• Ruby

m = a.group_by(&:id)

• C#

var m = a.ToDictionary(e => e.id, e => e);

• Dart

final m = {
  for (final e in a) e.id: e,
};

• Go

m := make(map[K]V, len(a))
for _, e := range a {
	m[e.id] = e
}

• JS

let m = new Map(a.map(e => [e.id, e]))

• Java

Map<Integer, E> m = new HashMap<>();
for (E e : a) m.put(e.id, e);

• Java

Map<Integer, E> m = new HashMap<>();
a.forEach(e -> m.put(e.id, e));

• Java

Map<Integer, E> m = a.stream()
    .collect(toMap(e -> e.id, e -> e));

• Lua

local m={}
for _,e in ipairs(a) do
 m[e.id]=e
end

• Pascal

type
  TList = specialize TFPGList<TData>;
  TMap = specialize TFPGMap<TKey, TData>;

var
  a: TList;
  m: TMap;
  idx: integer;

begin
  ....
  m := TMap.Create;
  for idx := 0 to a.count-1 do
  begin
    m.add(a.items[idx].e, a.items[idx]);
  end;
  ....
  m.Free;
  ....

• Perl

foreach my $e (@a) {
    $m{$e->id) = $e;
}

• Python

m = dict((x.id, x) for x in a)

• Python

m = {e.id:e for e in a}

• Rust

let mut m = HashMap::new();

for e in a {
	m.entry(e.id).or_insert_with(Vec::new).push(e);
}