# Step 2: Build the dictionary and replace rare words with UNK token.
vocabulary_size = 50000
def build_dataset(words, n_words):
  """Process raw inputs into a dataset."""
  count = [['UNK', -1]]
  count.extend(collections.Counter(words).most_common(n_words - 1))
  dictionary = dict()
  for word, _ in count:
    dictionary[word] = len(dictionary)
  data = list()
  unk_count = 0
  for word in words:
    if word in dictionary:
      index = dictionary[word]
    else:
      index = 0  # dictionary['UNK']
      unk_count += 1
    data.append(index)
  count[0][1] = unk_count
  reversed_dictionary = dict(zip(dictionary.values(), dictionary.keys()))
  return data, count, dictionary, reversed_dictionary
data, count, dictionary, reverse_dictionary = build_dataset(vocabulary,
                                                            vocabulary_size)

我正在学习使用Tensorflow的单词向量表示的基本示例。
这个步骤2的标题是“构建字典并用UNK令牌替换罕见单词”，然而，没有关于“UNK”所指的内容的预先定义过程。
要指定问题，请执行以下操作：
0)在NLP中，UNK通常指的是什么？
1)count = 'UNK'，-1是什么意思？我知道括号[]在python中是指list，但是，为什么我们要将它与-1搭配？