← Starting Henceforth
Episode 5
Moving numbers
The shuffle words change where numbers sit — never what they are.
1:35 · Brandenburg No. 3 · Allegro (BWV 1048 i)
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.
Code along
Tap to copy, then type it into Henceforth.
What you’ll learn
- ✓ The stack only ever hands you its top — so to use a number twice, in a new order, or from underneath, you reach for a mover: dup, swap, over, rot, drop.
- ✓ Movers change where numbers sit, never what they are. swap flips the top two, so 10 4 swap - leaves 4 - 10 = -6 — same numbers, new order.
- ✓ A mover still needs operands — swap on a one-item stack hits the floor (underflow). Bitcoin Script has the same move, OP_DUP, which you'll wield in episode ten.
Transcript
- last time — you copied the top.
- that was dup. it made square.
- dup's got a family — words that just move numbers.
- because the stack only ever hands you the top.
- dup — copy the top. you've met this one.
- what if they're backwards?
- i want 4 minus 10 — but 4's on top.
- swap — flip the top two.
- minus six. same numbers, new order.
- and the one underneath?
- over — copy the second one up top.
- buried deeper?
- rot — bring the bottom one up.
- and when you don't need it?
- drop — throw the top away. pretty simple, huh?
- but they still need numbers to move.
- give swap one — it hits the floor.
- movers need a stocked stack.
- watch — none of them did math.
- they only move numbers. never change what they are.
- bitcoin has this same move — op_dup.
- you'll wield it in episode ten.
- henceforth.