Insert array index into transposed array

Question

Suppose I have the array:

[[2,1,5,2], 
 [1,4,2,1],
 [4,5,5,7],
 [1,5,9,3]]

I am trying to transpose the array to shape (16, 3) where the first two elements in the resulting array are the index numbers, and the last is the value. eg:

[[0, 0, 2], [1, 0, 1], [2, 0, 5], [3, 0, 2], [0, 1, 4], ....]

Is this possible with a numpy function or similar? Or I have to do this with my own function?

Working example code:

import numpy as np

src = np.array([[2,1,5,2],
                [1,4,2,1],
                [4,5,5,7],
                [1,5,9,3]])

dst = np.array([])


for x in range(src.shape[0]):
    for y in range(src.shape[1]):
        dst = np.append(dst, [[y, x, src[x][y]]])

print(dst.reshape(16,3))

Note on terminology: "transpose" on an array means to swap the rows and columns. This isn't what you are asking here. — Code-Apprentice, Sep 19 '19 at 16:48

score 1 · Answer 1 · answered Sep 19 '19 at 16:49

I don't know if there is a function in numpy for that, but you can use list comprehension to easily build that array:

import numpy as np

src = np.array([[2,1,5,2],
                [1,4,2,1],
                [4,5,5,7],
                [1,5,9,3]])

dst = np.array([ [y, x, src[x][y]] 
                 for x in range(src.shape[0]) 
                 for y in range(src.shape[1])])

print(dst.reshape(16,3))

Hope this can help.

pault · Accepted Answer · 2019-09-19T17:05:02.120

Update: There is a numpy function for that:

You can use numpy.ndenumerate:

dst = np.array([[*reversed(x), y] for x, y in np.ndenumerate(src)])
print(dst)
#[[0 0 2]
# [1 0 1]
# [2 0 5]
# [3 0 2]
# [0 1 1]
# [1 1 4]
# [2 1 2]
# [3 1 1]
# [0 2 4]
# [1 2 5]
# [2 2 5]
# [3 2 7]
# [0 3 1]
# [1 3 5]
# [2 3 9]
# [3 3 3]]

ndenumerate will return an iterator yielding pairs of array coordinates and values. You will first need to reverse the coordinates for your desired output. Next unpack the coordinates into a list¹ with the value and use a list comprehension to consume the iterator.

Original Answer

You can try:

dst = np.column_stack(zip(*[*reversed(np.indices(src.shape)), src])).T
print(dst)
#[[0 0 2]
# [1 0 1]
# [2 0 5]
# [3 0 2]
# [0 1 1]
# [1 1 4]
# [2 1 2]
# [3 1 1]
# [0 2 4]
# [1 2 5]
# [2 2 5]
# [3 2 7]
# [0 3 1]
# [1 3 5]
# [2 3 9]
# [3 3 3]]

Explanation

First, use numpy.indices to get an array representing the indices of a grid with the shape of src.

print(np.indices(src.shape))
#[[[0 0 0 0]
#  [1 1 1 1]
#  [2 2 2 2]
#  [3 3 3 3]]
#
# [[0 1 2 3]
#  [0 1 2 3]
#  [0 1 2 3]
#  [0 1 2 3]]]

We can reverse these (since that's the order you want in your final output), and unpack into a list¹ that also contains src.

Then zip all of the elements of this list to get the (col, row, val) triples. We can stack these together using numpy.column_stack.

list(zip(*[*reversed(np.indices(src.shape)), src]))
#[(array([0, 1, 2, 3]), array([0, 0, 0, 0]), array([2, 1, 5, 2])),
# (array([0, 1, 2, 3]), array([1, 1, 1, 1]), array([1, 4, 2, 1])),
# (array([0, 1, 2, 3]), array([2, 2, 2, 2]), array([4, 5, 5, 7])),
# (array([0, 1, 2, 3]), array([3, 3, 3, 3]), array([1, 5, 9, 3]))]

Finally transpose (numpy.ndarray.T) to get the final output.

Notes:

Unpacking into a list is only available in python 3.5+

Insert array index into transposed array

2 Answers2