Splitting a List into N Parts of Approximately Equal Length

Splitting a list into N parts of approximately equal length

This code is broken due to rounding errors. Do not use it!!!

assert len(chunkIt([1,2,3], 10)) == 10  # fails

Here's one that could work:

def chunkIt(seq, num):
    avg = len(seq) / float(num)
    out = []
    last = 0.0

    while last < len(seq):
        out.append(seq[int(last):int(last + avg)])
        last += avg

    return out

Testing:

>>> chunkIt(range(10), 3)
[[0, 1, 2], [3, 4, 5], [6, 7, 8, 9]]
>>> chunkIt(range(11), 3)
[[0, 1, 2], [3, 4, 5, 6], [7, 8, 9, 10]]
>>> chunkIt(range(12), 3)
[[0, 1, 2, 3], [4, 5, 6, 7], [8, 9, 10, 11]]

How to divide a list into n equal parts, python

One-liner returning a list of lists, given a list and the chunk size:

>>> lol = lambda lst, sz: [lst[i:i+sz] for i in range(0, len(lst), sz)]

Testing:

>>> x = range(20, 36)
>>> print x
[20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35]

>>> lol(x, 4)
[[20, 21, 22, 23], 
 [24, 25, 26, 27], 
 [28, 29, 30, 31], 
 [32, 33, 34, 35]]

>>> lol(x, 7)
[[20, 21, 22, 23, 24, 25, 26], 
 [27, 28, 29, 30, 31, 32, 33], 
 [34, 35]]

Update:

I think the question is really asking is a function which, given a list and a number, returns a list containing $(number) lists, with the items of the original list evenly distributed. So your example of lol(x, 7) should really return [[20,21,22], [23,24,25], [26,27], [28,29], [30,31], [32,33], [34,35]]. – markrian

Well, in this case, you can try:

def slice_list(input, size):
    input_size = len(input)
    slice_size = input_size / size
    remain = input_size % size
    result = []
    iterator = iter(input)
    for i in range(size):
        result.append([])
        for j in range(slice_size):
            result[i].append(iterator.next())
        if remain:
            result[i].append(iterator.next())
            remain -= 1
    return result

I'm sure this can be improved but I'm feeling lazy. :-)

>>> slice_list(x, 7)
[[20, 21, 22], [23, 24, 25], 
 [26, 27], [28, 29], 
 [30, 31], [32, 33], 
 [34, 35]]

How do I split a list into equally-sized chunks?

Here's a generator that yields evenly-sized chunks:

def chunks(lst, n):
    """Yield successive n-sized chunks from lst."""
    for i in range(0, len(lst), n):
        yield lst[i:i + n]

import pprint
pprint.pprint(list(chunks(range(10, 75), 10)))
[[10, 11, 12, 13, 14, 15, 16, 17, 18, 19],
 [20, 21, 22, 23, 24, 25, 26, 27, 28, 29],
 [30, 31, 32, 33, 34, 35, 36, 37, 38, 39],
 [40, 41, 42, 43, 44, 45, 46, 47, 48, 49],
 [50, 51, 52, 53, 54, 55, 56, 57, 58, 59],
 [60, 61, 62, 63, 64, 65, 66, 67, 68, 69],
 [70, 71, 72, 73, 74]]

For Python 2, using xrange instead of range:

def chunks(lst, n):
    """Yield successive n-sized chunks from lst."""
    for i in xrange(0, len(lst), n):
        yield lst[i:i + n]

Below is a list comprehension one-liner. The method above is preferable, though, since using named functions makes code easier to understand. For Python 3:

[lst[i:i + n] for i in range(0, len(lst), n)]

For Python 2:

[lst[i:i + n] for i in xrange(0, len(lst), n)]