将两个变量放在python中的for-in循环中是什么意思 [英] what does it mean by putting two variable in a for-in loop in python

问题描述

我正在阅读动手学习机器Scikit-Learn和TensorFlow:构建智能系统的概念，工具和技术.在一个示例中，我在for循环中看到了这种语法.

I am reading Hands-On Machine Learning with Scikit-Learn and TensorFlow: Concepts, Tools, and Techniques to Build Intelligent Systems. In an example, I see this syntax in for loop.

from sklearn.model_selection import StratifiedShuffleSplit

split = StratifiedShuffleSplit(n_splits=1, test_size=0.2, random_state=42)
for train_index, test_index in split.split(housing, housing["income_cat"]):
    strat_train_set = housing.loc[train_index]
    strat_test_set = housing.loc[test_index]

我打印出train_index和test_index，它们是索引数组. 这个for循环是什么意思? train_index和test_index具有不同数量的元素，迭代如何工作? 这段代码是否等同于下面的代码?

I printed out the train_index and test_index, they are array of indices. What does this for loop mean? The train_index and test_index have different number of elements, how does iteration work? Is this code equivalent to the below?

from sklearn.model_selection import StratifiedShuffleSplit

split = StratifiedShuffleSplit(n_splits=1, test_size=0.2, random_state=42)
train_index, test_index = split.split(housing, housing["income_cat"]):
strat_train_set = housing.loc[train_index]
strat_test_set = housing.loc[test_index]

推荐答案

下面是一个简单的for循环中包含2个变量的情况:

Here's a simple case of 2 variables in a for loop:

In [173]: for a,b in [[0,1],[10,12]]:
     ...:     print(a,b)
     ...:     
0 1
10 12

如果出于以下原因而工作:

If works for the same reason that:

In [174]: a,b = [10,12]

迭代返回某种元组或列表，并且a,b in ...将2个值解压缩到匹配数量的变量中.

The iteration returns some sort of tuple or list, and the a,b in ... unpacks the 2 values into the the matching number of variables.

for i, v in enumerate(['a','b','c']):
    print(i,v)

是循环解包的另一种常见用法.

is another common use of unpacking in a loop.

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