putIfAbsent method - Map class - dart:core library (original) (raw)
putIfAbsent abstract method
V putIfAbsent(
- K key,
- V ifAbsent() )
Look up the value of key
, or add a new entry if it isn't there.
Returns the value associated to key
, if there is one. Otherwise calls ifAbsent
to get a new value, associates key
to that value, and then returns the new value.
That is, if the key is currently in the map,map.putIfAbsent(key, ifAbsent)
is equivalent to map[key]
. If the key is not currently in the map, it's instead equivalent to map[key] = ifAbsent()
(but without any guarantee that the []
and []=
operators are actually called to achieve that effect).
final diameters = <num, String>{1.0: 'Earth'};
final otherDiameters = <double, String>{0.383: 'Mercury', 0.949: 'Venus'};
for (final item in otherDiameters.entries) {
diameters.putIfAbsent(item.key, () => item.value);
}
print(diameters); // {1.0: Earth, 0.383: Mercury, 0.949: Venus}
// If the key already exists, the current value is returned.
final result = diameters.putIfAbsent(0.383, () => 'Random');
print(result); // Mercury
print(diameters); // {1.0: Earth, 0.383: Mercury, 0.949: Venus}
The ifAbsent
function is allowed to modify the map, and if so, it behaves the same as the equivalent map[key] = ifAbsent()
.
Implementation
V putIfAbsent(K key, V ifAbsent());