1 up to 10 — it stops just before the limit.
  • ✓ begin … until loops until a test comes true — the counter lives on the stack the whole way down, with dup to peek and 1 - to step it.
  • ✓ An if inside a loop asks a question every step: 101 0 do i prime? if i . then loop prints every prime under a hundred — a loop, a decision, and the stack.
Transcript
  • every word so far runs once. what if i want it again — and again?
  • a loop. i write the steps once, and say how many.
  • first, two numbers land on the stack — eleven, then one.
  • do takes them both: start at one, stop just before eleven.
  • one, two, three … ten. ten loops.
  • and each turn, i drops on the stack — the dot prints it, then clears it off.
  • and when i don't know how many times?
  • ten goes on the stack — and stays there, counting.
  • dup copies it to print; 1 - takes one off the top.
  • until checks the top: is it zero yet?
  • watch the stack count itself down — ten to one.
  • loops do real work. watch money grow.
  • twelve percent a year is times 112, over 100 — the */ word.
  • 100 sats sit on the stack — the running total.
  • the same do loop — one to ten. ten years.
  • each year, */ grows the total on the stack. 100 becomes 304 — it triples.
  • and a decision, inside a loop?
  • prime? is already written — it asks if a number has any divisor.
  • this loop runs zero to a hundred.
  • each turn, i goes on the stack; prime? takes it, leaves a yes or no.
  • if yes — print it.
  • every prime under a hundred — a loop, asking a question of the stack.
  • why teach the dog to repeat?
  • bitcoin script loops too — counting, checking, until a rule is met.
  • more in episode ten.
  • henceforth.
← Ep 7