I'm new to Python and numpy. I've figured out how to slice 1 dimensional sequence: arr[start:end], and access an element in the array: el = arr[row][col].
我是Python和numpy的新手。我已经想出如何切割1维序列：arr [start：end]，并访问数组中的元素：el = arr [row] [col]。
Trying something like slice = arr[0:2][0:2] (where arr is a numpy array) doesn't give me the first 2 rows and columns, but repeats the first 2 rows. What did I just do, and how do I slice along another dimension?
尝试像slice = arr [0：2] [0：2]（其中arr是一个numpy数组）并没有给我前两行和列，但重复前两行。我刚才做了什么，以及如何沿着另一个维度切片？
1 个解决方案
#1
41
If you use numpy, this is easy:
如果你使用numpy，这很容易：
slice = arr[:2,:2]
or if you want the 0's,
或者如果你想要0，
slice = arr[0:2,0:2]
You'll get the same result.
你会得到相同的结果。
*note that slice is actually the name of a builtin-type. Generally, I would advise giving your object a different "name".
*请注意，切片实际上是内置类型的名称。一般来说，我建议给你的对象一个不同的“名字”。
Another way, if you're working with lists of lists*:
另一种方法，如果你正在使用列表列表*：
slice = [arr[i][0:2] for i in range(0,2)]
(Note that the 0's here are unnecessary: [arr[i][:2] for i in range(2)] would also work.).
（注意这里的0是不必要的：[范围（2）中的i的[arr [i] [：2]]也可以。）。
What I did here is that I take each desired row 1 at a time (arr[i]). I then slice the columns I want out of that row and add it to the list that I'm building.
我在这里做的是我每次都获取每个所需的行1（arr [i]）。然后我将我想要的列切出该行并将其添加到我正在构建的列表中。
If you naively try: arr[0:2] You get the first 2 rows which if you then slice again arr[0:2][0:2], you're just slicing the first two rows over again.
如果你天真地尝试：arr [0：2]你获得前两行，如果你再次切片arr [0：2] [0：2]，你只需要再切两行。
*This actually works for numpy arrays too, but it will be slow compared to the "native" solution I posted above.
*这实际上也适用于numpy数组，但与我上面发布的“原生”解决方案相比，它会很慢。
#1
41
If you use numpy, this is easy:
如果你使用numpy，这很容易：
slice = arr[:2,:2]
or if you want the 0's,
或者如果你想要0，
slice = arr[0:2,0:2]
You'll get the same result.
你会得到相同的结果。
*note that slice is actually the name of a builtin-type. Generally, I would advise giving your object a different "name".
*请注意，切片实际上是内置类型的名称。一般来说，我建议给你的对象一个不同的“名字”。
Another way, if you're working with lists of lists*:
另一种方法，如果你正在使用列表列表*：
slice = [arr[i][0:2] for i in range(0,2)]
(Note that the 0's here are unnecessary: [arr[i][:2] for i in range(2)] would also work.).
（注意这里的0是不必要的：[范围（2）中的i的[arr [i] [：2]]也可以。）。
What I did here is that I take each desired row 1 at a time (arr[i]). I then slice the columns I want out of that row and add it to the list that I'm building.
我在这里做的是我每次都获取每个所需的行1（arr [i]）。然后我将我想要的列切出该行并将其添加到我正在构建的列表中。
If you naively try: arr[0:2] You get the first 2 rows which if you then slice again arr[0:2][0:2], you're just slicing the first two rows over again.
如果你天真地尝试：arr [0：2]你获得前两行，如果你再次切片arr [0：2] [0：2]，你只需要再切两行。
*This actually works for numpy arrays too, but it will be slow compared to the "native" solution I posted above.
*这实际上也适用于numpy数组，但与我上面发布的“原生”解决方案相比，它会很慢。