How do I get an empty array of any size in python?

Question

I basically want a python equivalent of this in C:

int a[x];

but in python I declare an array like:

a = []

and the problem is I want to assign random slots with values like:

a[4] = 1

but I can't do that with python, since the array is empty.

Answer 1

If by "array" you actually mean a Python list, you can use

a = [0] * 10

or

a = [None] * 10

Answer 2

You can't do exactly what you want in Python (if I read you correctly). You need to put values in for each element of the list (or as you called it, array).

But, try this:

a = [0 for x in range(N)]  # N = size of list you want
a[i] = 5  # as long as i < N, you're okay

For lists of other types, use something besides 0. None is often a good choice as well.

Answer 3

You can use numpy:

import numpy as np

Example from Empty Array:

np.empty([2, 2])
array([[ -9.74499359e+001,   6.69583040e-309],
       [  2.13182611e-314,   3.06959433e-309]])  

Answer 4

also you can extend that with extend method of list.

a= []
a.extend([None]*10)
a.extend([None]*20)

Answer 5

Just declare the list and append each element. For ex:

a = []
a.append('first item')
a.append('second item')

Answer 6

If you (or other searchers of this question) were actually interested in creating a contiguous array to fill with integers, consider bytearray and memoryivew:

# cast() is available starting Python 3.3
size = 10**6 
ints = memoryview(bytearray(size)).cast('i') 

ints.contiguous, ints.itemsize, ints.shape
# (True, 4, (250000,))

ints[0]
# 0

ints[0] = 16
ints[0]
# 16

Answer 7

If you actually want a C-style array

import array
a = array.array('i', x * [0])
a[3] = 5
try:
   [5] = 'a'
except TypeError:
   print('integers only allowed')

Note that there's no concept of un-initialized variable in python. A variable is a name that is bound to a value, so that value must have something. In the example above the array is initialized with zeros.

However, this is uncommon in python, unless you actually need it for low-level stuff. In most cases, you are better-off using an empty list or empty numpy array, as other answers suggest.