How to set first N elements of array to zero?

Question

I have a long array:

x= ([2, 5, 4, 7, ...])

for which I need to set the first N elements to 0. So for N = 2, the desired output would be:

x = ([0, 0, 4, 7, ...])

Is there an easy way to do this in Python? Some numpy function?

Indexing and assigning is covered pretty thoroughly in NumPy's [documentation](http://docs.scipy.org/doc/numpy/user/basics.indexing.html). — Alex Riley, Jun 25 '15 at 11:54
possible duplicate of [Python/Numpy: Setting values to index ranges](http://stackoverflow.com/questions/20812787/python-numpy-setting-values-to-index-ranges) — jhoepken, Jun 25 '15 at 11:55

syntonym · Accepted Answer · 2018-09-27T23:01:30.683

40

Pure python:

x[:n] = [0] * n

with numpy:

y = numpy.array(x)
y[:n] = 0

also note that x[:n] = 0 does not work if x is a python list (instead of a numpy array).

It is also a bad idea to use [{some object here}] * n for anything mutable, because the list will not contain n different objects but n references to the same object:

>>> a = [[],[],[],[]]
>>> a[0:2] = [["a"]] * 2
>>> a
[['a'], ['a'], [], []]
>>> a[0].append("b")
>>> a
[['a', 'b'], ['a', 'b'], [], []]

edited Sep 27 '18 at 23:01

answered Jun 25 '15 at 11:52

syntonym

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Note that `0` is the default `start`, and is therefore redundant. – jonrsharpe Jun 25 '15 at 11:53
I guess leaving out the first index looks better. – xrisk Jun 25 '15 at 11:53
1

Well I guess it's a matter of taste. I think `a[0:n]` is more symmetric and "easier" to generalize to `a[x:y]`, but it's probably "more pythonic" to use `a[:n]`. – syntonym Jun 25 '15 at 11:56
Thanks for [0]*n, this is awesome. – Cadoiz Jun 04 '20 at 17:28

score 5 · Answer 2 · answered Jun 25 '15 at 11:54

5

Just set them explicitly:

x[0:2] = 0

answered Jun 25 '15 at 11:54

jhoepken

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you will get a typeerror you can assign only lists to lists. (this is for normal lists in python, dont know about numpy) – bindingofisaac Jun 25 '15 at 12:00
3

And we are talking about numpy here. This works, I checked it in a console. – jhoepken Jun 25 '15 at 12:01
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@bindingofisaac, it's part of Numpy's magic to make vector arithmetic easier. – leewz Jun 25 '15 at 17:15
np_reshaped[0:100] = 32001 sets the first 100 rows to 32001 but I want to set the first 100 columns to 32001, thoughts? – Mona Jalal Aug 05 '17 at 03:19
`np_reshaped.transpose()[0:100] = 32001` or `np_reshaped.T[0:100] = 32001` should switch axes – Zulfiqaar Dec 05 '18 at 13:25

How to set first N elements of array to zero?

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