Enumerate or map through a list with index and value in Dart

Question

In dart there any equivalent to the common:

enumerate(List) -> Iterator((index, value) => f)
or 
List.enumerate()  -> Iterator((index, value) => f)
or 
List.map() -> Iterator((index, value) => f)

It seems that this is the easiest way but it still seems strange that this functionality wouldn't exist.

Iterable<int>.generate(list.length).forEach( (index) => {
  newList.add(list[index], index)
});

Answer 1

There is an asMap method which converts the List to a Map where the keys are the indices and the values are the elements. Please take a look at the documentation for asMap.

Example:

final sample = ['a', 'b', 'c'];
sample.asMap().forEach((index, value) => f(index, value));

---

If you are using Dart 3, you can alternatively use the indexed property. Please take a look at the documentation for indexed.

Example:

final sample = ['a', 'b', 'c'];
for (final (index, item) in sample.indexed) {
  f(index, item);
}

Answer 2

You can use the mapIndexed or forEachIndexed extension methods from the collection package. Note that unlike javascript's array.map() or C#'s IEnumerable.Select(), the index is the first argument, not the second argument of the callback:

import 'package:collection/collection.dart';

void main() {
  final inputs = ['a', 'b', 'c', 'd', 'e', 'f'];
  final indexes = inputs.mapIndexed((index, element) => index).toList();
  
  inputs.forEachIndexed((index, element) {
    print('index: $index, element: $element');
  });

  print(indexes);
}

Live Demo

---

Old answer

Starting with Dart 2.7, you can use extension methods to extend the functionalities of Iterable instead of having to write helper functions:

extension ExtendedIterable<E> on Iterable<E> {
  /// Like Iterable<T>.map but the callback has index as second argument
  Iterable<T> mapIndexed<T>(T Function(E e, int i) f) {
    var i = 0;
    return map((e) => f(e, i++));
  }

  void forEachIndexed(void Function(E e, int i) f) {
    var i = 0;
    forEach((e) => f(e, i++));
  }
}

Usage:

final inputs = ['a', 'b', 'c', 'd', 'e', 'f'];
final results = inputs
  .mapIndexed((e, i) => 'item: $e, index: $i')
  .toList()
  .join('\n');

print(results);

// item: a, index: 0
// item: b, index: 1
// item: c, index: 2
// item: d, index: 3
// item: e, index: 4
// item: f, index: 5

inputs.forEachIndexed((e, i) => print('item: $e, index: $i'));

// item: a, index: 0
// item: b, index: 1
// item: c, index: 2
// item: d, index: 3
// item: e, index: 4
// item: f, index: 5

Live Demo

Answer 3

There is no built-in function to get the iteration index.

If like me you don't like the idea to build a Map (the data structure) just for a simple index, what you probably want is a map (the function) which gives you the index. Let's call it mapIndexed (like in Kotlin):

children: mapIndexed(
  list,
  (index, item) => Text("event_$index")
).toList();

The implementation of mapIndexed is simple:

Iterable<E> mapIndexed<E, T>(
    Iterable<T> items, E Function(int index, T item) f) sync* {
  var index = 0;

  for (final item in items) {
    yield f(index, item);
    index = index + 1;
  }
}

Answer 4

Building on @Hemanth Raj answer.

To convert it back you could do

List<String> _sample = ['a', 'b', 'c'];
_sample.asMap().values.toList(); 
//returns ['a', 'b', 'c'];

Or if you needed the index for a mapping function you could do this:

_sample
.asMap()
.map((index, str) => MapEntry(index, str + index.toString()))
.values
.toList();
// returns ['a0', 'b1', 'c2']

Answer 5

I initially thought ['one', 'two', 'three'].asMap().forEach((index, value) { ... }); would be really inefficient because it looks like it is converting the list to a map. Actually it isn't - the documentation says it creates an immutable view of the list. I double checked with the dart2js of this code:

void main() {
  final foo = ['one', 'two', 'three'];
  foo.asMap().forEach((idx, val) {
    print('$idx: $val');
  });
}

It generates lot of code! But the gist is this:

  main: function() {
    var foo = H.setRuntimeTypeInfo(["one", "two", "three"], ...);
    new H.ListMapView(foo, ...).forEach$1(0, new F.main_closure());
  },

  H.ListMapView.prototype = {
    forEach$1: function(_, f) {
      var t1, $length, t2, i;
      ...
      t1 = this._values;
      $length = t1.length;
      for (t2 = $length, i = 0; i < $length; ++i) {
        if (i >= t2)
          return H.ioore(t1, i);
        f.call$2(i, t1[i]);
        t2 = t1.length;
        if ($length !== t2)
          throw H.wrapException(P.ConcurrentModificationError$(t1));
      }
    },
    ...
  },

  F.main_closure.prototype = {
    call$2: function(idx, val) {
      ...
      H.printString("" + idx + ": " + H.S(val));
    },
    $signature: 1
  };

So it is smart enough to do the efficient thing! Pretty clever.

Of course you can also just use a normal for loop:

for (var index = 0; index < values.length; ++index) {
  final value = values[index];

Answer 6

Use asMap to convert List to map first. The index of element is the key. The element becomes value. Use entries to map the key and value to anything you want.

List rawList = ["a", "b", "c"];
List<String> argList = rawList.asMap().entries.map((e) => '${e.key}:${e.value}').toList();
print(argList);

Output:

[0:a, 1:b, 2:c]

Answer 7

use -> mapIndexed(index, Element) Function

to map each element and its index to a new value.

import 'package:collection/collection.dart';

and use the map index as follows

(List).mapIndexed<Widget>(
 (mapIndex, mapElement) => Positioned(
  left: mapIndex.toDouble() * 5,
  child: Card(
   color: Colors.blue,
    child: Image.network(
     '${mapElement.ImageURL}',
      height: 80,
      width: 80))))

Please Refer: https://pub.dev/documentation/collection/latest/collection/IterableExtension/mapIndexed.html

Answer 8

Lukas Renggli's more package includes many useful tools including 'indexed' which does exactly what you want. From the docs:

indexed(['a', 'b'], offset: 1)
  .map((each) => '${each.index}: ${each.value}')
  .join(', ');

(You can ignore the offset argument unless you have a Smalltalk background :-).

Answer 9

You can use the mapIndexed extension from the collections package:

import 'package:collection/collection.dart';

void main() {
  final nums = [1, 2, 3];
  final strs = nums.mapIndexed((index, element) => index.toString() + '_' + element.toString()).toList();

  print(strs); //  [0_1, 1_2, 2_3]
}

Answer 10

For convenience you can use this extension method.

extension CollectionUtil<T> on Iterable<T>  {

  Iterable<E> mapIndexed<E, T>(E Function(int index, T item) transform) sync* {
    var index = 0;

    for (final item in this) {
      yield transform(index, item as T);
      index++;
    }
  }
}

Answer 11

You can use Iterable.generate factory. The following code would map an Iterable using indexes and values.

extension IterableMapIndex<T> on Iterable<T> {
  Iterable<E> mapIndexed<E>(E f(int index, T t)) {
    return Iterable.generate(this.length, (index)=>f(index, elementAt(index)));
  }
}

Answer 12

Using dart collection package you can access various list extensions

one is mapIndexed:

Iterable<R> mapIndexed<R>(R Function(int, E) convert)

list of all iterable extensions

Answer 13

With records in Dart 3, a new extension property indexed is added to the IterableExtensions which can be used like this:

final iterable = [1,2,3,4];
for(var (index, value) in iterable.indexed) {
    print('$index => $value)
}

Answer 14

package:quiver includes an enumerate function that is similar to Python's enumerate function.

(Note that although package:quiver isn't from the Dart team, it is owned and maintained by Google, and many of its contributors are members of Google's Dart and Flutter teams.)

Answer 15

Here's a solution from 2023 that uses the new Dart 3 record type.

You can use it like this:

for (final (index, item) in enumerate(items)) {
  print('$index: $item');
}

And this is the helper function:

Iterable<(int, T)> enumerate<T>(Iterable<T> items) sync* {
  var index = 0;
  for (final item in items) {
    yield (index, item);
    index++;
  }
}

It is similar to the enumerate function in Python.

Answer 16

You can create another variable to get index.

    int index = 0;

    array?.forEach((element) {
    
    // your code is here

    index++; // you should add this at end of the for loop
    });