Lesson 12
Changing a variable
After this lesson: You'll change a variable's value: increase it, decrease it and swap two variables.
Video
The video for this lesson is coming soon.
Explanation
In a game you collect points - the score keeps growing. See how to change what's in the box!
A new value throws out the old one
You can put a new value into a box at any time. Then the old value disappears - a box holds only one thing at a time:
Replacing the contents
Watch the box x: every new number drops in and the old one falls out. A box holds only one thing.
Click the arrow and watch the values move between the boxes.
1x = 52print(x)3x = 84print(x)5x = 1006print(x)
▍The weather during the day ☀️🌧️
One box, three different values. Watch the variables panel - the old value disappears.
Click the arrow and watch the values move between the boxes.
1weather = "sun"2print("Morning:", weather)3weather = "rain"4print("Noon:", weather)5weather = "snow"6print("Evening:", weather)
▍A backpack for a trip 🎒
Two boxes: food and drink. We change only one of them - watch whether the other one changes too.
Click the arrow and watch the values move between the boxes.
1food = "sandwich"2drink = "water"3print("Food:", food)4print("Drink:", drink)5food = "apple"6print("Food:", food)7print("Drink:", drink)
▍Did you notice? When the apple went into the box food, the box drink still had the water. Every variable is a separate box - changing one doesn't touch the other.
A match: two teams' goals ⚽
Two boxes for the match score. After every goal a new number goes into the team's box - and the old one falls out.
Click the arrow and watch the values move between the boxes.
1red = 02blue = 03print("Red:", red, "Blue:", blue)4red = 15print("Red:", red, "Blue:", blue)6blue = 17print("Red:", red, "Blue:", blue)8blue = 29print("Red:", red, "Blue:", blue)
▍x = x + 1
In maths x = x + 1 would be impossible - no number equals itself plus 1! 🤔 But in Python the = sign doesn't mean "is equal to". It means "put into the box".
See how Python reads x = x + 1 when the box x holds 5:
- First it looks at the right side:
x + 1. - It looks into the box x - there's 5 in it.
- It calculates: 5 + 1 = 6.
- It puts 6 into the box x. The old five falls out - a box holds only one thing.
Read it like this: "the new x is the old x plus 1". It's like a points counter in a game: after every point the score goes up by one.
coins = coins + 1 step by step 🪙
A coin hangs above Bajtek. When coins = coins + 1 runs, Bajtek jumps, grabs it - and the coin disappears! The box coins held 5, and now it holds 6: the new coins are the old coins plus 1.
1coins = 52print("Before the jump:", coins)3coins = coins + 14print("After the jump:", coins)
▍Game points
You collect coins worth 10 points and a star worth 5. After every change we print the score - watch it grow: 0, 10, 20, 25.
1points = 02print("Start:", points)3points = points + 104print("After the first coin:", points)5points = points + 106print("After the second coin:", points)7points = points + 58print("After the star:", points)
▍Lives and coins 🎮
1lives = 32lives = lives - 13print("Lives after a mistake:", lives)4coins = 105coins = coins + 56print("Coins after a treasure:", coins)7coins = coins * 28print("Coins after a x2 bonus:", coins)
▍Piggy bank
A real story: you deposit every week and sometimes spend.
1piggy_bank = 202print("At the start:", piggy_bank)3# deposit pocket money4piggy_bank = piggy_bank + 155print("After pocket money:", piggy_bank)6# buy ice cream7piggy_bank = piggy_bank - 88print("After ice cream:", piggy_bank)9# grandma doubles the savings10piggy_bank = piggy_bank * 211print("After grandma's gift:", piggy_bank)
▍A birthday every year 🎂
The age grows by 1 - each time we take the old value and add 1.
1age = 92age = age + 13print(age)4age = age + 15print(age)6age = age + 17print(age)
▍Baubles for the tree 🎄
Everyone in the family hangs their baubles: mum 5, dad 3, me 7. After each turn we count the baubles on the tree: 5, 8, 15.
1baubles = 02baubles = baubles + 53print("Mum hung 5, on the tree:", baubles)4baubles = baubles + 35print("Dad hung 3, on the tree:", baubles)6baubles = baubles + 77print("I hung 7, on the tree:", baubles)
▍A snail on a wall 🐌
During the day it climbs 3 cm up, at night it slides 1 cm down. Where will it be after 3 days?
1height = 02height = height + 33print("Day 1:", height)4height = height - 15print("Night 1:", height)6height = height + 37print("Day 2:", height)8height = height - 19print("Night 2:", height)10height = height + 311print("Day 3:", height)
▍A magic bean 🌱
The bean doubles its height every night: bean = bean * 2. Watch how fast it grows: 2, 4, 8, 16.
1bean = 12bean = bean * 23print("Night 1:", bean)4bean = bean * 25print("Night 2:", bean)6bean = bean * 27print("Night 3:", bean)8bean = bean * 29print("Night 4:", bean)
▍Swapping two variables
Want to swap what's in the boxes a and b? If you wrote a = b, the old contents of a would be gone and you'd have nothing to put into b. You need a third box:
- put a aside in a helper box
temp, - put b into a,
- put what you set aside in
tempinto b.
It's like swapping drinks between two cups - you need a third, helper cup.
Two cups 🥤
To swap juice and water, you need a third cup - a helper. It holds the juice for a moment.
Click the arrow and watch the values move between the boxes.
1cup1 = "juice"2cup2 = "water"3helper = cup14cup1 = cup25cup2 = helper6print(cup1, cup2)
▍Wrong - without a third cup
After cup1 = cup2 the juice is lost forever!
Click the arrow and watch the values move between the boxes.
1cup1 = "juice"2cup2 = "water"3cup1 = cup24cup2 = cup15print(cup1, cup2)
▍Swapping with a helper box
Now the same with numbers: the box temp is our helper cup. First we print a and b before the swap, and at the end after it - compare both lines.
Click the arrow and watch the values move between the boxes.
1a = 12b = 23print("Before the swap:", a, b)4temp = a5a = b6b = temp7print("After the swap:", a, b)
▍Swapping without a helper - doesn't work!
After a = b both boxes hold 2 - the one is lost forever.
Click the arrow and watch the values move between the boxes.
1a = 12b = 23a = b4b = a5print(a, b)
▍What will this program print?
x = 2x = x + 3x = x * 2print(x)
What will this program print?
x = 10x = 3print(x)
Common mistakes
x + 1withoutx =- Python calculates the result and throws it away, x doesn't change.- Swapping with
a = b,b = awithout a helper variable - both variables end up the same.
A score counter in a game
The player collects coins: +10, +10, +50 for a treasure, then loses 20 to a trap. Watch the points box change. 🎮
1points = 02points = points + 103print("After the first coin:", points)4points = points + 105print("After the second coin:", points)6points = points + 507print("After the treasure:", points)8points = points - 209print("After the trap:", points)
▍A growing plant
The plant is 3 cm tall and grows 2 cm every day. After the third day we double it with fertiliser. 🌱
1height = 32height = height + 23print("Day 1:", height)4height = height + 25print("Day 2:", height)6height = height + 27print("Day 3:", height)8height = height * 29print("With fertiliser:", height)
▍Why does // 10 cut off the last digit? Imagine you're packing sweets into packs of 10. 🍬
You have 47 sweets. That makes 4 full packs (40 sweets) and 7 loose sweets. Look: 4 and 7 are exactly the digits of 47!
47 // 10says how many full packs there are: 4 - the number without its last digit,47 % 10says how many are left loose: 7 - the last digit.
The last digit is always left loose, because it's smaller than 10 - not enough for a new pack. And // counts only full packs, so it skips it. ✂️
Packs of 10 🍬
The same number of sweets: how many full packs and how many loose.
1print("Full packs:", 47 // 10)2print("Loose:", 47 % 10)
▍2026 // 10 step by step
Cutting off digits
x = x // 10 cuts off the last digit. Three times - and 2026 becomes 2.
Click the arrow - every x = x // 10 cuts one digit off the end.
1x = 20262x = x // 103print(x)4x = x // 105print(x)6x = x // 107print(x)
▍What does the program print?
a = 4a = a * 3a = a - 2print(a)
What does the program print?
a = 5b = aa = 9print(b)
Remember
- A new value in the box throws out the old one.
x = x + 1- the new x is the old x plus 1.- Swapping two variables needs a third, helper variable.
Now you change what's in the boxes. Let's go!
Tasks from the video
We solve them together in the video - pause it and try first on your own.
Practice tasks
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