Dart Data Types and Variable Declarations

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🎯 Dart Data Types and Variable Declarations – Explained with Examples

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💡 Introduction

Every programming language uses data types to know what kind of data is being stored and manipulated. In Dart, a strongly-typed language, understanding data types helps you write more powerful and safer code.

Think of data types as the labels on storage boxes: some boxes store numbers, some hold text, others may contain just true/false switches.

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📋 Keywords

• int – Integer numbers

• double – Decimal numbers (floating-point)

• num – Can be either int or double

• String – Text data

• bool – Boolean (true or false)

• var – Type inferred automatically

• dynamic – Type that can change at runtime

• const / final – Immutable variables

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🧠 Dart Variables and Data Types Explained

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🧮 1. int – Whole numbers

Stores non-decimal numbers (positive or negative).

int age = 25;

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🧊 2. double – Decimal numbers

Stores floating-point numbers (like 3.14).

double pi = 3.1416;

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➕ 3. num – Either int or double

Can hold both integer and decimal values.

num x = 10;
x = 3.14; // Valid

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💬 4. String – Text

Used to store words, names, sentences.

String name = 'Shubham';

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✅ 5. bool – True or False

For logical conditions.

bool isStudent = true;

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📦 6. var – Type inference

Dart guesses the type from the right-hand value.

var city = 'Varanasi'; // Inferred as String

Once assigned, you can’t change its type:

// city = 123; ❌ Error

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🔄 7. dynamic – Flexible type

Type can change at runtime.

dynamic value = 'Hello';
value = 42; // OK

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🔐 8. final and const – Immutable variables

• final: Value set once at runtime

• const: Value set at compile time

final dob = '2000-01-01';
const pi = 3.14;

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⚠️ Difference between var, final, and const

Keyword	Can Reassign	Fixed Type	Immutable
var	✅	✅	❌
final	❌	✅	✅
const	❌	✅	✅ (at compile time)

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✅ Solved Examples

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1. Declare your name, age, and student status

void main() {
  String name = 'Shubham';
  int age = 22;
  bool isStudent = true;
  print('Name: $name');
  print('Age: $age');
  print('Is Student: $isStudent');
}

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2. Calculate area of a circle using const pi

void main() {
  const double pi = 3.1416;
  double radius = 5;
  double area = pi * radius * radius;
  print('Area = $area');
}

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3. Use of dynamic to change types

void main() {
  dynamic x = 'Hello';
  print(x);
  x = 123;
  print(x);
}

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4. Type inference with var

void main() {
  var city = 'Delhi'; // inferred as String
  // city = 123; ❌ Error: type mismatch
  print(city);
}

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5. Error: Assigning wrong type

void main() {
  int age = 25;
  // age = 'twenty-five'; ❌ Compile-time error
}

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❓ Unsolved Practice Problems

Try these in DartPad or your local Dart editor.

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1. Create a program that stores:

• Your full name

• Your percentage in graduation (e.g., 82.5)

• Whether you have programming experience (true/false)

Print the data in one sentence.

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2. Ask the user for a product name and its price (use String and double) and print a receipt-like line:

Product: Pen | Price: ₹10.50

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3. Use dynamic to store and print:

• A name

• Then a number

• Then a boolean

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4. Create a final variable for your birth year and try to reassign it. What happens?

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5. Write a program where you use:

• int for count

• String for item name

• bool for availability
  Then print a short inventory line like:

5 x Notebook - Available

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🧾 Summary

• Dart is strongly typed, but can be flexible using var and dynamic.

• Use final or const for values that never change.

• Use the correct data types to make your code clean, efficient, and bug-free.

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