I have a numpy array, for example
a = np.arange(10)
how can I move the first n elements to the end of the array?
I found this roll function but it seems like it only does the opposite, which shifts the last n elements to the beginning.
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LWZ
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2Isn't `roll` exactly what you want? Instead of moving the first `n` elements to the end, move the last `len(a)-n` elements to the beginning? – Sajjan Singh Apr 03 '13 at 16:09
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Post some data of what you want vs. what you are finding. – BlackVegetable Apr 03 '13 at 16:10
2 Answers
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Why not just roll with a negative number?
>>> import numpy as np
>>> a = np.arange(10)
>>> np.roll(a,2)
array([8, 9, 0, 1, 2, 3, 4, 5, 6, 7])
>>> np.roll(a,-2)
array([2, 3, 4, 5, 6, 7, 8, 9, 0, 1])
mgilson
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you can use negative shift
a = np.arange(10)
print(np.roll(a, 3))
print(np.roll(a, -3))
returns
[7, 8, 9, 0, 1, 2, 3, 4, 5, 6]
[3, 4, 5, 6, 7, 8, 9, 0, 1, 2]
Francesco Montesano
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Can you tell me ,why circshift (eye (m), 2) and numpy.roll(numpy.eye(m), -2)) yield different result? – Gunjan naik Sep 02 '15 at 12:36
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11.) what is circshift? 2) comments it's not the correct place to ask questions disconnected with the question – Francesco Montesano Sep 02 '15 at 15:06
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@FrancescoMontesano `circshift` is a [MATLAB function](https://nl.mathworks.com/help/matlab/ref/circshift.html). – Zoltán Csáti May 22 '20 at 09:53
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I don't know Matlab.My wild guess is: index from 0 or 1; column- vs. row-first ordering; ... – Francesco Montesano May 22 '20 at 10:50
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@Gunjannaik numpy.roll uses a peculiar default behaviour for 'axis': "By default, the array is flattened before shifting, after which the original shape is restored.". You'll probably need axis=(0,1) to have the behaviour of the two functions match. – Giorgos Sfikas May 17 '22 at 05:15