Split array into smaller arrays.

Question

I am looking for a way to easily split a python array in half.

So that if I have an array:

A = [0,1,2,3,4,5]

I would be able to get:

B = [0,1,2]

C = [3,4,5]

Answer 1

def splitter(A):
    B = A[0:len(A)//2]
    C = A[len(A)//2:]

 return (B,C)

I tested, and the double slash is required to force int division in python 3. My original post was correct, although wysiwyg broke in Opera, for some reason.

Answer 2

A = [1,2,3,4,5,6]
B = A[:len(A)/2]
C = A[len(A)/2:]

If you want a function:

def split_list(a_list):
    half = len(a_list)/2
    return a_list[:half], a_list[half:]

A = [1,2,3,4,5,6]
B, C = split_list(A)

Answer 3

B,C=A[:len(A)/2],A[len(A)/2:]

Answer 4

Using list slicing. The syntax is basically my_list[start_index:end_index]

>>> i = [0,1,2,3,4,5]
>>> i[:3] # same as i[0:3] - grabs from first to third index (0->2)
[0, 1, 2]
>>> i[3:] # same as i[3:len(i)] - grabs from fourth index to end
[3, 4, 5]

To get the first half of the list, you slice from the first index to len(i)/2...

>>> i[:len(i)/2]
[0, 1, 2]

..and the swap the values around to get the second half:

>>> i[len(i)/2:]
[3, 4, 5]

Answer 5

A little more generic solution (you can specify the number of parts you want, not just split 'in half'):

EDIT: updated post to handle odd list lengths

EDIT2: update post again based on Brians informative comments

def split_list(alist, wanted_parts=1):
    length = len(alist)
    return [ alist[i*length // wanted_parts: (i+1)*length // wanted_parts] 
             for i in range(wanted_parts) ]

A = [0,1,2,3,4,5,6,7,8,9]

print split_list(A, wanted_parts=1)
print split_list(A, wanted_parts=2)
print split_list(A, wanted_parts=8)

Answer 6

While the answers above are more or less correct, you may run into trouble if the size of your array isn't divisible by 2, as the result of a / 2, a being odd, is a float in python 3.0, and in earlier version if you specify from __future__ import division at the beginning of your script. You are in any case better off going for integer division, i.e. a // 2, in order to get "forward" compatibility of your code.

Answer 7

f = lambda A, n=3: [A[i:i+n] for i in range(0, len(A), n)]

f(A)

n - the predefined length of result arrays