Moving the first element to end in Numpy array

Question

I have a NumPy array, which I want to move the first element to the end. I know how to do it by converting to the list and do the insert operation but I want to avoid this and do it in a better and optimized way.

Suppose this is my array and I want to move 0 to the end:

x = [[ 0 388.13 19.346 412.38 39.594]
    [ 0 388.15 8.5168 416 38.972] 
    [ 0 223.14 156.21 317.91 193.46]]

I know I can do it in this way by converting to the list:

X = X.tolist()
for ele in x:
    ele.insert(len(ele), ele.pop(0))

Is there any way to do it without converting it to a list (using numpy)? and also, the way I'm doing is not efficient, What is a possible way to make my code efficient in term of speed even by converting it to list?

Expected output:

x = [[388.13 19.346 412.38 39.594 0]
    [388.15 8.5168 416 38.972 0] 
    [223.14 156.21 317.91 193.46 0]]

You can store the first element in temp variable then do np.delete() then np.append() just remember to reshape your array. — WinterSoldier
– WinterSoldier, Commented Nov 15, 2020 at 19:25
One way or other you'll have to copy all values of the array to a new location, either in this array or a new one. numpy does not a use a linked list storage where such a move would just require changing a few pointers. — hpaulj
– hpaulj, Commented Nov 15, 2020 at 22:05

mathfux · Accepted Answer · 2020-11-15 19:22:43Z

11

Try np.roll :np.roll(x, -1, axis=1).tolist()

answered Nov 15, 2020 at 19:22

mathfux

5,9792 gold badges20 silver badges38 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Onyambu · Accepted Answer · 2020-11-15 19:28:28Z

6

you could also try np.hstack:

np.hstack([x[:, 1:], x[:, :1]])

or even np.concatenate:

np.concatenate([x[:, 1:], x[:, :1]],  axis=1)

or even np.c_

np.c_[x[:, 1:], x[:, :1]]

all the above gives you:

array([[388.13  ,  19.346 , 412.38  ,  39.594 ,   0.    ],
       [388.15  ,   8.5168, 416.    ,  38.972 ,   0.    ],
       [223.14  , 156.21  , 317.91  , 193.46  ,   0.    ]])

edited Nov 15, 2020 at 19:28

answered Nov 15, 2020 at 19:24

Onyambu

80.3k3 gold badges29 silver badges65 bronze badges

1 Comment

s3dev Over a year ago

You beat me to it! Well played.

Collectives™ on Stack Overflow

Moving the first element to end in Numpy array

2 Answers 2

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related