Variables Explained With Simple Examples

A variable is a name you attach to a piece of information so your program can use it again later. That is it. Everything else is detail.

age = 12
print(age)

You gave the number 12 the name age. From now on, writing age means 12.

Why bother naming things?#

Compare these two lines:

print(40 * 12 * 0.8)

hours = 40
rate = 12
after_tax = 0.8
print(hours * rate * after_tax)

Both print the same number. Only the second one tells you what the number means, and only the second one is easy to change later. Variables are mostly about making code readable by a human — usually you, in two weeks, having forgotten everything.

Creating a variable#

In Python you just pick a name and assign a value:

city = "Lahore"
temperature = 31
is_raining = False

In JavaScript you add a keyword in front, usually let or const:

let city = "Lahore";
let temperature = 31;
const isRaining = false;

const means “I do not intend to reassign this name”. let means “this may change”. More detail in JavaScript variables explained.

The equals sign does not mean equals#

This trips up nearly everyone who did algebra at school. In most programming languages, = means “put the value on the right into the name on the left”. It is an instruction, not a statement of fact.

count = 5
count = count + 1
print(count)   # 6

In maths, count = count + 1 is nonsense. In code it reads as: take the current value of count, add one, and store the result back under the same name.

To ask whether two things are equal, you use == instead. Mixing these two up is one of the most common beginner errors — see if statements explained.

Changing what a variable holds#

A variable holds one value at a time. Assigning a new value replaces the old one.

score = 0
score = 10
score = 25
print(score)   # 25

The earlier values are gone. If you need to keep several values, you want a list — see arrays and lists explained.

Naming variables well#

Every language has rules and conventions. The rules stop your code running if you break them. The conventions just make you easier to work with.

Rules#

• Start with a letter or an underscore, never a number. 2nd_place is invalid; second_place is fine.
• No spaces. Use an underscore or capital letters instead.
• No reserved words. You cannot name a variable if, for or class.
• Case matters. total and Total are two different variables.

Conventions#

• Python uses snake_case: user_name, total_price.
• JavaScript uses camelCase: userName, totalPrice.
• Say what the thing is, not what type it is. email beats str1.
• Short names are fine for short-lived things. A loop counter can be i. A value used across fifty lines should not be.

A worked example#

A small program that uses variables for everything it might want to change later:

item = "Notebook"
unit_price = 250
quantity = 3
discount = 0.1

subtotal = unit_price * quantity
saving = subtotal * discount
total = subtotal - saving

print("Item:", item)
print("Subtotal:", subtotal)
print("You saved:", saving)
print("Total:", total)

Change quantity to 10 and everything below updates on its own. That is the payoff: one edit in one place.

Where to go next#

• Data types explained — why "2" + "2" is not 4
• Python variables in more depth
• JavaScript variables: let, const and var