Block coding lesson 17: Changing a variable (and a first look at Python)

US version. US grades and US spelling. · UK version of this lesson

change size by 1 adds 1 to the number in the box. Inside a repeat, the variable grows each time round – each line is longer than the last, which draws a spiral. A complete 40-minute block coding lesson (Grades 4–5), from Unit 6: Variables. Myrtle the Turtle runs right here in your browser – nothing to install.

Time: 40 minutes + 20 min homework
Grade: Grades 4–5 (ages 9–11)
You need: A computer or tablet, squared paper and a pencil
Graded: Instantly, with explanations

For parents: how to use this lesson

You don't need to know anything about coding. The lesson explains each step on screen, the way Mr Babbage, our Head of Computing, likes it: watch Myrtle first, then have a go. Programs are made from blocks – no typing – and every answer is graded straight away.

Before the screen: Climb the stairs saying the step number each time: "change step by 1".
Press Start the lesson below and watch the worked examples together. Press Run and say what each block does before Myrtle does it.
Let your child do the practice on their own. When Myrtle goes the wrong way, that's a clue, not a failure – press Run again and watch which block it is on.
Finish with the talk question: “How does changing a variable make a spiral?”
Later that day or the next day: the homework (20 minutes), then a practice test.

Watch out for

Thinking change size by 1 sets size to 1. It adds 1 to what is already there.
Forgetting the change runs every time round the loop. Write the value down after each time round.

Miss Lovelace's tip for home: ask your child to be Myrtle. Read out the blocks and let them walk it on the floor. If they can act it out, they understand it.

What your child will learn

Learning goal: change a variable while the program runs.

Success looks like

I can change a variable while the program runs.
I can predict what a variable holds at the end.
I can match blocks to the same program in Python.

Key words

change · increase · spiral · Python

The big ideas (for the adult)

change size by 1 adds 1 to the number in the box. Inside a repeat, the variable grows each time round – each line is longer than the last, which draws a spiral.

The same program can be typed in Python: forward(size), right(90), size = size + 1. Spelling trap: Python always writes color, the American spelling, even on a UK computer.

A worked example

set size to 1
repeat 5 times
    move forward size
    turn left 90°
    change size by 1
end

Press Start the lesson to watch the turtle run it.

The lesson, step by step

40 minutes in 5 parts
Part	Time	How it is checked
Starter	5 min	Graded by Classfern
Teach	8 min	Watch, talk and try
Worked example and try it	8 min	Graded by Classfern
Practice	14 min	Graded by Classfern
Wrap up	5 min	Graded by Classfern + self-check

The lesson

It runs here on the page. No sign-up and no names – progress is kept only in this browser. Each visit picks a new mix of questions.

Homework

20 minutes: 6 questions – work out what Myrtle does, then build or fix a program – and a short reflection. New questions every time, all graded instantly. Away from the screen: climb the stairs saying the step number each time: "change step by 1".

Practice tests

All computer science practice tests · All 18 block coding lessons

Curriculum

CSTA K–12 Computer Science Standards (2017), grades 3–5: 1B-AP-09 – Build programs that store and change data using variables; 1B-AP-12 – Modify, remix or reuse parts of an existing program to add features.

The standard codes are CSTA's; the descriptions are our own words.

Our lessons, blocks and questions are our own. They are not written or endorsed by any government department, testing agency or exam board. Python is a registered trademark of the Python Software Foundation; Classfern is not affiliated with or endorsed by it.