# putIfAbsent method

V putIfAbsent (K key, V ifAbsent())

Look up the value of `key`, or add a new value 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.

    Map<String, int> scores = {'Bob': 36};
    for (var key in ['Bob', 'Rohan', 'Sophena']) {
      scores.putIfAbsent(key, () => key.length);
    }
    scores['Bob'];      // 36
    scores['Rohan'];    //  5
    scores['Sophena'];  //  7

Calling `ifAbsent` must not add or remove keys from the map.

## Implementation

    V putIfAbsent(K key, V ifAbsent()) {
      if (containsKey(key)) {
        return this[key];
      }
      return this[key] = ifAbsent();
    }