Variables & Data Types

Basics 5 examples

Variables are fundamental building blocks in programming. They allow us to store, retrieve, and manipulate data in our programs. In this lesson, you'll learn what variables are, how to declare and initialize them, and the different data types available in most programming languages.

What are Variables?

A variable is a named storage location that contains a value. You can think of variables as labeled containers that hold data which can be changed throughout the program's execution.

Key Concepts:

Variables have a name (identifier)
Variables store data of specific types
Variables can be reassigned with new values
Variables must be declared before use

Here's how variables work in most programming languages:


// Declaring a variable
let age;

// Initializing a variable
age = 25;

// Declaring and initializing in one step
let name = "John";

// Changing a variable's value
age = 26;

Common Data Types

Different programming languages support various data types. Here are the most common ones:

Numeric Types

Integer: Whole numbers without decimals
Float/Double: Numbers with decimal points


let count = 42;         // Integer
let price = 9.99;       // Float

Text Types

String: Text data enclosed in quotes
Character: Single character (in some languages)


let message = "Hello, World!";  // String
let grade = 'A';                // Character

Boolean Type

Boolean: Logical values (true/false)


let isActive = true;
let hasPermission = false;

Complex Types

Array: Ordered collection of values
Object: Collection of key-value pairs


let colors = ["red", "green", "blue"];  // Array
let person = {                          // Object
  name: "John",
  age: 30
};

Type Conversion

Sometimes you need to convert values from one data type to another. This is called type conversion or type casting.


// String to Number
let numStr = "42";
let num = Number(numStr);  // Explicit conversion
let num2 = +numStr;        // Shorthand conversion

// Number to String
let count = 100;
let countStr = String(count);  // Explicit conversion
let countStr2 = count + "";    // Implicit conversion

// To Boolean
let active = Boolean(1);      // true
let inactive = Boolean(0);    // false
let emptyCheck = Boolean(""); // false

Common Pitfalls:

Be careful with automatic type conversion, especially in loosely typed languages. It can lead to unexpected results:


console.log("5" + 2);    // Outputs: "52" (string concatenation)
console.log("5" - 2);    // Outputs: 3 (numeric subtraction)

Variable Scope

Variable scope determines where in your code a variable can be accessed or modified.

Common Scope Types:

Global scope: Variables accessible throughout the program
Local/function scope: Variables accessible only within the function they're declared in
Block scope: Variables accessible only within the block they're declared in (using let/const in JavaScript)


// Global variable
let globalVar = "I'm global";

function exampleFunction() {
  // Local variable
  let localVar = "I'm local";
  
  // Block scope
  if (true) {
    let blockVar = "I'm in a block";
    console.log(blockVar);  // Accessible
  }
  
  console.log(localVar);    // Accessible
  console.log(globalVar);   // Accessible
  // console.log(blockVar); // Error: blockVar is not defined
}

exampleFunction();
console.log(globalVar);     // Accessible
// console.log(localVar);   // Error: localVar is not defined

Constants

Constants are like variables, but their values cannot be changed after initialization.


// Declaring a constant
const PI = 3.14159;
const BASE_URL = "https://api.example.com";

// This would cause an error
// PI = 3.14;  // Error: Assignment to constant variable

Best Practice:

Use constants for values that should not change throughout your program. Common examples include configuration values, mathematical constants, and API endpoints.

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