For Loops and While Loops

What if you wanted to place ten boxes in a row? Or a hundred stars scattered across the sky? Writing the same code over and over would take forever — and one typo ruins everything.

A loop lets you repeat code automatically. Python gives you two kinds:

A for loop runs a block of code once for each item in a sequence.
A while loop keeps running as long as a condition stays true.

Four Boxes the Hard Way

Run the cell below. Four boxes appear in a row. Each box requires three nearly identical lines of code. Imagine having to do this for twenty boxes!

import scene3d

scene = scene3d.Scene()
scene.set_sky('#1a1a2e')
scene.set_ground(length=12, width=8)

box1 = scene3d.Shapes.Box(width=1, height=1, depth=1)
box1.set_color('#e94560')
box1.set_position(-3, 0.5, 0)
scene.add(box1)

box2 = scene3d.Shapes.Box(width=1, height=1, depth=1)
box2.set_color('#e94560')
box2.set_position(-1, 0.5, 0)
scene.add(box2)

box3 = scene3d.Shapes.Box(width=1, height=1, depth=1)
box3.set_color('#e94560')
box3.set_position(1, 0.5, 0)
scene.add(box3)

box4 = scene3d.Shapes.Box(width=1, height=1, depth=1)
box4.set_color('#e94560')
box4.set_position(3, 0.5, 0)
scene.add(box4)

Four Boxes the Smart Way

A for loop does the same job in just a few lines. Run the cell below — you’ll get the exact same four boxes.

import scene3d

scene = scene3d.Scene()
scene.set_sky('#1a1a2e')
scene.set_ground(length=12, width=8)

for i in range(4):
  box = scene3d.Shapes.Box(width=1, height=1, depth=1)
  box.set_color('#e94560')
  box.set_position(i * 2 - 3, 0.5, 0)
  scene.add(box)

How the `for` Loop Works

for i in range(4):
    box = scene3d.Shapes.Box(width=1, height=1, depth=1)
    box.set_position(i * 2 - 3, 0.5, 0)
    ...

Part	What it does
`for`	The keyword that starts the loop
`i`	A variable that takes each number from `range`, one at a time
`range(4)`	Produces the numbers 0, 1, 2, 3 — four values total
`:`	Marks the end of the `for` line
Indented body	The code that repeats — one run per value in `range`

On the first pass, i is 0. Second: i is 1. And so on through 3. The expression i * 2 - 3 gives x-positions of -3, -1, 1, 3 — spacing the boxes evenly.

Indentation matters!

Just like with if statements, everything indented under the for line is inside the loop. Return to the left margin and you’re outside again.

{ “question_type”: “multiple_choice”, “question”: “What values does range(4) produce?”, “options”: [ { “key”: “a”, “text”: “1, 2, 3, 4” }, { “key”: “b”, “text”: “0, 1, 2, 3” }, { “key”: “c”, “text”: “0, 1, 2, 3, 4” }, { “key”: “d”, “text”: “4, 3, 2, 1” } ], “answer”: “b”, “submitted_answer”: “” }

{ “question_type”: “true_false”, “question”: “If you write ‘for i in range(3):’, the loop body will run exactly 3 times.”, “answer”: “True”, “submitted_answer”: “” }

Using `i` to Create Variety

The loop variable i isn’t just a counter — you can use its value to make each object different. The scene below uses i to control both the height and the color of each box, creating a staircase.

import scene3d

scene = scene3d.Scene()
scene.set_sky('#0f3460')
scene.set_ground(length=15, width=8)

colors = ['#e94560', '#f5a623', '#44cc88', '#4488ff', '#cc44ff']

for i in range(5):
  box_height = i + 1
  box = scene3d.Shapes.Box(width=1.2, height=box_height, depth=1.2)
  box.set_color(colors[i])
  box.set_position(i * 2.5 - 5, box_height / 2, 0)
  scene.add(box)

{ “question_type”: “multiple_choice”, “question”: “In the staircase example, what is the value of i during the very first pass through the loop?”, “options”: [ { “key”: “a”, “text”: “1” }, { “key”: “b”, “text”: “-1” }, { “key”: “c”, “text”: “0” }, { “key”: “d”, “text”: “5” } ], “answer”: “c”, “submitted_answer”: “” }

`while` Loops — Repeat Until a Condition Changes

A for loop is great when you know exactly how many times to repeat. But sometimes you want to keep going until something happens — and you don’t know in advance how many steps that takes.

That’s what a while loop is for:

while condition:
    # runs as long as condition is True

The scene below builds a tower by stacking boxes. The loop keeps adding a box while the current height is less than 5 — and stops the moment that’s no longer true.

Hint: Use the scroll wheel to zoom out and see the full tower.

import scene3d

scene = scene3d.Scene()
scene.set_sky('#87CEEB')
scene.set_ground(length=12, width=12)

height = 0.5
while height < 5:
  box = scene3d.Shapes.Box(width=1.5, height=1.0, depth=1.5)
  box.set_color('#cc4400')
  box.set_position(0, height, 0)
  scene.add(box)
  height += 1.0

How the `while` Loop Works

height = 0.5
while height < 5:
    ...
    height += 1.0

Step	What happens
1	Python checks: is `height < 5`?
2	If True → run the indented body
3	After the body, go back to step 1
4	If False → exit the loop and continue

Warning — Infinite Loops! If the condition never becomes False, the loop runs forever and your program hangs. Always make sure something inside the loop changes the condition. Here, height += 1.0 keeps growing height until it reaches 5.

`for` vs `while` — which one to use?

Use `for` when…	Use `while` when…
You know exactly how many times to repeat	You repeat until a condition changes
Stepping through a fixed sequence	The number of repetitions isn’t known upfront
`range(n)` fits naturally	You’re waiting for something to happen

{ “question_type”: “true_false”, “question”: “A while loop automatically stops after running exactly 10 times, no matter what the condition says.”, “answer”: “False”, “submitted_answer”: “” }

{ “question_type”: “multiple_choice”, “question”: “Which loop is the most natural choice for placing exactly 8 spheres in a scene?”, “options”: [ { “key”: “a”, “text”: “A while loop” }, { “key”: “b”, “text”: “A for loop with range(8)” }, { “key”: “c”, “text”: “Neither — just write 8 separate lines of code” }, { “key”: “d”, “text”: “It doesn’t matter — for and while always do the same thing” } ], “answer”: “b”, “submitted_answer”: “” }

A Note on `do/while`

Some languages have a do/while loop that always runs the body at least once before checking the condition. Python doesn’t have this keyword, but you can get the same effect using while True: with break:

while True:
    # this always runs at least once
    count += 1
    if count >= 5:
        break   # exit the loop immediately

break exits the loop right away, jumping to the first line after it. while True: would run forever on its own — break is how you decide exactly when to stop.

Try It Yourself

Use the slider to choose how many spheres appear in the scene. The for loop adapts automatically — change the count, rerun the cell, and see the result.

import scene3d

COUNT = 5 #@param {type:"slider", min:1, max:10, step:1}

scene = scene3d.Scene()
scene.set_sky('#0f3460')
scene.set_ground(length=25, width=8)

colors = ['#e94560', '#f5a623', '#4488ff', '#44cc88', '#cc44ff',
      '#ff44cc', '#44ffcc', '#ffcc44', '#ff8844', '#88ff44']

spacing = 20 / COUNT

for i in range(COUNT):
  sphere = scene3d.Shapes.Sphere(diameter=1.2, segments=16)
  sphere.set_color(colors[i % len(colors)])
  sphere.set_position(i * spacing - 10 + spacing / 2, 0.7, 0)
  scene.add(sphere)

Think of a real scene you’d like to build — a forest of trees, a city block of buildings, a night sky full of stars, a row of traffic cones.

Which kind of loop would you use, and why? What code would go inside the loop? Write a short description of your scene and how the loop helps build it.

Four Boxes the Hard Way

Four Boxes the Smart Way

How the for Loop Works

Indentation matters!

Using i to Create Variety

while Loops — Repeat Until a Condition Changes

How the while Loop Works

for vs while — which one to use?

A Note on do/while