python - 类型错误 : '<' not supported between instances of 'tuple' and 'str'

Question

我有一个构建哈夫曼树的方法如下:

def buildTree(tuples) :
    while len(tuples) > 1 :
        leastTwo = tuple(tuples[0:2])                  # get the 2 to combine
        theRest  = tuples[2:]                          # all the others
        combFreq = leastTwo[0][0] + leastTwo[1][0]     #enter code here the branch points freq
        tuples   = theRest + [(combFreq,leastTwo)]     # add branch point to the end
        tuples.sort()                                  # sort it into place
    return tuples[0]            # Return the single tree inside the list

但是当我为函数提供以下参数时:

[(1, 'b'), (1, 'd'), (1, 'g'), (2, 'c'), (2, 'f'), (3, 'a'), (5, 'e')]

我得到的错误是

  File "<stdin>", line 7, in buildTree
    tuples.sort()
TypeError: '<' not supported between instances of 'tuple' and 'str'

调试时我发现错误在 tuples.sort() 中。

Answer 1

抛出错误是因为您正在以 (priority, (node, node)) 形式创建内部节点。对于相同的优先级，Python 然后尝试将来自叶节点的符号(因此 (priority, symbol) 节点元组中的第二个元素)与 (node, node) 来自内部节点的元组:

>>> inner = (combFreq, leastTwo)
>>> inner
(2, ((1, 'b'), (1, 'd')))
>>> theRest[1]
(2, 'c')
>>> theRest[1] < inner
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: '<' not supported between instances of 'str' and 'tuple'

为了构建霍夫曼树，如果你想对节点数组进行排序，你只需要根据优先级排序，忽略其余元组(符号或子节点):

tuples.sort(key=lambda t: t[0])

通过该更正，您的 buildTree() 函数会生成一棵树:

>>> buildTree([(1, 'b'), (1, 'd'), (1, 'g'), (2, 'c'), (2, 'f'), (3, 'a'), (5, 'e')])
(15, ((6, ((3, 'a'), (3, ((1, 'g'), (2, 'c'))))), (9, ((4, ((2, 'f'), (2, ((1, 'b'), (1, 'd'))))), (5, 'e')))))

就个人而言，我会改用优先级队列，避免每次都排序。参见 How to implement Priority Queues in Python?