How to declare an array in python

Question

I want to assign values to different indices and not in a sequential manner(by using append), like we would do in a hash table. How to initialize the array of a given size? What I do:

a=[]
for x in range(0,10000):
       a.append(0)

Is there a better way? Also, is there any function like memset() in c++?

Solution 1

As noted in the comments, initializing a list (not an array, that's not the type in Python, though there is an array module for more specialized use) to a bunch of zeroes is just:

a = [0] * 10000

If you want an equivalent to memset for this purpose, say, you want to zero the first 1000 elements of an existing list, you'd use slice assignment:

a[:1000] = [0] * 1000

Solution 2

You can initialize a list of a given size in several ways.

Python has list comprehensions, which create lists from other lists inline. So, if you make a list with 10000 elements (range(10000)) you can easily make from this a list with 10000 zeroes:

[0 for _ in range(10000)]

This is pretty close to your original solution.

Probably a more efficient approach is to multiply a list with a single zero by 10000:

[0]*10000

Both will yield a list with 10000 zeroes.

Solution 3

There is a call to "memset" from CPython:

    from cpython cimport array
    import array

    cdef array.array a = array.array('i', [1, 2, 3])

    # access underlying pointer:
    print a.data.as_ints[0]

    from libc.string cimport memset
    memset(a.data.as_voidptr, 0, len(a) * sizeof(int))