CODEWITHSUNDEEP
pythonnumpy
brainless
brainless

Reputation: 89

Python with numpy: How to delete an element from each row of a 2-D array according to a specific index

Say I have a 2-D numpy array A of size 20 x 10.

I also have an array of length 20, del_ind.

I want to delete an element from each row of A according to del_ind, to get a resultant array of size 20 x 9.

How can I do this?

I looked into np.delete with a specified axis = 1, but this only deletes element from the same position for each row.

Thanks for the help

Upvotes: 5

Views: 1465

Answers (3)

Denizantip
Denizantip

Reputation: 1

If you need to remove each element of a row according to its index in the row, you can achieve this in Python using NumPy. For example, if you want to delete the maximum element in each row, you can follow the steps below:

import numpy as np

# Your 2D NumPy array
a = np.random.random((10, 3))

# Find indices of max elements along each row
max_idx = a.argmax(axis=1) + (np.arange(a.shape[0]) * a.shape[1])

# Delete elements at specified indices
a = np.delete(a, max_idx).reshape(a.shape[0], -1)

In this example, max_idx is calculated by adding the row-wise maximum indices to the corresponding row indices adjusted for the array shape. The np.delete function is then used to remove elements at these indices, and the array is reshaped to the expected shape.

Upvotes: -1

wim
wim

Reputation: 362687

You will probably have to build a new array.

Fortunately you can avoid python loops for this task, using fancy indexing:

h, w = 20, 10
A = np.arange(h*w).reshape(h, w)
del_ind = np.random.randint(0, w, size=h)
mask = np.ones((h,w), dtype=bool)
mask[range(h), del_ind] = False
A_ = A[mask].reshape(h, w-1)

Demo with a smaller dataset:

>>> h, w = 5, 4
>>> %paste
A = np.arange(h*w).reshape(h, w)
del_ind = np.random.randint(0, w, size=h)
mask = np.ones((h,w), dtype=bool)
mask[range(h), del_ind] = False
A_ = A[mask].reshape(h, w-1)

## -- End pasted text --
>>> A
array([[ 0,  1,  2,  3],
       [ 4,  5,  6,  7],
       [ 8,  9, 10, 11],
       [12, 13, 14, 15],
       [16, 17, 18, 19]])
>>> del_ind
array([2, 2, 1, 1, 0])
>>> A_
array([[ 0,  1,  3],
       [ 4,  5,  7],
       [ 8, 10, 11],
       [12, 14, 15],
       [17, 18, 19]])

Upvotes: 5

user2993124
user2993124

Reputation: 51

Numpy isn't known for inplace edits; it's mainly intended for statically sized matrices. For that reason, I'd recommend doing this by copying the intended elements to a new array.

Assuming that it's sufficient to delete one column from every row:

def remove_indices(arr, indices):
    result = np.empty((arr.shape[0], arr.shape[1] - 1)) 

    for i, (delete_index, row) in enumerate(zip(indices, arr)):
        result[i] = np.delete(row, delete_index)

    return result

Upvotes: 0

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