HH\Map::lazy
Returns a lazy, access elements only when needed view of the current
Map
public function lazy(): KeyedIterable<Tk, Tv>;
Normally, memory is allocated for all of the elements of the Map. With
a lazy view, memory is allocated for an element only when needed or used
in a calculation like in map() or filter().
Guide
Returns
KeyedIterable<Tk,Tv>- aKeyedIterablerepresenting the lazy view into the currentMap.
Examples
This example shows you how to use lazy() on a rather large Map and the time for both a strict and non-strict version. Since we only need 5 of the elements in the end, the lazy view actually allows us to stop after we meet our required 5 without having to actually filter and allocate all 1000000 elements up front.
$map = new Map(\range(0, 1000000));
$s = \microtime(true);
$non_lazy = $map->filter($x ==> $x % 2 === 0)->take(5);
$e = \microtime(true);
\var_dump($non_lazy);
echo "Time non-lazy: ".\strval($e - $s).\PHP_EOL;
// Using a lazy view of the Map can save us a bunch of time, possibly even
// cutting this call time by 90%.
$s = \microtime(true);
$lazy = $map->lazy()->filter($x ==> $x % 2 === 0)->take(5);
$e = \microtime(true);
\var_dump(new Map($lazy));
echo "Time lazy: ".\strval($e - $s).\PHP_EOL;
```.hhvm.expectf
object(HH\Map) (5) {
[0]=>
int(0)
[2]=>
int(2)
[4]=>
int(4)
[6]=>
int(6)
[8]=>
int(8)
}
Time non-lazy: %f
object(HH\Map) (5) {
[0]=>
int(0)
[2]=>
int(2)
[4]=>
int(4)
[6]=>
int(6)
[8]=>
int(8)
}
Time lazy: %f
```.example.hhvm.out
object(HH\Map) (5) {
[0]=>
int(0)
[2]=>
int(2)
[4]=>
int(4)
[6]=>
int(6)
[8]=>
int(8)
}
Time non-lazy: 0.11302947998047
object(HH\Map) (5) {
[0]=>
int(0)
[2]=>
int(2)
[4]=>
int(4)
[6]=>
int(6)
[8]=>
int(8)
}
Time lazy: 0.0068733692169189