問題描述

我正在做這個代碼練習(xí)以嘗試在 python 中進(jìn)行函數(shù)式編程，但我遇到了 sum()、list() 和 map 對象的問題.我不明白我做錯了什么，但 list() 函數(shù)似乎與我的地圖對象搞砸了.

I'm doing this code excercise for trying out functional programming in python, and I ran into problems with sum(), list(), and map objects. I don't understand what I'm doing wrong, but the list() function seems to be screwing with my map object.

這是我的代碼:

people = [{'name': 'Mary', 'height': 160},
          {'name': 'Isla', 'height': 80},
          {'name': 'Sam'}]

heights = map(lambda x: x['height'], filter(lambda x: 'height' in x, people))

print(len(list(heights)))
print(sum(list(heights)))
print(len(list(heights)))

average_height = sum(list(heights)) / len(list(heights))
print(average_height)

heights 應(yīng)該是一個地圖對象，包含(或產(chǎn)生)兩個現(xiàn)有高度條目的列表:[160, 80].

heights should be a map object containing (or resulting in) a list of the two existing height entries: [160, 80].

打印長度應(yīng)該是2，兩者的和顯然應(yīng)該是240，平均應(yīng)該是120.

printing the length should give 2, the sum of the two should obviously be 240, and the average should be 120.

不過，我面臨的問題是我收到以下錯誤消息:

The problem I'm facing, though, is that I get this error message:

2
0
0
Traceback (most recent call last):
  File "C:UsersHugoDropboxProgrammeringPythonProjektexercise2.py", line 12, in <module>
    average_height = sum(list(heights)) / len(list(heights))
ZeroDivisionError: division by zero

是的，長度是對的，但是總和是0，第二個長度打印也是0.整個零除錯誤必須來自那里的某些東西，并且似乎是 list() 函數(shù)導(dǎo)致了它.更改打印順序仍然只能讓第一個打印語句正確:

Yes, the length is right, but the sum is 0, and the second length print is 0 aswell. The whole zero-division fault must come from something there, and it seems that the list() function is causing it. Changing the print order still only lets through the first print statement correctly:

print(sum(list(heights)))
print(len(list(heights)))
print(len(list(heights)))

給予:

240
0
0
Traceback (most recent call last):
  File "C:UsersHugoDropboxProgrammeringPythonProjektexercise2.py", line 12, in <module>
    average_height = sum(list(heights)) / len(list(heights))
ZeroDivisionError: division by zero

并刪除 list() 函數(shù):

and removing the list() function:

print(sum(list(heights)))
print(len(heights))
print(len(list(heights)))

給我:

240
Traceback (most recent call last):
  File "C:UsersHugoDropboxProgrammeringPythonProjektexercise2.py", line 9, in <module>
    print(len(heights))
TypeError: object of type 'map' has no len()

所以我不知道發(fā)生了什么.list() 函數(shù)不應(yīng)該以任何方式更改地圖對象，對吧?它仍然是一個地圖對象，但不止一次調(diào)用 list() 似乎會改變它的行為.我很困惑.

So I have no idea what's going on. The list() function should not be changing the map object in any way, right? And it stays a map object, but calling list() on it more than once seems to change its behaviour. I am confuse.

推薦答案

實(shí)際上，調(diào)用 list 確實(shí)會改變你的 map 對象.map 是一個迭代器，而不是一個數(shù)據(jù)結(jié)構(gòu)(在 Python3 中).在它被循環(huán)一次之后，它就被耗盡了.要重現(xiàn)結(jié)果，您需要重新創(chuàng)建地圖對象.

Actually, calling list does change your map object. map is an iterator, not a data structure (in Python3). After it has been looped over once, it is exhausted. To reproduce the result, you need to recreate the map object.

在您的情況下，您可能想要做的是從地圖創(chuàng)建一個列表來存儲結(jié)果，然后執(zhí)行 sum.

In your case, what you probably want to do is create a list from the map to store the result, then perform the sum.

heights = list(heights)
average = sum(heights) / len(heights)

另一種方法.

您可以使用統(tǒng)計(jì)模塊直接從迭代器計(jì)算算術(shù)平均值(和其他統(tǒng)計(jì)數(shù)據(jù)).查看文檔.

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