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

问题

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?

回答1:

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);

回答2:

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);

回答3:

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);

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

回答4:

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.

回答5:

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);

回答6:

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

来源：https://stackoverflow.com/questions/10405348/what-is-the-cleanest-way-to-get-the-sum-of-numbers-in-a-collection-list-in-dart

标签

iteration

dart