問題描述
我正在做這個代碼練習以嘗試在 python 中進行函數式編程，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 應該是一個地圖對象，包含(或產生)兩個現有高度條目的列表:[160, 80].
heights should be a map object containing (or resulting in) a list of the two existing height entries: [160, 80].
打印長度應該是2，兩者的和顯然應該是240，平均應該是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() 函數導致了它.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:
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()
所以我不知道發生了什么.list() 函數不應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
實際上，調用 list 確實會改變你的 map 對象.map 是一個迭代器，而不是一個數據結構(在 Python3 中).在它被循環一次之后，它就被耗盡了.要重現結果，您需要重新創建地圖對象.
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建一個列表來存儲結果，然后執行 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.
您可以使用統計模塊直接從迭代器計算算術平均值(和其他統計數據).查看文檔.
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