How can I create a list
which contains only zeros? I want to be able to create a zeros list
for each int
in range(10)
For example, if the int
in the range was 4
I will get:
[0,0,0,0]
and for 7
:
[0,0,0,0,0,0,0]
This question is related to
python
zeros=[0]*4
you can replace 4
in the above example with whatever number you want.
Here is the xrange way:
list(0 for i in xrange(0,5))
If you want a function which will return an arbitrary number of zeros in a list, try this:
def make_zeros(number):
return [0] * number
list = make_zeros(10)
# list now contains: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
zlists = [[0] * i for i in range(10)]
zlists[0]
is a list of 0 zeroes, zlists[1]
is a list of 1 zero, zlists[2]
is a list of 2 zeroes, etc.
$ python3
>>> from itertools import repeat
>>> list(repeat(0, 7))
[0, 0, 0, 0, 0, 0, 0]
The easiest way to create a list where all values are the same is multiplying a one-element list by n
.
>>> [0] * 4
[0, 0, 0, 0]
So for your loop:
for i in range(10):
print [0] * i
$python 2.7.8
from timeit import timeit
import numpy
timeit("list(0 for i in xrange(0, 100000))", number=1000)
> 8.173301935195923
timeit("[0 for i in xrange(0, 100000)]", number=1000)
> 4.881675958633423
timeit("[0] * 100000", number=1000)
> 0.6624710559844971
timeit('list(itertools.repeat(0, 100000))', 'import itertools', number=1000)
> 1.0820629596710205
You should use [0] * n
to generate a list with n
zeros.
See why [] is faster than list()
There is a gotcha though, both itertools.repeat
and [0] * n
will create lists whose elements refer to same id
. This is not a problem with immutable objects like integers or strings but if you try to create list of mutable objects like a list of lists ([[]] * n
) then all the elements will refer to the same object.
a = [[]] * 10
a[0].append(1)
a
> [[1], [1], [1], [1], [1], [1], [1], [1], [1], [1]]
[0] * n
will create the list immediately while repeat
can be used to create the list lazily when it is first accessed.
If you're dealing with really large amount of data and your problem doesn't need variable length of list or multiple data types within the list it is better to use numpy
arrays.
timeit('numpy.zeros(100000, numpy.int)', 'import numpy', number=1000)
> 0.057849884033203125
numpy
arrays will also consume less memory.
Source: Stackoverflow.com