嵌套列表中第一个值的总和

在传统的python中，sum函数给出一个list的总和:

In traditional python, the sum function gives the sum of a list:

sum([0,1,2,3,4])=10

另一方面，如果你有一个嵌套列表怎么办:

On the other hand, what if you have a nested list as so:

sum([[1,2,3],[4,5,6],[7,8,9]])

我们发现错误:

Traceback (most recent call last):
  File "<input>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'list'

除此之外，我们如何在嵌套列表中找到第一个值(索引 0)的 sum?如:

In addition to this, how could we find the sum of the first values (index 0) in a nested list? Such as:

something([[1,2,3],[4,5,6],[7,8,9]])=12

要获得所有第一个元素的总和，您需要有一个生成器表达式

To get the sum of all the first elements you need to have a generator expression

>>> a = [[1,2,3],[4,5,6],[7,8,9]]
>>> sum(i[0] for i in a)
12

您得到 unsupported operand type(s) for +: 'int' and 'list' 因为您尝试添加三个列表，这不是所需的行为.

You are getting unsupported operand type(s) for +: 'int' and 'list' because you are trying to add the three lists which is not the desired behavior.

如果您想要一个第一个元素的列表，然后找到它们的总和，您可以尝试使用列表推导

If you want a list of first elements and then find their sum, you can try a list comprehension instead

>>> l = [i[0] for i in a]
>>> l
[1, 4, 7]
>>> sum(l)
12

或者您可以调用 __next__ 方法，因为列表是可迭代的(如果 Py3)

Or you can call the __next__ method as list is an iterable (If Py3)

>>> sum(zip(*a).__next__())
12

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