python - 如何使用 sorted() 方法对成对列表进行排序？

Question

Define a function, histo(s) computing the histogram of a given string. Characters must appear in the list ordered from least frequent to most frequent. For example, histo('implemented') is [('t', 1), ('p', 1), ('n', 1), ('l', 1), ('i', 1), ('d', 1), ('m',
2), ('e', 3)]. (Characters with the same frequency must appear in decreasing alphabetical order.) To implement the sorting, use the python built-in function sorted.

这是我的作品:

def keyf(p):
    x,y = p
    return (y,x)

s = "implemented"
d = {}
for c in s:
    d[c] = d.get(c, 0) + 1
dlist = list(d.items())
dlist = sorted(dlist, key = keyf, reverse = False)

在我运行之后，dlist 是 [('d', 1), ('i', 1), ('l', 1), ('n', 1), ('p ', 1), ('t', 1), ('m', 2), ('e', 3)]。问题是具有相同频率的对中的第一个元素不是按降序排列的，我该如何解决这个问题？我试着把

dlist = sorted(dlist, reverse = True)

最后，还是不行。

Answer 1

可以在键函数中反转字符的顺序:

def keyf(p):
    x,y = p
    return (y,-ord(x))