maps

on Saturday, 25th of July, 2020

Dart supports many different types of collections, or data types that contain multiple values. The most common collections:

  • List (known as an array in some languages)
  • Set
  • Map

Maps

A map, also known commonly as a dictionary or hash, is an unordered collection of key-value pairs. Maps pair a key with a value for easy, fast retrieval. Like lists and sets, maps can be declared with a constructor or literal syntax.

Sets and maps have the the same syntax for their literal implementation. When you define a set literal, you must annotate the type of the variable. Otherwise, it will default to a `Map`.
// literal syntax
var dogsIvePet = {
  'Golden Retriever': ['Cowboy', 'Jack'],
  'Laberdoodles': ['Wallace', 'Phoebe'], 
  'Shepards': ['Doug', 'Steak'],
}

// constructor syntax
var dogsIWantToPet = Map();

// create a key called 'Border Collie' and assign it's value
dogsIWantToPet['Border Collie'] = ['Mike', 'Jackson'];

Like other collections, there are many class members on the Map class that make it more convenient to manipulate. A couple of them are:

  • remove(key)
  • keys
  • values
  • length
  • isEmpty and isNotEmpty
  • containsKey(key)
var dogsIvePet = {
  'Golden Retriever': ['Cowboy', 'Jack'],
  'Laberdoodles': ['Wallace', 'Phoebe'], 
  'Shepherds': ['Doug', 'Steak'],
}

// remove an item
dogsIvePet.remove('Laberdoodles');

// length gives the number of key-value pairs
print(dogsIvePet.length);
// output: 2

print(dogsIvePet.keys);
// output: ['Golden Retriever', 'Shepherds']
Like lists, maps support collection-if, collection-for, and spread operators.


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