.putIfAbsent()

The .putifabsent() method in Dart is used to insert a key-value pair into a map if the key is not already present. If the key is already present, it does nothing and returns the existing value associated with the key.

Syntax

V putIfAbsent(K key, V ifAbsent())

• key: The key to check for or add to the map.
• ifAbsent: A function that returns the value associated with the key if it is not already in the map.

Example

In this example, the key key3 is not present in the map, so the ifAbsent() function () => 3 is invoked to provide the value 3. The keys key1 and key2 are present so the ifAbsent() function is not invoked and the existing values are returned.

void main() {
  var map = {'key1': 1, 'key2': 2};
  // 'key3' is not present, so the value 3 will be provided by the ifAbsent function
  map.putIfAbsent('key3', () => 3); // {'key1': 1, 'key2': 2, 'key3': 3}
  // 'key1' is present, so the ifAbsent function is not invoked
  print(map.putIfAbsent('key1', () => 5)); // Notice the output is the existing value associated with key1
  print(map); // output {'key1': 1, 'key2': 2, 'key3': 3}
}

Here is the output for the above code:

1
{key1: 1, key2: 2, key3: 3}