What is the cleanest way to get the sum of numbers in a collection/list in Dart?

I don't like using an indexed array for no reason other than I think it looks ugly. Is there a clean way to sum with an anonymous function? Is it possible to do it without using any outside variables?

Dart iterables now have a reduce function (https://code.google.com/p/dart/issues/detail?id=1649), so you can do a sum pithily without defining your own fold function:

var sum = [1, 2, 3].reduce((a, b) => a + b);

int sum = [1, 2, 3].fold(0, (previous, current) => previous + current);

or with shorter variable names to make it take up less room:

int sum = [1, 2, 3].fold(0, (p, c) => p + c);

This is a very old question but

In 2022 there is actually a built-in package.

Just import

import 'package:collection/collection.dart';

and call the .sum extension method on the Iterable.

FULL EXAMPLE

import 'package:collection/collection.dart';

void main() {
  final list = [1, 2, 3, 4];
  final sum = list.sum;
  print(sum); // prints 10
}

If the list is empty, .sum returns 0.

You might also be interested in list.average...

I still think this is cleaner and easier to understand for this particular problem.

num sum = 0;
[1, 2, 3].forEach((num e){sum += e;});
print(sum);

or

num sum = 0;
for (num e in [1,2,3]) {
  sum += e;
}

There is not a clean way to do it using the core libraries as they are now, but if you roll your own foldLeft then there is

main() {
  var sum = foldLeft([1,2,3], 0, (val, entry) => val + entry);
  print(sum);
}

Dynamic foldLeft(Collection collection, Dynamic val, func) {
  collection.forEach((entry) => val = func(val, entry));
  return val;
}

I talked to the Dart team about adding foldLeft to the core collections and I hope it will be there soon.

Starting with Dart 2.6 you can use extensions to define a utility method on the List. This works for numbers (example 1) but also for generic objects (example 2).

extension ListUtils<T> on List<T> {
  num sumBy(num f(T element)) {
    num sum = 0;
    for(var item in this) {
      sum += f(item);
    }
    return sum;
  }
}

Example 1 (sum all the numbers in the list):

var numbers = [1, 2, 3];
var sum = numbers.sumBy((number) => number);

Example 2 (sum all the Point.x fields):

var points = [Point(1, 2), Point(3, 4)];
var sum = points.sumBy((point) => point.x);

I suggest you to create this function in any common utility file.

T sum<T extends num>(T lhs, T rhs) => lhs + rhs;

int, double, float extends num class so you can use that function to sum any numbers.

e.g.,

List<int> a = [1,2,3];
int result = a.reduce(sum);
print(result); // result will be 6

Herewith sharing my Approach:

void main() {
  int value = sumTwo([1, 4, 3, 43]);
  print(value);
}

int sumTwo(List < int > numbers) {
  int sum = 0;

  for (var i in numbers) {
    sum = sum + i;
  }
  return sum;
}

I'd just like to add some small detail to @tmaihoff's answer (about using the collection.dart package):

The sum getter he talks about only works for iterables of num values, like List<int> or Set<double>.

If you have a list of other object types that represent values (like Money, Decimal, Rational, or any others) you must map it to numbers. For example, to count the number of chars in a list of strings you can do:

// Returns 15.
['a', 'ab', 'abc', 'abcd', 'abcde'].map((e) => e.length).sum;

As of 2022, another way of doing it, is using the sumBy() method of the fast_immutable_collections package:

// Returns 15.
['a', 'ab', 'abc', 'abcd', 'abcde'].sumBy((e) => e.length), 15);

Note: I'm the package author.

If you are planning on doing a number of mathematical operations on your list, it may be helpful to create another list type that includes .sum() and other operations by extending ListBase. Parts of this are inspired by this response with performance tweaks from this response.

import 'dart:collection';
import 'dart:core';

class Vector<num> extends ListBase<num> {
  
  List<num> _list;
  Vector() : _list = new List<num>();
  Vector.fromList(List<num> lst): _list = lst;
  
  void set length(int l) {
    this._list.length=l;
  }

  int get length => _list.length;
  
  num operator [](int index) => _list[index];

  void operator []=(int index, num value) {
    _list[index]=value;
  }
  // Though not strictly necessary, for performance reasons
  // you should implement add and addAll.

  void add(num value) => _list.add(value);

  void addAll(Iterable<num> all) => _list.addAll(all);
  
  num sum() => _list.fold(0.0, (a, b) => a + b) as num;
  /// add additional vector functions here like min, max, mean, factorial, normalize etc

}

And use it like so:

Vector vec1 = Vector();
vec1.add(1); 
print(vec1); // => [1]
vec1.addAll([2,3,4,5]); 
print(vec1); // => [1,2,3,4,5]
print(vec1.sum().toString()); // => 15
Vector vec = Vector.fromList([1.0,2.0,3.0,4.0,5.0]); // works for double too.
print(vec.sum().toString()); // => 15

If when using fold gives a double TypeError, you can use reduce:

var sum = [0.0, 4.5, 6.9].reduce((a, b) => a + b);

A solution that has worked cleanly for me is:

var total => [1,2,3,4].fold(0, (e, t) => e + t); // result 10

Different ways to find the sum of all dart list elements,

Method 1: Using a loop : This is the most commonly used method. Iterate through the list using a loop and add all elements of the list to a final sum variable. We are using one for loop here :

main(List<String> args) {
  var sum = 0;
  var given_list = [1, 2, 3, 4, 5];

  for (var i = 0; i < given_list.length; i++) {
    sum += given_list[i];
  }

  print("Sum : ${sum}");
}

Method 2: Using forEach : forEach is another way to iterate through a list. We can also use this method to find out the total sum of all values in a dart list. It is similar to the above method. The only difference is that we don’t have to initialize another variable i and list.length is not required.

main(List<String> args) {
  var sum = 0;
  var given_list = [1, 2, 3, 4, 5];

  given_list.forEach((e) => sum += e);

  print("Sum : ${sum}");
}

Method 3: Using reduce : reduce method combines all elements of a list iteratively to one single value using a given function. We can use this method to find out the sum of all elements as like below :

main(List<String> args) {
  var given_list = [1, 2, 3, 4, 5];

  var sum = given_list.reduce((value, element) => value + element);

  print("Sum : ${sum}");
}

Method 4: Using fold : fold() is similar to reduce. It combines all elements of a list iteratively to one single value using a function. It takes one initial value and calculates the final value based on the previous value.

main(List<String> args) {
  var sum = 0;
  var given_list = [1,2,3,4,5];

  sum = given_list.fold(0, (previous, current) => previous + current);

  print("Sum : ${sum}");
}

for more details:https://www.codevscolor.com/dart-find-sum-list-elements