How to transform an object into an array

Question

I am learning python and I am having a little bit of trouble while handling an object. I have tried to look for a solution but it led nowhere, so I am asking you guys.

I want to get the first X columns of an object, but I can't as it doesn't have the same size in every row.

I have this object:

array([[45, 45, 45, 50, 51, 50, 50, 50, 51, 50, 52],
       [45, 45, 45, 50, 51, 50, 50, 50, 51, 50, 51, 52, 55],
       [45, 45, 45, 50, 51, 50, 52, 50, 50, 50, 51],
       [50, 51, 52, 55, 50, 52, 50, 50, 50, 51, 50, 51]], dtype=object)

And I would like to get something like this:

array([[45, 45, 45, 50],
       [45, 45, 45, 50],
       [45, 45, 45, 50],
       [50, 51, 52, 55]])

What could I do to solve this? Thanks for your help

Alvaro

You have a 1d 4 element array of dtype=object. Each element is a list. This is basically the same as a list of lists; and it's usually best to treat it as such. The array wrapper does not add much, and may even slow things down. — hpaulj
– hpaulj, Commented Oct 29, 2016 at 17:33

Guillaume Jacquenot · Accepted Answer · 2016-10-29 17:04:37Z

3

What about

import numpy as np
data = np.array([[45, 45, 45, 50, 51, 50, 50, 50, 51, 50, 52],
                 [45, 45, 45, 50, 51, 50, 50, 50, 51, 50, 51, 52, 55],
                 [45, 45, 45, 50, 51, 50, 52, 50, 50, 50, 51],
                 [50, 51, 52, 55, 50, 52, 50, 50, 50, 51, 50, 51]], dtype=object)
newData = np.array([d[:4] for d in data])

answered Oct 29, 2016 at 17:04

Guillaume Jacquenot

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1 Comment

Alvaro Silva Over a year ago

Thanks, that works! Do you think there is any way to do it without a for? For example like an array function or something like that?

ettanany · Accepted Answer · 2016-10-29 17:27:16Z

You may need a more generic solution where you can also specifiy the number of rows as follows:

import numpy as np

arr = np.array([[45, 45, 45, 50, 51, 50, 50, 50, 51, 50, 52],
       [45, 45, 45, 50, 51, 50, 50, 50, 51, 50, 51, 52, 55],
       [45, 45, 45, 50, 51, 50, 52, 50, 50, 50, 51],
       [50, 51, 52, 55, 50, 52, 50, 50, 50, 51, 50, 51]], dtype=object)

def slice_array(arr, num_cols, num_rows=None):
    if num_rows:
        return np.array([row[:num_cols] for row in arr[:num_rows]])
    return np.array([row[:num_cols] for row in arr])

res1 = slice_array(arr, 4, 2)
res2 = slice_array(arr, 4)  # Like in your case

Results:

>>> res1
array([[45, 45, 45, 50],
       [45, 45, 45, 50]])
>>> res2
array([[45, 45, 45, 50],
       [45, 45, 45, 50],
       [45, 45, 45, 50],
       [50, 51, 52, 55]])

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