# putIfAbsent method

V putIfAbsent (K key, V ifAbsent())
override

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 (key == null) throw new ArgumentError(key);
      int comp = _splay(key);
      if (comp == 0) {
        return _root.value;
      }
      int modificationCount = _modificationCount;
      int splayCount = _splayCount;
      V value = ifAbsent();
      if (modificationCount != _modificationCount) {
        throw new ConcurrentModificationError(this);
      }
      if (splayCount != _splayCount) {
        comp = _splay(key);
        // Key is still not there, otherwise _modificationCount would be changed.
        assert(comp != 0);
      }
      _addNewRoot(new _SplayTreeMapNode(key, value), comp);
      return value;
    }