How to count items' occurence in a List

Question

I am new to Dart. Currently I have a List of duplicate items, and I would like to count the occurence of them and store it in a Map.

var elements = ["a", "b", "c", "d", "e", "a", "b", "c", "f", "g", "h", "h", "h", "e", "a"];

I want to have a result like:

{
  "a": 3,
  "b": 2,
  "c": 2,
  "d": 2,
  "e": 2,
  "f": 1,
  "g": 1,
  "h": 3
}

I did some research and found a JavaScript solution, but I don't know how to translate it to Dart.

var counts = {};
your_array.forEach(function(x) { counts[x] = (counts[x] || 0)+1; });

Answer 1

Play around with this:

  var elements = ["a", "b", "c", "d", "e", "a", "b", "c", "f", "g", "h", "h", "h", "e"];
  var map = Map();

  elements.forEach((element) {
    if(!map.containsKey(element)) {
      map[element] = 1;
    } else {
      map[element] +=1;
    }
  });

  print(map);

What this does is:

• loops through list elements
• if your map does not have list element set as a key, then creates that element with a value of 1
• else, if element already exists, then adds 1 to the existing key value

Or if you like syntactic sugar and one liners try this one:

  var elements = ["a", "b", "c", "d", "e", "a", "b", "c", "f", "g", "h", "h", "h", "e"];
  var map = Map();

  elements.forEach((x) => map[x] = !map.containsKey(x) ? (1) : (map[x] + 1));

  print(map);

There are many ways to achieve this in all programming languages!

Answer 2

The shorter way to count items' occurrence in a List

List of items. Count items equal 1.

List<int> lst = [0,1,1,1,0,8,8,9,1,0];
int res = lst.map((element) => element == 1 ? 1 : 0).reduce((value, element) => value + element);

List of objects. Count objects, which property age equals 1.

class Person {
   int age;
   
   Person(this.age);
}

List<Person> lst2 = [Person(1), Person(0), Person(1), Person(0)];
int res2 = lst2.map((element) => element.age == 1 ? 1 : 0).reduce((value, element) => value + element);