Row swapping in np array python

Question

I want to perform a swap between lines in np.array in python. What I want is to take the first line of the array and put it in the end of the array. I have the code that you can swap two rows which is the following:

import numpy as np
my_array = np.arrange(25).reshape(5, 5)
print my_array, '\n'

def swap_rows(arr, frm, to):
   arr[[frm, to],:] = arr[[to, frm],:]

//swap_rows(my_array, 0, 8)
//print my_array
my_array[-1] = my_array[0]
print my_array

But this code performs the swap between first and last row. I want just to put the first line in the end. How can I do so?

The initial matrix is the following:

[[ 0  1  2  3  4]
[ 5  6  7  8  9]
[10 11 12 13 14]
[15 16 17 18 19]
[20 21 22 23 24]]

The desired outcome is the following:

[[ 5  6  7  8  9]
[10 11 12 13 14]
[15 16 17 18 19]
[20 21 22 23 24]
[ 0  1  2  3  4]]

EDIT: I am trying to do the same in my matrix which is the following:

But it doesnt change anything. My code is the following:

initial_data.append(copy.deepcopy(tab)) 
initial_data2 = np.asarray(initial_data)
initial_data3 = np.roll(initial_data2, -1, axis=0)

I am getting the same array.

I add a pic with all my arrays and lists. Actually tab contains a less bracket '['. The initial_data variable is in my posted image from debug. — konstantin
– konstantin, Commented Feb 10, 2017 at 14:15
Initial i have tab which is list of lists. I copy the values into initial_data. Then i convert it into np.asarray: initial_data2 = np.asarray(initial_data) and in the end initial_data3= np.roll(initial_data2,-1,axis=0). But still initial_data3 has the same values. — konstantin
– konstantin, Commented Feb 10, 2017 at 14:25

Divakar · Accepted Answer · 2017-02-10 13:37:38Z

2

You can use np.roll -

np.roll(my_array,-1,axis=0)

Sample run -

In [53]: my_array
Out[53]: 
array([[ 0,  1,  2,  3,  4,  5,  6,  7,  8],
       [ 9, 10, 11, 12, 13, 14, 15, 16, 17],
       [18, 19, 20, 21, 22, 23, 24, 25, 26],
       [27, 28, 29, 30, 31, 32, 33, 34, 35],
       [36, 37, 38, 39, 40, 41, 42, 43, 44],
       [45, 46, 47, 48, 49, 50, 51, 52, 53],
       [54, 55, 56, 57, 58, 59, 60, 61, 62],
       [63, 64, 65, 66, 67, 68, 69, 70, 71],
       [72, 73, 74, 75, 76, 77, 78, 79, 80]])

In [54]: np.roll(my_array,-1,axis=0)
Out[54]: 
array([[ 9, 10, 11, 12, 13, 14, 15, 16, 17],
       [18, 19, 20, 21, 22, 23, 24, 25, 26],
       [27, 28, 29, 30, 31, 32, 33, 34, 35],
       [36, 37, 38, 39, 40, 41, 42, 43, 44],
       [45, 46, 47, 48, 49, 50, 51, 52, 53],
       [54, 55, 56, 57, 58, 59, 60, 61, 62],
       [63, 64, 65, 66, 67, 68, 69, 70, 71],
       [72, 73, 74, 75, 76, 77, 78, 79, 80],
       [ 0,  1,  2,  3,  4,  5,  6,  7,  8]])

answered Feb 10, 2017 at 13:37

Divakar

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3 Comments

konstantin Over a year ago

I tried to do the same, in my list of lists which is converted into an np.array but it didnt work.

konstantin Over a year ago

Check in my edit. I convert the list into np array and then I have the output of the debug. I am getting the same result.

konstantin Over a year ago

No but I dont need to turn it again into list. Is there a chance that this can do the trick?

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