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What are Variables?

Variables are containers that store data values. Think of them like labeled boxes where you can put information and retrieve it later.

Declaring Variables in Dart

Basic Syntax

A variable declaration starts with the data type, followed by the variable name, and the value assigned using =.

dataType variableName = value;

Common Data Types

void main() {
  int age = 25;           // Integer (whole numbers)
  double height = 5.9;    // Double (decimal numbers)
  String name = 'John';   // String (text)
  bool isStudent = true;  // Boolean (true/false)

  print('Name: $name');
  print('Age: $age');
  print('Height: $height');
  print('Is Student: $isStudent');
}

This is called statically typed - we explicitly specify the data type during declaration.

Dynamic Variable Keywords

Dart provides several keywords for declaring variables without explicit types:

Using `var`

Use when you want to declare a variable that can be reassigned but maintains its inferred type.

void main() {
  var city = 'New York';  // Inferred as String
  print('City: $city');

  city = 'Los Angeles';   // OK - same type
  print('Updated City: $city');
  
  // city = 123;  // ERROR - can't change type!
}

Using `dynamic`

Use when you need a variable that can hold any type and can change types at runtime.

void main() {
  dynamic data = 42;       // Integer
  print('Data: $data');

  data = 'Hello';          // Now a String - OK!
  print('Updated Data: $data');

  data = true;             // Now a Boolean - OK!
  print('Final Data: $data');
}

Use dynamic sparingly - you lose type safety and compile-time error checking!

Using `const`

Use for values that are compile-time constants and will never change.

void main() {
  const pi = 3.14159;     // Constant value
  print('Value of pi: $pi');

  // pi = 3.14;  // ERROR - cannot reassign const!
}

Using `final`

Similar to const, but the value can be determined at runtime.

void main() {
  final currentTime = DateTime.now();  // Set at runtime
  print('Current Time: $currentTime');

  // currentTime = DateTime.now();  // ERROR - cannot reassign!
}

const vs final: Use const when the value is known at compile time. Use final when it's determined at runtime.

Null Safety in Dart

Null safety means variables cannot be null by default unless you explicitly allow it.

Bonus Tips

Embed variables in strings using `$`

void main() {
  String name = 'Alice';
  int age = 25;
  
  print('Hello, $name!');
  print('You are $age years old.');
  
  // For expressions, use ${}
  print('Next year you will be ${age + 1}');
}

Check variable types

void main() {
  var number = 42;
  print('Type: ${number.runtimeType}');  // Output: int

  dynamic data = 'Hello';
  print('Is String: ${data is String}');  // Output: true
}

Summary

Keyword	Mutable?	Type Changes?	When Assigned?
`var`	✅ Yes	❌ No	Runtime
`dynamic`	✅ Yes	✅ Yes	Runtime
`final`	❌ No	❌ No	Runtime
`const`	❌ No	❌ No	Compile time

Up Next: In the next lesson, we'll explore Control Flow in Dart - if statements, loops, and more!

Variables and Null Safety in Dart