Array In Dart

[1].  Arrays are Lists

List basics

Dart supports List literals like JavaScript. A simple Dart list:

main() {

  var list = [1,2,3];

  print( list is List ); // true

}

Get the list's length, or number of elements inside of the list:

main() {

  var list = [1,2,3];

  print( list.length ); // 3

}

Access the second element in the list: (notice how Dart list indexes are 0 based, i.e. 0 is the first element, 1 is the second element, etc)

main() {

  var list = [1,2,3];

  print( list[1] ); // 2

}

If you try to reference an element that is outside the length of the list, you'll get an IndexOutOfRangeException.

main() {

  var list = [1,2,3];

  print( list[5] );  

  // Unhandled exception: IndexOutOfRangeException

}

Add an element to a list constructed from a list literal:

main() {

  List list = [1,2,3];

  list.add(4);

  print( list.length ); // 4

}

Now is a good time to point out that the add(object) method is optional. That is, not all implementations of List have to support it. I don't personally like optional methods

An example of a List that you can't add to is the fixed size List, as constructed by:

main() {

  var list = new List(3);

  list.add(4); 

// this throws an UnsupportedOperationException: 

//Cannot add to a non-extendable array

// exception thrown for Lists constructed with an initial size

}

It's important to know that when you construct a fixed size List, that the List itself is allocated to that size and filled with nulls for the size of the List. For example:

main() {

  var list = new List(5); // [null,null,null,null,null]

  print(list[0]);  // null

}

 Lists in Dart are not associative arrays. That is, the following code will NOT work in Dart:

main() {

  var list = new List(); // no initial size! aka EMPTY

  list[3] = 'hello'; // IndexOutOfRangeException

}

Remove an element from a list: 

main() {

  List list = [1,2,3];

  list.removeRange(1, 1); // remove only the second element

  print( list.length ); // 2

}

To print the elements of a List, we need to add a map() function to eventually convert a List to a List<String> because Strings.join only takes a List<String>. The Dart libs are lacking a map function on collections (see bug  945).

List map(List list, converter(x)) {

  final List result = [];

  for (final x in list) {

    result.add(converter(x));

  }

  return result;

}

List<String> listToStrings(List list) {

  return map(list, (x) => x.toString());

}

main() {

  List list = [1,2,3];

  print( Strings.join( listToStrings(list), ',' )); // 1,2,3

}

Sorting

Sorting a List takes advantage of Dart's function support.

main() {

  var list = [4,1,2];

  list.sort(compare(a,b) {

    if (a == b) {

      return 0;

    } else if (a > b) {

      return 1;

    } else {

      return -1;

    }

  });

  // list is now 1,2,4

}

Iterating

Iterating over a List can be done in at least four ways. The standard, most familiar way is the for loop:

main() {

  var list = [4,1,2];

  for (var x = 0; x < list.length; x++) {

    print(x);

  }

}

There's a more terse way of using for if you don't need the current iteration index:

main() {

  var list = [4,1,2];

  for (final x in list) {

    print(x);

  }

}

The verbose way to say the above is to use a formal Iterator from the List:

main() {

  var list = [4,1,2];

  Iterator i = list.iterator();

  while (i.hasNext()) {

    print(i.next());

  }

}

And finally, if you just want to apply a function to each element of the List, use forEach:

main() {

  var list = [4,1,2];

  list.forEach(f(e) => print(e));

}

Filtering

The Collection interface, which List extends, provides a filter method, which returns a new collection with only the elements that satisfy a condition.

An example of filtering:

main() {

  var list = [4,1,2];

  var evens = list.filter(f(e) => e % 2 == 0);

  printList(evens); // 4,2

}

Of course, you don't need to inline the filtering function. Passing in a function also works:

isEven(x) => x % 2 == 0;

main() {

  var list = [4,1,2];

  var evens = list.filter(isEven);

  printList(evens);

}

[2]. Top 10 Array utility methods you should know

#1. forEach()

Runs a function on each element in the list

var fruits = [‘banana’, ‘pineapple’, ‘watermelon’];

fruits.forEach((fruit) => print(fruit));

 // => banana pineapple watermelon

#2. map()

Produces a new list after transforming each element in a given list

var mappedFruits = fruits.map((fruit) => ‘I love $fruit’).toList();

print(mappedFruits); 

// => ['I love banana', ‘I love pineapple’, ‘I love watermelon’]

#3. contains()

Checks to confirm that the given element is in the list

var numbers = [1, 3, 2, 5, 4];

print(numbers.contains(2)); // => true

#4. sort()

Order the elements based on the provided ordering function

numbers.sort((num1, num2) => num1 - num2); // => [1, 2, 3, 4, 5]

#5. reduce(), fold()

Compresses the elements to a single value, using the given function

var sum = numbers.reduce((curr, next) => curr + next);

print(sum); // => 15

const initialValue = 10;

var sum2 = numbers.fold(initialValue, (curr, next) => curr + next);

print(sum2); // => 25

#6. every()

Confirms that every element satisfies the test

List<Map<String, dynamic>> users = [

  { “name”: ‘John’, “age”: 18 },

  { “name”: ‘Jane’, “age”: 21 },

  { “name”: ‘Mary’, “age”: 23 },

];

var is18AndOver = users.every((user) => user[“age”] >= 18);

print(is18AndOver); // => true

var hasNamesWithJ = users.every((user) => user[“name”].startsWith('J'));

print(hasNamesWithJ); // => false

#7. where(), firstWhere(), singleWhere()

Returns a collection of elements that satisfy a test.

// See #6 for users list

var over21s = users.where((user) => user[“age”] > 21);

print(over21s.length); // => 1

var nameJ = users.firstWhere((user) => user[“name”].startsWith(‘J’), orElse: () => null);

print(nameJ); // => {name: John, age: 18}

var under18s = users.singleWhere((user) => user[“age”] < 18, orElse: () => null);

print(under18s); // => null

firstWhere() returns the first match in the list, while singleWhere() returns the first match provided there is exactly one match.

#8. take(), skip()

Returns a collection while including or skipping elements

var fiboNumbers = [1, 2, 3, 5, 8, 13, 21];

print(fiboNumbers.take(3).toList()); // => [1, 2, 3]

print(fiboNumbers.skip(5).toList()); // => [13, 21]

print(fiboNumbers.take(3).skip(2).take(1).toList()); // => [3]

#9. List.from()

Creates a new list from the given collection

var clonedFiboNumbers = List.from(fiboNumbers);

print(‘Cloned list: $clonedFiboNumbers’);

As of Dart 2.0, the new keyword is optional when instantiating objects.

#10. expand()

Expands each element into zero or more elements

var pairs = [[1, 2], [3, 4]];

var flattened = pairs.expand((pair) => pair).toList();

print(‘Flattened result: $flattened’); // => [1, 2, 3, 4]

var input = [1, 2, 3];

var duplicated = input.expand((i) => [i, i]).toList();

print(duplicated); // => [1, 1, 2, 2, 3, 3]

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