read_key_or_create.dart

Map<String, int> map = {};
int id = 0;

int getIdOrCreate(String key) {
  if (map.containsKey(key)) {
    return map[key];
  } else {
    id++;
    map[key] = id;
    return id;
  }
}

int getIdOrCreate_short(String key) {
  return map[key] ??= ++id;
}

void main() {
  print(getIdOrCreate('a') == 1);
  print(getIdOrCreate('a') == 1);

  print(getIdOrCreate_short('b') == 2);
  print(getIdOrCreate_short('b') == 2);
}