Maps - Dart - OneCompiler

Maps are like dictionaries or phone books - they store data in key-value pairs where each key is unique and maps to a specific value. Think of it as a way to organize information where you can quickly look up a value using its corresponding key.

What is a Map?

A Map is a collection of key-value pairs where:

Each key is unique
Keys can be of any type (String, int, etc.)
Values can be of any type and don't need to be unique
Maps are unordered collections

Creating Maps

You can create maps in several ways:

Using Map Literal Syntax:

// String keys with String values
var studentGrades = {
  'Alice': 'A',
  'Bob': 'B+',
  'Charlie': 'A-'
};

// Mixed types
var userInfo = {
  'name': 'John',
  'age': 25,
  'isActive': true
};

// Empty map
var emptyMap = <String, int>{};

Using Map Constructor:

// Creating an empty map
var colors = Map<String, String>();

// Creating from another map
var originalMap = {'a': 1, 'b': 2};
var copiedMap = Map.from(originalMap);

Accessing Map Elements

You can access values using square brackets with the key:

var fruits = {
  'apple': 'red',
  'banana': 'yellow',
  'grape': 'purple'
};

print(fruits['apple']);    // Output: red
print(fruits['banana']);   // Output: yellow
print(fruits['orange']);   // Output: null (key doesn't exist)

Adding and Updating Elements

var inventory = <String, int>{};

// Adding new elements
inventory['apples'] = 50;
inventory['oranges'] = 30;

// Updating existing elements
inventory['apples'] = 45;

print(inventory); // {apples: 45, oranges: 30}

Useful Map Properties and Methods

Property/Method	Description	Example
`length`	Returns number of key-value pairs	`map.length`
`isEmpty`	Checks if map is empty	`map.isEmpty`
`isNotEmpty`	Checks if map is not empty	`map.isNotEmpty`
`keys`	Returns all keys	`map.keys`
`values`	Returns all values	`map.values`
`containsKey()`	Checks if key exists	`map.containsKey('key')`
`containsValue()`	Checks if value exists	`map.containsValue('value')`
`remove()`	Removes key-value pair	`map.remove('key')`
`clear()`	Removes all elements	`map.clear()`

Working with Map Methods

var bookPrices = {
  'Dart Guide': 29.99,
  'Flutter Basics': 34.99,
  'Mobile Development': 39.99
};

// Check if a book exists
if (bookPrices.containsKey('Dart Guide')) {
  print('Dart Guide costs \$${bookPrices['Dart Guide']}');
}

// Get all book titles
print('Available books: ${bookPrices.keys.toList()}');

// Get all prices
print('Prices: ${bookPrices.values.toList()}');

// Remove a book
bookPrices.remove('Flutter Basics');
print('After removal: $bookPrices');

Iterating Through Maps

You can loop through maps in different ways:

var cityPopulations = {
  'New York': 8419000,
  'Los Angeles': 3980000,
  'Chicago': 2716000
};

// Iterate through key-value pairs
cityPopulations.forEach((city, population) {
  print('$city has $population people');
});

// Iterate through keys only
for (var city in cityPopulations.keys) {
  print('City: $city');
}

// Iterate through values only
for (var population in cityPopulations.values) {
  print('Population: $population');
}

Practical Example: Student Management

// Student management system
var students = <String, Map<String, dynamic>>{};

// Adding student information
students['S001'] = {
  'name': 'Alice Johnson',
  'grade': 'A',
  'subjects': ['Math', 'Science', 'English']
};

students['S002'] = {
  'name': 'Bob Smith',
  'grade': 'B+',
  'subjects': ['History', 'Art', 'Math']
};

// Accessing nested information
print('Student S001: ${students['S001']!['name']}');
print('Grade: ${students['S001']!['grade']}');

// Check if student exists
if (students.containsKey('S003')) {
  print('Student found');
} else {
  print('Student not found');
}

Maps are incredibly useful for organizing related data and creating efficient lookup systems. They're perfect when you need to associate unique identifiers with specific information!

Maps in Dart