Use the counter

The loop counter i changes every pass, so a line can grow without you typing each number.

Progress bars, animation frames and game levels all count. The loop counter is how code counts.

You have met i in every loop so far, but never used it. It is a counter, and it changes on every pass.

Predict: Read the code and say what it will do before you run anything. A wrong guess is fine: it shows you what to look for.
Run: Run it and compare the result with your prediction.
Investigate: Trace how each line of code connects to what you see.
Modify: Change the code on purpose and check the result.

PredictRead the code and say what it will do before you run anything. A wrong guess is fine: it shows you what to look for.

Predict

counter.py

from turtle import *

for i in range(30):
    forward(i * 5)
    right(90)

forward(i * 5) uses the counter to work out how far to go. Read the code. What will the turtle draw?

What will the turtle draw?a spiral that grows outwarda squarea circlenothing: i is never stored

RunRun it and compare the result with your prediction.

Run

Run it and compare it with your prediction.

Here is what each line does.

Line	What it does	Why
`for i in range(30):`	Runs 30 times and makes a counter called `i`	`i` is 0 on the first pass, 1 on the second, up to 29
`forward(i * 5)`	Draws a line `i * 5` long	The lines are 0, 5, 10, 15 and so on, so they grow
`right(90)`	Turns a quarter turn	The same turn every pass

The first line is 0 steps long, because i is 0 on the first pass. Counting starts at 0, not 1.

counter.py

from turtle import *

for i in range(30):
    forward(i * 5)
    right(90)

Try it yourselfYour edits are saved in this browser.

InvestigateTrace how each line of code connects to what you see.

Investigate

Follow the counter round the loop, then use what you know about names.

counter.py

from turtle import *

for i in range(30):
    forward(i * 5)
    right(90)

Work out i and the length of the line on each pass.

A name holds a value you can calculate with.

Fill in the counter and the length of the line on each pass.

pass	i	line length (i * 5)
1	0	0
2
3
4

A program has sides = 8 and turns right(360 / sides) on every pass of its loop. How many degrees is each turn?

ModifyChange the code on purpose and check the result.

Modify

Change one number so the spiral grows faster.

Need a hint?

Only one number needs to change. Which number is multiplied by i?

Use the counter to make the line thicker on every pass.

Need a hint?

pensize( ) sets how thick the line is. The pen needs a number that changes each pass.

Another hint

The counter changes each pass, so use it. pensize(i + 1) starts at 1.

Try it yourselfYour edits are saved in this browser.

I changed one number so the spiral grows faster.

Use the counter to change the pen size.

Run your program. This ticks when your code does all of the above.

For teachers

Students use the loop counter i to work out something different on every pass, and meet the fact that counting starts at 0. About 30 minutes.

Open the teacher guide: answers, solutions, slides and a worksheet.

Students will be able to

I can say what i holds on each pass of a loop.
I can use i in a calculation inside a loop.
I can change a drawing by changing one number in that calculation.

Before class

Run the spiral, then try pensize(i + 1) yourself. Notice the first line is invisible because i is 0.

Running the lesson

Predict: show the loop with forward(i * 5) and ask what it draws. Take a vote before anyone runs it; the wrong options are a square (the counter does not change), a circle, and an error because i was never stored.
Run: students run it and compare. Ask: how long is the first line, and why?
Investigate: trace the counter 0, 1, 2, 3 and say that counting starts at 0, because later lessons rely on it. The second task checks that a name still holds a value you can calculate with.
Modify: students change the multiplier, then use the counter for the pen. Let them try pensize(i) first: it starts at 0, so the first line is thin. That is a good moment to talk about starting at 0.

The hard part (on purpose)

Believing the counter is the same number every pass, or that it starts at 1. Ask what i is on the first pass.

When students get stuck

The spiral is a square, with every line the same length. Ask: Is the length worked out from i, or typed as a number? Fix: The length must use i, for example forward(i * 5).
A student expects the first line to be 5 long. Ask: What is i on the very first pass? Fix: i is 0 on the first pass, so i * 5 is 0. Counting starts at 0.
An error says i is not defined. Ask: Is the line that uses i indented inside the for loop? Fix: i only exists inside the loop. Indent the line under the for line.

Stretch and support

Stretch: Draw a spiral that turns 91 degrees instead of 90. What happens, and why?

Support: Give the first three values of i * 5 and ask the student to continue the pattern before running.

Where it leads

Next lesson makes a value grow without the counter: a variable that adds to itself. Counters are used for scores, timers and animations. Class ideas: a spiral gallery with different multipliers.

What each step is for

Investigate: The counter takes a new value on every pass, so any line that uses it does something different each time round.
Modify: Because the line length is worked out from i, changing one number changes every line of the spiral.

Now you can

Say what i holds on each pass of a loop.
Use i in a calculation inside a loop.
Change a drawing by changing one number in that calculation.

Next lesson5. A value that growsA variable can change itself, so a value grows each time round a loop.

Source: content/code/python-foundations/turtle-counter/lesson.md