---
title: "Repeat with a loop"
subtitle: "Python Foundations with Turtle · Commands, loops and values · Lesson 2"
---

# Repeat with a loop

**Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}}

> **Key question:** Loops are how a program draws 100 tiles, sends 100 emails or reads 100 sensors without 100 lines of code.


:::teacher-note

**In one sentence:** Students see that the square they drew with eight lines can be written with three, using a loop, and find out how to change the count and the turn.

**Students should be able to say:** I can use a loop to repeat lines instead of copying them. I can say which lines belong to a loop. I can work out the turn for a shape with any number of sides.

**Width:** the unmarked parts are the floor every student does (predict, run, investigate and make). Typical adds the modify practice; wider adds the real-world contexts. They add contexts, not difficulty. Students work in the online tutorial alongside this sheet.

:::


## Predict

The line `for i in range(4):` is new. Read the code. What will the turtle draw?

```python
from turtle import *

for i in range(4):
    forward(100)
    right(90)
```

:::keep-together

**What will the turtle draw?**

☐ nothing, because i is never used

☐ one side, then a turn

☐ one long line

☐ a square

My prediction, and why:

{{lines-3}}

:::


:::teacher-note

**What happens:** A square. The two indented lines run 4 times: a side of 100, then a quarter turn, four times over. The turtle finishes where it started, facing right.

:::


## Run

Run the program in the tutorial. Was your prediction right? What was different?

{{lines-2}}

## Investigate

Compare the two programs. They draw the same square.

```python
# Without a loop
forward(100)
right(90)
forward(100)
right(90)
forward(100)
right(90)
forward(100)
right(90)

# With a loop
for i in range(4):
    forward(100)
    right(90)
```

:::keep-together

### Count the lines

**A square drawn one command at a time uses 8 lines of drawing code. A 20-sided shape drawn the same way would use how many lines?**

Answer: {{field-15}}

:::


:::teacher-note

**Answer:** 40

Each side needs 2 lines (forward and a turn), so 20 sides need 40.

:::


:::keep-together

### Spot the bug

**This was meant to draw a square. Instead the turtle draws one long line and turns once at the very end. Click the wrong line.**

```
 1  from turtle import *
 2  
 3  for i in range(4):
 4      forward(100)
 5  right(90)
```

Line: {{field-15}}

Why is it wrong?

{{lines-2}}

:::


:::teacher-note

**Answer:** line 5

right(90) is not indented, so it is not part of the loop. The loop draws four sides in a straight line, and the turn runs once afterwards. Indent it under the for line so both lines repeat.

:::


:::keep-together

### Follow the counter

**for i in range(4): makes a counter called i. It starts at 0 and goes up by 1 each time round the loop. Fill in the table for the square.**

{{rows 1}}

| pass | i | turned so far |
|---|---|---|
| 1 | 0 | 90 |
| 2 |   |   |
| 3 |   |   |
| 4 |   |   |

:::


:::teacher-note

**Answer:** 1, 180, 2, 270, 3, 360

i counts 0, 1, 2, 3: four passes, starting from 0. This square never uses i, but it is there, and later programs will use it. After four turns of 90 the turtle has turned 360 degrees, a full turn.

:::


:::typical

## Modify

Change the loop so it draws a triangle instead of a square. Run after each change.

Work in the tutorial. Plan first: what will you change, and how will you test it?

{{box-4}}

:::teacher-note

**One possible solution.** Other correct programs are fine; check that the program runs and does what the task asks.

```python
from turtle import *

for i in range(3):
    forward(100)
    right(120)
```

:::

:::


## Make

A loop works for any pattern, not only shapes. Find the part that repeats, write it once, and say how many times.

Draw a staircase with 5 steps, like the picture. When it works, change one number to make it 10 steps. Then try a pattern of your own: a zigzag, a row of battlements or a dashed line. Find the part that repeats first.

Next lesson adds variables. Then you put loops and variables to work on a whole program: a street of houses.

Work in the tutorial. Plan first: what will you change, and how will you test it?

{{box-4}}

:::teacher-note

**One possible solution.** Other correct programs are fine; check that the program runs and does what the task asks.

```python
from turtle import *

for i in range(5):
    left(90)
    forward(20)
    right(90)
    forward(20)
```

:::


## Exit ticket

:::keep-together

**A turtle draws a shape with 5 equal sides using a loop. How many times does the loop repeat, and how far does the turtle turn each time?**

{{lines-3}}

:::


:::teacher-note

**Answer:** 5 times, turning 72 degrees each time (360 divided by 5).

:::