Python:next()函数 [英] Python: next() function

问题描述

我正在从一本书中学习Python，并且遇到了以下示例:

I'm learning Python from a book, and I came across this example:

M = [[1,2,3],
     [4,5,6],
     [7,8,9]]

G = (sum(row) for row in M) # create a generator of row sums
next(G) # Run the iteration protocol

由于我是一个绝对的初学者，并且作者没有提供有关示例或next()函数的任何解释，所以我不理解代码在做什么.

Since I'm an absolute beginner, and the author hasn't provided any explanation of the example or the next() function, I don't understand what the code is doing.

推荐答案

表达式(sum(row) for row in M)创建所谓的生成器.该生成器将对M中的每一行一次计算表达式(sum(row)).但是，生成器尚未执行任何操作，我们已经对其进行了设置.

The expression (sum(row) for row in M) creates what's called a generator. This generator will evaluate the expression (sum(row)) once for each row in M. However, the generator doesn't do anything yet, we've just set it up.

语句next(G)实际上在M上运行 生成器.因此，如果您运行一次next(G)，您将获得第一行的总和.如果再次运行它，将得到第二行的总和，依此类推.

The statement next(G) actually runs the generator on M. So, if you run next(G) once, you'll get the sum of the first row. If you run it again, you'll get the sum of the second row, and so on.

>>> M = [[1,2,3],
...      [4,5,6],
...      [7,8,9]]
>>> 
>>> G = (sum(row) for row in M) # create a generator of row sums
>>> next(G) # Run the iteration protocol
6
>>> next(G)
15
>>> next(G)
24

另请参见:

• 关于生成器的文档
• 关于收益表达式的文档(以及有关生成器的一些信息)

See also:

• Documentation on generators
• Documentation on yield expressions (with some info about generators)

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