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pythonarraysnumpy
waspinator
waspinator

Reputation: 6826

Flip or reverse columns in numpy array

I want to flip the first and second values of arrays in an array. A naive solution is to loop through the array. What is the right way of doing this?

import numpy as np
contour = np.array([[1, 4],
                    [3, 2]])

flipped_contour = np.empty((0,2))
for point in contour:
    x_y_fipped = np.array([point[1], point[0]])
    flipped_contour = np.vstack((flipped_contour, x_y_fipped))

print(flipped_contour)

[[4. 1.]
[2. 3.]]

Upvotes: 10

Views: 22579

Answers (6)

Sem_jong
Sem_jong

Reputation: 61

Another way would be using permutation matrices

import numpy as np

contour = np.array([[1, 4],
                [3, 2]])

P = np.array([[0, 1],
         [1, 0]])#permutation matrix that will 
                 #swap column 1 with column 2 if right multiplied

contour = contour @ P
print(contour)

Output:

array([[4, 1],
   [2, 3]])

Upvotes: 0

Omri Alcobi
Omri Alcobi

Reputation: 1

>>> your_array[indices_to_flip] = np.flip(your_array[indices_to_flip], axis=1)

Upvotes: 0

ducminh
ducminh

Reputation: 1352

In addition to COLDSPEED's answer, if we only want to swap the first and second column only, not to flip the entire array:

contour[:, :2] = contour[:, 1::-1]

Here contour[:, 1::-1] is the array formed by first two columns of the array contour, in the reverse order. It then is assigned to the first two columns (contour[:, :2]). Now the first two column are swapped.

In general, to swap the ith and jth columns, do the following:

contour[:, [i, j]] = contour[:, [j, i]]

Upvotes: 7

Paul Panzer
Paul Panzer

Reputation: 53029

Here are two non-inplace ways of swapping the first two columns:

>>> a = np.arange(15).reshape(3, 5)
>>> a[:, np.r_[1:-1:-1, 2:5]]
array([[ 1,  0,  2,  3,  4],
       [ 6,  5,  7,  8,  9],
       [11, 10, 12, 13, 14]])

or

>>> np.c_[a[:, 1::-1], a[:, 2:]]
array([[ 1,  0,  2,  3,  4],
       [ 6,  5,  7,  8,  9],
       [11, 10, 12, 13, 14]])

Upvotes: 3

jpp
jpp

Reputation: 164703

You can use numpy indexing:

contour[:, ::-1]

Upvotes: 7

cs95
cs95

Reputation: 402613

Use the aptly named np.flip:

np.flip(contour, axis=1)

Or,

np.fliplr(contour)


array([[4, 1],
       [2, 3]])

Upvotes: 17

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