Manipulating array elements in NumPy

Question

I have a given array 'a' as follows:

import numpy as np

a = np.arange(-100.0, 110.0, 20.0, dtype=float) #increase 20
a = np.tile(a, 4)
a = a.reshape(4,11)

[[-100.  -80.  -60.  -40.  -20.    0.   20.   40.   60.   80.  100.]
 [-100.  -80.  -60.  -40.  -20.    0.   20.   40.   60.   80.  100.]
 [-100.  -80.  -60.  -40.  -20.    0.   20.   40.   60.   80.  100.]
 [-100.  -80.  -60.  -40.  -20.    0.   20.   40.   60.   80.  100.]]

From array 'a', I need to make new array 'b' which looks as follows:

[[ 100.   80.   60.   40.   20.    0.  -20.  -40.  -60.  -80. -100.]
 [ 100.   80.   60.   40.   20.    0.  -20.  -40.  -60.  -80. -100.]
 [ 100.   80.   60.   40.   20.    0.  -20.  -40.  -60.  -80. -100.]
 [ 100.   80.   60.   40.   20.    0.  -20.  -40.  -60.  -80. -100.]]

Actually, array 'b' is:

b =  np.arange(100.0, -110.0, -20.0, dtype=float)
b = np.tile(b, 4)
b = b.reshape(4,11)

However, in my real problem, the actual data are not fixed, only the index of a i.e., a[0,0] is fixed. Therefore, I have to produce the array 'b' from array 'a' by reshaping/rearranging its elements using indices.

I tried as follows, but could not imagine how to get the correct answer:

b = np.flipud(a)
print b

[[-100.  -80.  -60.  -40.  -20.    0.   20.   40.   60.   80.  100.]
 [-100.  -80.  -60.  -40.  -20.    0.   20.   40.   60.   80.  100.]
 [-100.  -80.  -60.  -40.  -20.    0.   20.   40.   60.   80.  100.]
 [-100.  -80.  -60.  -40.  -20.    0.   20.   40.   60.   80.  100.]]

b = np.rot90(a,1)
print b

[[ 100.  100.  100.  100.]
 [  80.   80.   80.   80.]
 [  60.   60.   60.   60.]
 [  40.   40.   40.   40.]
 [  20.   20.   20.   20.]
 [   0.    0.    0.    0.]
 [ -20.  -20.  -20.  -20.]
 [ -40.  -40.  -40.  -40.]
 [ -60.  -60.  -60.  -60.]
 [ -80.  -80.  -80.  -80.]
 [-100. -100. -100. -100.]]

What numpy functions are suitable for this problem?

Alex Riley · Accepted Answer · 2014-08-27 11:59:35Z

2

You could use np.fliplr:

b = np.fliplr(a)
print b

[[ 100.   80.   60.   40.   20.    0.  -20.  -40.  -60.  -80. -100.]
 [ 100.   80.   60.   40.   20.    0.  -20.  -40.  -60.  -80. -100.]
 [ 100.   80.   60.   40.   20.    0.  -20.  -40.  -60.  -80. -100.]
 [ 100.   80.   60.   40.   20.    0.  -20.  -40.  -60.  -80. -100.]]

This function flips the array in a left/right direction.

The documentation also suggests the following equivalent slice operation:

a[:,::-1]

answered Aug 27, 2014 at 11:59

Alex Riley

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4 Comments

phinn Over a year ago

Thanks +1, sorry I did not used np.fliplr

phinn Over a year ago

Sorry waiting for 15 reputation for +1.

Alex Riley Over a year ago

No problem - glad the answer was helpful!

phinn Over a year ago

Glad to be able to +1 finally!

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