Basics · 02

Variables and values

Give a value a name so you can reuse and change it.

A variable is a name tag on a value.
The example below stores a name and an age, prints them, then adds 1 to the age and prints it again.

name = "Lara"age = 10print(name + " " + str(age))age = age + 1print(age)

const name = "Lara";let age = 10;console.log(name + " " + age);age = age + 1;console.log(age);

const name: string = "Lara";let age: number = 10;console.log(name + " " + age);age = age + 1;console.log(age);

public class Main {    public static void main(String[] args) {        String name = "Lara";        int age = 10;        System.out.println(name + " " + age);        age = age + 1;        System.out.println(age);    }}

using System; class Program{    static void Main()    {        string name = "Lara";        int age = 10;        Console.WriteLine(name + " " + age);        age = age + 1;        Console.WriteLine(age);    }}

#include <iostream>#include <string> int main() {    std::string name = "Lara";    int age = 10;    std::cout << name << " " << age << '\n';    age = age + 1;    std::cout << age << '\n';}

#include <stdio.h> int main(void) {    const char *name = "Lara";    int age = 10;    printf("%s %d\n", name, age);    age = age + 1;    printf("%d\n", age);    return 0;}

name = "Lara";age = 10;Console.WriteLine(name + " " + age);age = age + 1;Console.WriteLine(age);

stdout

Lara 10
11

Key points

= does not mean "is equal to".
It means "put the value on the right into the name on the left", which is why age = age + 1 makes sense.
Some languages make you write a type (integer, text, …) when you create a variable.
In others the value knows its own type, so you don't.
A value that should never change is a constant.
Assigning to a constant again is an error.
Integers have a size limit.
A 32-bit int goes up to 2,147,483,647 (about 2.1 billion); for bigger numbers use a 64-bit integer (up to about 9.2×10^18).

How each language writes it

Python	`age = 10` — no type. There is no constant syntax, only the convention of an uppercase name like `MAX_SCORE`. Integers have no size limit. Swap with `a, b = b, a`
JavaScript	`let age = 10`, constants with `const` (reassigning one throws `TypeError: Assignment to constant variable.` at run time). Numbers are exact only up to 2^53 (about 9×10^15). Swap with `[a, b] = [b, a]`; put a value in text with `${age}`
TypeScript	`let age: number = 10` — leave the type out and it is inferred from the value. Reassigning a `const` is a compile error. Otherwise the same as JavaScript
Java	`int age = 10;`, text is `String`, and `var` lets the compiler infer. Constants use `final`, big numbers `long`. Swap with a temporary
C#	`int age = 10;`, text is `string`, `var` also works. Constants use `const`, big numbers `long`. Swap with `(a, b) = (b, a)`; put a value in text with `$"{age}"`
C++	`int age = 10;`, text is `std::string`, and `auto` lets the compiler infer. Constants use `const`, big numbers `long long`. Swap with `std::swap(a, b)`
C	`int age = 10;`, text is `const char *` (or a `char` array). Constants use `const`, big numbers `long long`. Swap with a temporary. Put text and numbers into slots: `printf("%s %d\n", name, age)`
Craykon	`age = 10;` — no type, and no constants. Numbers are exact up to 2^53. Swap with a temporary. `me` is a keyword, so it can't be a variable name

Common mistakes

Swapping with `a = b; b = a;`

a = 1; b = 2;
a = b;
b = a;

→ Both a and b are 2: a's original value was lost on the first line

Use a temporary: t = a; a = b; b = t;

Adding to a variable with no starting valueC · C++

int total;
total += 5;
printf("%d\n", total);

→ It starts from whatever garbage was in memory, so the sum is wrong

Give it a starting value: int total = 0;.
Java and C# reject such code at compile time.

Using a variable outside the block that created itCraykon

if (true) {
    y = 1;
}
Console.WriteLine(y);

→ An error saying the name y hasn't been made yet

Create it before the block with y = 0;.
(Python, by contrast, still sees it outside.)

How languages differ
Python can't join text and a number with + ("Lara " + 10 → TypeError: can only concatenate str (not "int") to str).
Convert with str(age) or use f"{name} {age}".

Variables and values

Key points

How each language writes it

Common mistakes

Swapping with a = b; b = a;

Adding to a variable with no starting valueC · C++

Using a variable outside the block that created itCraykon

Practice

Swapping with `a = b; b = a;`