Lesson 11
Variables - labelled boxes
After this lesson: You'll store numbers and text in variables and calculate with them.
Video
The video for this lesson is coming soon.
Explanation
Imagine a box with a label. You write a name on the label and put a number or text inside. In programming such a box is a variable.
Creating a variable
age = 11 means: take a box labelled "age" and put 11 in it.
- On the left of
=is the variable's name, on the right - the value. - When you later write
age(without quotes!), Python looks into the box and uses what's inside. - The
=sign in Python doesn't mean "equals" like in maths - it means "put into the box".
A box with a label 🎬
Click Next and see how a variable works.
An empty box
Bajtek is 11 🎂 and wants the program to remember it. He takes an empty box 📦 and sticks a label on it that says age.
One box 📦
Line 1: we put the number 5 into the box labelled number. Line 2: print looks into the box and prints what's in it - 5.
Click the arrow and watch the values move between the boxes.
1number = 52print(number)
▍Boxes in the computer's memory
Step through the program and watch the boxes with variables appear.
The memory is still empty. Click the arrow - every line with = puts down a new box.
1name = "Bajtek"2age = 33print(name)4print(age + 1)5print(name, age)
▍A pet shop 🐶
Three boxes: dogs, cats and fish. First we print how many of each pet there are, and at the end we count them all together: 3 + 5 + 12.
The memory is still empty. Click the arrow - every line with = puts down a new box.
1dogs = 32cats = 53fish = 124print("Dogs:", dogs)5print("Cats:", cats)6print("Fish:", fish)7print("Pets in total:", dogs + cats + fish)
▍What can go into a box?
- a number:
points = 100, - text:
name = "Ola"(text in quotes!), - the result of a calculation:
area = 4 * 3- Python first calculates 12 and then puts 12 into the box, - the contents of another box:
copy = points.
Calculating with variables
Notice that the area box holds the already calculated number 12, not the calculation. Below is the same rectangle as a picture: the area is the squares inside, and the perimeter is the fence around.
The memory is still empty. Click the arrow - every line with = puts down a new box.
1length = 42width = 33area = length * width4perimeter = 2 * length + 2 * width5print("The area is:", area)6print("The perimeter is:", perimeter)
▍A game character card 🧙
Text and two numbers - each in its own box. Every box fills one field of the character card.
Click the arrow - every variable fills one field of the character card.
1name = "Merlin"2level = 73health = 854print("Character:", name)5print("Level:", level)6print("Health:", health)
▍Why do we need variables?
Imagine the rectangle now has length 5 instead of 4. Without variables you'd have to fix every calculation. With variables you change one line length = 5 - and everything is recalculated. Variables also have names that say what you're calculating - the code reads like a sentence.
A tree with a variable 🎄
We keep the tree's character in the box mark. Every line of the tree takes it from the box.
Click the arrow - first the character goes into the box, then every line of the tree takes it from there.
1mark = "*"2print(" " + mark)3print(" " + mark * 3)4print(" " + mark * 5)5print(" |")
▍We change only the first line ✨
The same program - we changed only mark = "*" to mark = "o". We don't touch the rest of the code, and the whole tree is now made of baubles! That's what variables are for: change it in one place, and it changes everywhere.
mark = "*"➡️📦 mark = ?the rest of the code is the sameClick the arrow - first the character goes into the box, then every line of the tree takes it from there.
1mark = "o"2print(" " + mark)3print(" " + mark * 3)4print(" " + mark * 5)5print(" |")
▍Pancakes for 1 person 🥞
The recipe for one person: 50 g of flour, 100 ml of milk and 1 egg. We keep the number of people in the box people.
Click the arrow - first the number of people goes into the box.
1people = 12print("Flour (g):", 50 * people)3print("Milk (ml):", 100 * people)4print("Eggs:", 1 * people)
▍Pancakes for a family (4 people) 👨👩👧👦
The same program - we changed only people = 1 to people = 4. All the ingredients recalculated themselves!
people = 1➡️📦 people = ?Click the arrow - first the number of people goes into the box.
1people = 42print("Flour (g):", 50 * people)3print("Milk (ml):", 100 * people)4print("Eggs:", 1 * people)
▍Pancakes for grandparents and two families (10 people) 👵👴
Grandparents and two families of 4 make 2 + 4 + 4 = 10 people. Again we change only the first line - the rest of the code stays the same.
people = 4➡️📦 people = ?Click the arrow - first the number of people goes into the box.
1people = 102print("Flour (g):", 50 * people)3print("Milk (ml):", 100 * people)4print("Eggs:", 1 * people)
▍Good variable names
- small letters, no spaces:
age,number_of_cats, - digits are fine, but not at the start:
player1- good,1player- wrong, - instead of a space - an underscore
_, - the name should say what's inside:
priceis better thanx.
age or "age"?
print(age) prints what's in the box (e.g. 11), while print("age") prints the word age. Quotes make a huge difference!
With and without quotes
Line 1: we put 11 into the box labelled age. Line 2: age without quotes - Python looks into the box and prints what's inside: 11. Line 3: "age" in quotes - it's plain text, Python copies it letter by letter: age. Line 4: both at once - the text as a label, and next to it what's in the box. Hint: in the code, text in quotes is orange.
Click the arrow - first a number goes into the box age.
1age = 112print(age)3print("age")4print("age:", age)
▍A variable that doesn't exist
A typo in the name: there's no box called agee, so Python reports a NameError.
Click the arrow. For every name Python checks its list to see if it knows it.
1age = 112print(agee)
▍Ola has as many as Kuba 🍬
Click Next and see what ola = kuba does.
Code
🖥️ Screen
📦 kuba = 5Grandma brings sweets
Grandma 👵 came to visit and gave Kuba a box with 5 sweets 🍬. That's the first line: kuba = 5 - we put 5 into the box labelled kuba.
A draw ⚽
Ola scored 3 goals, and Kuba the same. kuba_goals = ola_goals looks into the box ola_goals and puts the same 3 into a new box.
1ola_goals = 32kuba_goals = ola_goals3print("Ola:", ola_goals)4print("Kuba:", kuba_goals)5print("Draw", ola_goals, ":", kuba_goals)
▍The same colour 🎨
A box can hold text too. kuba_colour = ola_colour copies the text "green" into the second box - both boxes now have the same colour.
1ola_colour = "green"2kuba_colour = ola_colour3print("Ola picks the colour:", ola_colour)4print("Kuba picks the colour:", kuba_colour)
▍What will this program print?
a = 4b = aprint(a + b)
Which variable name is correct?
What's in the area box after this program?
side = 6area = side * side
Imagine a cabinet with drawers. Every drawer has a label with a name, and something lies inside. A variable is such a drawer! 🗄️
A backpack for a trip
Watch the variables panel: every line with = creates a new box.
The memory is still empty. Click the arrow - every line with = puts down a new box.
1sandwiches = 32bottles = 23name = "Ola"4print(name, "has", sandwiches, "sandwiches")5print("Items in total:", sandwiches + bottles)
▍Guess what this calculates
Both programs calculate the same thing. Which one is easier to understand?
Click the arrow - first the first program, then the second.
1a = 122b = 33print(a * b)4notebook_price = 125notebook_count = 36print(notebook_price * notebook_count)
▍A variable or just text?
This is a very important difference:
| Code | Screen | Why |
|---|---|---|
print(name) | Ola | no quotes - the contents of the box |
print("name") | name | in quotes - plain text |
A name without quotes is a label on a box - Python looks inside. 📦
A rectangle
The same program as above - we changed only length = 4 to length = 5. Dimensions stored once are used in several calculations, so changing one number changes every result: the rectangle is longer, and the area and the perimeter are recalculated by themselves.
The memory is still empty. Click the arrow - every line with = puts down a new box.
1length = 52width = 33area = length * width4perimeter = 2 * length + 2 * width5print("The area is:", area)6print("The perimeter is:", perimeter)
▍Solving a task with variables
Task: A cinema ticket costs 18 zl, popcorn 12 zl. 4 friends go and each buys a ticket and popcorn. How much do they pay in total?
- I put the prices in boxes:
ticket = 18,popcorn = 12. - The number of people:
people = 4. - One person pays:
per_person = ticket + popcorn- that is 30. - Everyone together:
total = per_person * people- that is 30 * 4 = 120. Only the ready results are printed with print. 🍿
The cinema in code
Click the arrow - first the prices and the number of people go into boxes.
1ticket = 182popcorn = 123people = 44per_person = ticket + popcorn5total = per_person * people6print("One person pays:", per_person)7print("Everyone together:", total)
▍What does the program print?
cat = "Tom"print("cat")
Which variable name is valid?
Remember
name = valueputs the value into the box labelled name.=means "put into the box", not "equals".- A variable name is written without quotes, text - in quotes.
- Names: small letters, no spaces, don't start with a digit.
Now you create the boxes! In the tasks, first store the numbers in variables, then calculate with them.
Tasks from the video
We solve them together in the video - pause it and try first on your own.
Practice tasks
Try to solve them without the video. Stuck? Every task has hints and an editorial.
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