Reference

How the pen works

Drawing with code is just a pen that goes forward and turns. Once you can picture where the pen is and which way it faces, you can read a loop and SEE the shape before you run it. Here’s the model. Open the drawing lab →

• The pen has a place and a heading

  Think of a pen (a “turtle”) sitting on the paper, pointing in some direction. Everything you draw is relative to where it is and which way it faces right now.

• forward draws, turn rotates

  forward(50) slides the pen 50 steps the way it faces, leaving a line. turn(90) spins the pen in place by 90° — it draws nothing on its own, it just changes the direction of the NEXT forward. A turn goes clockwise, like a clock’s hand.

• A loop repeats to build a shape

  for i in 1...4 { … } with forward(60) and turn(90) on their own lines inside does “forward, turn” four times. Four equal sides and four right-angle turns close into a square. The number of repeats is the number of sides.

• The turn is the corner

  A gentler turn makes a wider corner and a rounder shape: 90° → square, 72° → pentagon, 60° → hexagon. The turns of a closed shape always add up to one full 360° trip around.

• A star is a turn that overshoots

  Turn MORE than the polygon angle and the pen skips across the middle: five sides with a 144° turn (not 72°) cross into a five-pointed star instead of a pentagon.

• penUp lifts the pen (a gap)

  penUp() picks the pen off the paper so the next forward MOVES without drawing; penDown() puts it back. That is how you make gaps — dashes, or separate pieces.

• Change something each loop

  If a length GROWS each time (d = d + 10), the sides get longer and the path winds outward into a spiral. Draw a whole shape many times, turning a little between each, and you get a flower.

The pen commands

for i in 1...4 {
  forward(60)
  turn(90)
}

• forward(n)Move n steps the way the pen faces, drawing a line (if the pen is down).
• turn(deg)Rotate the pen in place by deg degrees, clockwise like a clock’s hand. Draws nothing — it aims the next forward.
• penUp()Lift the pen: the next forward moves but leaves no line (a gap).
• penDown()Put the pen back down so forward draws again.
• for i in 1...n { … }Repeat the body n times — the heart of drawing a shape with a loop.

The trick to reading a pen program is to trace the loop one step at a time: forward moves, turn aims, repeat. The count tells you the sides; the turn tells you the corner. Read a pen program →