How to declare array of zeros in python (or an array of a certain size)
Question:
I am trying to build a histogram of counts… so I create buckets.
I know I could just go through and append a bunch of zeros i.e something along these lines:
buckets = []
for i in xrange(0,100):
buckets.append(0)
Is there a more elegant way to do it? I feel like there should be a way to just declare an array of a certain size.
I know numpy has numpy.zeros but I want the more general solution
Answers:
buckets = [0] * 100
Careful – this technique doesn’t generalize to multidimensional arrays or lists of lists. Which leads to the List of lists changes reflected across sublists unexpectedly problem
You can multiply a list by an integer n to repeat the list n times:
buckets = [0] * 100
The simplest solution would be
"x00" * size # for a buffer of binary zeros
[0] * size # for a list of integer zeros
In general you should use more pythonic code like list comprehension (in your example: [0 for unused in xrange(100)]) or using string.join for buffers.
Depending on what you’re actually going to do with the data after it’s collected, collections.defaultdict(int) might be useful.
Use this:
bucket = [None] * 100
for i in range(100):
bucket[i] = [None] * 100
OR
w, h = 100, 100
bucket = [[None] * w for i in range(h)]
Both of them will output proper empty multidimensional bucket list 100×100
Just for completeness: To declare a multidimensional list of zeros in python you have to use a list comprehension like this:
buckets = [[0 for col in range(5)] for row in range(10)]
to avoid reference sharing between the rows.
This looks more clumsy than chester1000’s code, but is essential if the values are supposed to be changed later. See the Python FAQ for more details.
Well I would like to help you by posting a sample program and its output
Program :-
t=input("")
x=[None]*t
y=[[None]*t]*t
for i in range(1,t+1):
x[i-1]=i;
for j in range(1,t+1):
y[i-1][j-1]=j;
print x
print y
Output :-
2
[1, 2]
[[1, 2], [1, 2]]
I hope this clears some very basic concept of yours regarding their declaration. To initialize them with some other specific values,like initializing them with 0..you can declare them as :
x=[0]*10
Hope it helps..!! 😉
If you need more columns:
buckets = [[0., 0., 0., 0., 0.] for x in range(0)]
The question says “How to declare array of zeros …” but then the sample code references the Python list:
buckets = [] # this is a list
However, if someone is actually wanting to initialize an array, I suggest:
from array import array
my_arr = array('I', [0] * count)
The Python purist might claim this is not pythonic and suggest:
my_arr = array('I', (0 for i in range(count)))
The pythonic version is very slow and when you have a few hundred arrays to be initialized with thousands of values, the difference is quite noticeable.
I am trying to build a histogram of counts… so I create buckets.
I know I could just go through and append a bunch of zeros i.e something along these lines:
buckets = []
for i in xrange(0,100):
buckets.append(0)
Is there a more elegant way to do it? I feel like there should be a way to just declare an array of a certain size.
I know numpy has numpy.zeros but I want the more general solution
buckets = [0] * 100
Careful – this technique doesn’t generalize to multidimensional arrays or lists of lists. Which leads to the List of lists changes reflected across sublists unexpectedly problem
You can multiply a list by an integer n to repeat the list n times:
buckets = [0] * 100
The simplest solution would be
"x00" * size # for a buffer of binary zeros
[0] * size # for a list of integer zeros
In general you should use more pythonic code like list comprehension (in your example: [0 for unused in xrange(100)]) or using string.join for buffers.
Depending on what you’re actually going to do with the data after it’s collected, collections.defaultdict(int) might be useful.
Use this:
bucket = [None] * 100
for i in range(100):
bucket[i] = [None] * 100
OR
w, h = 100, 100
bucket = [[None] * w for i in range(h)]
Both of them will output proper empty multidimensional bucket list 100×100
Just for completeness: To declare a multidimensional list of zeros in python you have to use a list comprehension like this:
buckets = [[0 for col in range(5)] for row in range(10)]
to avoid reference sharing between the rows.
This looks more clumsy than chester1000’s code, but is essential if the values are supposed to be changed later. See the Python FAQ for more details.
Well I would like to help you by posting a sample program and its output
Program :-
t=input("")
x=[None]*t
y=[[None]*t]*t
for i in range(1,t+1):
x[i-1]=i;
for j in range(1,t+1):
y[i-1][j-1]=j;
print x
print y
Output :-
2
[1, 2]
[[1, 2], [1, 2]]
I hope this clears some very basic concept of yours regarding their declaration. To initialize them with some other specific values,like initializing them with 0..you can declare them as :
x=[0]*10
Hope it helps..!! 😉
If you need more columns:
buckets = [[0., 0., 0., 0., 0.] for x in range(0)]
The question says “How to declare array of zeros …” but then the sample code references the Python list:
buckets = [] # this is a list
However, if someone is actually wanting to initialize an array, I suggest:
from array import array
my_arr = array('I', [0] * count)
The Python purist might claim this is not pythonic and suggest:
my_arr = array('I', (0 for i in range(count)))
The pythonic version is very slow and when you have a few hundred arrays to be initialized with thousands of values, the difference is quite noticeable.