问题描述
嗨:我正在尝试以自定义方式对元组列表进行排序:
例如:
lt = [(2,4), (4,5), (5,2)]必须排序:
lt = [(5,2), (2,4), (4,5)]规则:
* 如果 a[1] == b[0]
，则 b 元组大于元组* 如果 a[0] == b[1] 则 a 元组大于 b 元组
我已经实现了一个这样的 cmp 函数:
def tcmp(a, b):如果 a[1] == b[0]:返回 -1elif a[0] == b[1]:返回 1别的:返回 0但对列表进行排序:
lt.sort(tcmp)给我看:
lt = [(2, 4), (4, 5), (5, 2)]我做错了什么?
我不确定您的比较函数在数学意义上是否有效，即传递性.给定 a, b, c 一个比较函数，表示 a >b 和 b >c 意味着 a >c.排序过程依赖于这个属性.
更不用说根据你的规则，对于 a = [1, 2] 和 b = [2, 1] 你有两个 a[1] == b[0] 和 a[0] == b[1] 这意味着 a 既大于也小于 b.p>
Hi:
I'm trying to sort a list of tuples in a custom way:
For example:
lt = [(2,4), (4,5), (5,2)]
must be sorted:
lt = [(5,2), (2,4), (4,5)]
Rules:
* b tuple is greater than a tuple if a[1] == b[0]
* a tuple is greater than b tuple if a[0] == b[1]
I've implemented a cmp function like this:
def tcmp(a, b):
if a[1] == b[0]:
return -1
elif a[0] == b[1]:
return 1
else:
return 0
but sorting the list:
lt.sort(tcmp)
lt show me:
lt = [(2, 4), (4, 5), (5, 2)]
What am I doing wrong?
I'm not sure your comparison function is a valid one in a mathematical sense, i.e. transitive. Given a, b, c a comparison function saying that a > b and b > c implies that a > c. Sorting procedures rely on this property.
Not to mention that by your rules, for a = [1, 2] and b = [2, 1] you have both a[1] == b[0] and a[0] == b[1] which means that a is both greater and smaller than b.
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