flutter - how do calculate average the data in list? - list

Question

I have a List<Comment> about food from users.
It looks like this:
[
{userId: 1,
rating: 4.5
},
{userId: 2,
rating: 4.0
},
{userId: 3,
rating: 3.5
},
{userId: 4,
rating: 3.0
}
...
];
I want to get the average rating. AVERAGE = Number of ratings : total user, how do I apply this in dart?

Answer 1

var values = [
{'userId': 1, 'rating': 4.5},
{'userId': 2, 'rating': 4.0},
{'userId': 3, 'rating': 3.5},
{'userId': 4, 'rating': 3.0}
];
var result = values.map((m) => m['rating']).reduce((a, b) => a + b) / values.length;
print(result);

Answer 2

In 2021 dart has the built-in average method from the collection package which can be used on an Iterable.
import 'package:collection/collection.dart';
void main() {
var values = [
{'userId': 1, 'rating': 4.5},
{'userId': 2, 'rating': 4.0},
{'userId': 3, 'rating': 3.5},
{'userId': 4, 'rating': 3.0}
];
var result = values.map((m) => m['rating']!).average;
print(result); // prints 3.75
}
You might also be interested in the .sum method from the same package.

Answer 3

I did this by adding the values with the fold function and then dividing by the total. Here is an example:
var values = [
{'userId': 1, 'rating': 4.5},
{'userId': 2, 'rating': 4.0},
{'userId': 3, 'rating': 3.5},
{'userId': 4, 'rating': 3.0}
];
List<double> ratings = values.map((e) => e.price!).toList();
double sum = ratings.fold(0, (p, c) => p + c);
if (sum > 0) {
double average = sum / ratings.length;
}