使用reduce（）在python中计算平均值

Question

Given an sequence of numbers, for example, seq = [2, 4, 2, 4] Then calculate its mean value using reduce and lambda in python. I've already know one solution:

def my_mean(seq):
    return reduce(lambda x, y: x + y, seq) / float(len(seq))

And then another idea comes to me:

def my_mean(seq):
    return reduce(lambda x, y: (x * seq.index(y) + y) / float(seq.index(y) + 1))

But the second solution doesn't work. bad case: seq = [2, 4, 2] returns 2.0 while seq = [2, 2, 4] returns 3.0.In fact, the return values should be the same. Could you please give me some hints whats wrong with my second solution? Thank you and sorry for my broken English.

Answer 1

First of all, you're missing the second argument of reduce : seq .

The problem is, index will find the first entry. If you have duplicates, that will mess up the calculation.

You can use enumerate to fix it, but it's simply not worth it..