Slicing specific rows and columns within a numpy matrix

Question

I have a list of lists that I convert into a matrix.

m = [[0, 2, 1, 0, 0], [0, 0, 0, 3, 4], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]]

n = np.matrix(m)

How do I go about creating a new matrix based on the first three rows and columns?

Specifically, this:

I = [[0, 2, 1], [0, 0, 0], [0, 0, 0]]

I thought the following line would work

I = m[0:3, 0:3]

but I get the error

TypeError: list indices must be integers or slices, not tuple

plasmon360 · Accepted Answer · 2017-04-19 01:01:51Z

3

you are slicing a list instead of a matrix. list cannot take tuple as an argument. Use n[0:3,0:3] instead of m[0:3, 0:3].

answered Apr 19, 2017 at 0:41

plasmon360

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2 Comments

It may benefit the asker to have an explanation of why this is correct.

Looks more like a typo. Why make n and not use it? The subject line also mentions the matrix.

Taejun · Accepted Answer · 2017-04-19 01:49:09Z

3

In addition to user1753919's answer, you can slice the list with following code:

In [10]: [row[0:3] for row in m[0:3]]
Out[10]: [[0, 2, 1], [0, 0, 0], [0, 0, 0]]

But I recommend you to do that with the matrix. Because it's more simple.

answered Apr 19, 2017 at 1:49

Taejun

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