how to select random element from Map in dart? - dart

Question

How to select random key (element) from Map?
I can do it using map.keys.toList(), as in the code below, but I wonder if there is more direct way?
import "dart:math";
void main() {
var map = {'a' :1, 'b':2, 'c':3};
final _random = new Random();
var keys = map.keys.toList();
var element = keys[_random.nextInt(keys.length)];
var r = map[element];
print(r);
}

Answer 1

There is no simple way to pick a "random" key from a map.
I assume that "random" here means to pick it uniformly at random among the keys of the map.
For that, you need to pick a random number in the range 0..map.length - 1. Then you need to get the corresponding key. Since Map.key is an iterable, you can't assume that you can do constant-time lookup in it, but you can use elementAt to get a specific iterable item without creating a new list.
So, basically:
randomKey(Map map) =>
map.keys.elementAt(new Random().nextInt(map.length));
(like you do it, but without the toList).
If you need more than one key, you are probably better off converting the keys to a list once, and then do lookups in the list in constant time. Example:
Iterable randomKeys(Map map) sync* {
var keys = map.keys.toList();
var rnd = new Random();
while (keys.length > 0) {
var index = rnd.nextInt(keys.length);
var key = keys[index];
keys[index] = keys.last;
keys.length--;
yield key;
}
}
On top of getting better performance, taking a copy of the keys also avoids concurrent modification errors.

Answer 2

If you are selecting multiple random values from the list and you want to make sure you never select an entry more than once, you can take the keys or values as a list, shuffle it then iterate through it.
This is not very efficient if only a small fraction of the entries in the map are to be selected.
void main() {
var map = { 'a' :1, 'b':2, 'c':3 };
// Keys
var keys = map.keys.toList()..shuffle();
for(var k in keys) {
print('$k, ${map[k]}');
}
// Values
var values = map.values.toList()..shuffle();
for(var v in values) {
print(v);
}
}
https://dartpad.dartlang.org/e49012d93f7451af1662ad113f0aab95

Answer 3

I guess this is not what you're looking for, but actually it's a line shorter ;-)
void main() {
var map = {'a' :1, 'b':2, 'c':3};
final _random = new Random();
var values = map.values.toList();
var element = values[_random.nextInt(values.length)];
print(element);
}
DartPad example

Answer 4

You can use the dart_random_choice package to help you. While Map itself is not an iterable, you can use the Map.keys method to get an iterable and do the following:
import 'package:dart_random_choice/dart_random_choice.dart';
void main() {
var map = { 'a': 1, 'b': 2, 'c':3 };
var r = map[randomChoice(map.keys)];
print(r);
}