How to sort an integer array in-place in Python?

Question

how can one sort an integer array (not a list) in-place in Python 2.6? Is there a suitable function in one of the standard libraries?

In other words, I'm looking for a function that would do something like this:

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

Thanks in advance!

Why have you ruled out using a list? Did you profile and find it lacking? If you must use an array and need it sorted, I suggest using numpy, which has a sort method for arrays. — Steven Rumbalski
– Steven Rumbalski, Commented Apr 4, 2011 at 14:51
@Steven Yes, I have profiled it. I'm working on large collections of integers that barely fit in RAM. Lists of integers are at least 3 times larger than arrays of integers, so I cannot use them. Could you (or anyone else) point to a relevant function in SciPy/NumPy? — Bolo
– Bolo, Commented Apr 4, 2011 at 14:59
Start at docs.scipy.org/doc/numpy/reference/routines.sort.html. Note that the default sort returns a copy, but ndarray.sort sorts in-place. (I have no experience with SciPy/NumPy, but do know that it is well respected and highly optimized.) — Steven Rumbalski
– Steven Rumbalski, Commented Apr 4, 2011 at 15:03

Andrew Jaffe · Accepted Answer · 2011-04-04 15:00:36Z

10

Well, you can't do it with array.array, but you can with numpy.array:

In [3]: a = numpy.array([0,1,3,2], dtype=numpy.int)

In [4]: a.sort()

In [5]: a
Out[5]: array([0, 1, 2, 3])

Or you can convert directly from an array.array if you have that already:

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

answered Apr 4, 2011 at 15:00

Andrew Jaffe

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Steven Rumbalski Over a year ago

And this appears to be an in-place sort. Good answer.

lysdexia · Accepted Answer · 2011-04-04 15:01:08Z

2

@steven mentioned numpy.

Copies vs. in-place operation
-----------------------------
Most of the functions in `numpy` return a copy of the array argument
(e.g., `sort`).  In-place versions of these functions are often
available as array methods, i.e. ``x = np.array([1,2,3]); x.sort()``.
Exceptions to this rule are documented.

answered Apr 4, 2011 at 15:01

lysdexia

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GreenMatt · Accepted Answer · 2011-04-04 15:13:49Z

1

Looking at the array docs, I don't see a method for sorting. I think the following is about as close as you can get using standard functions, although it is really clobbering the old object with a new one with the same name:

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

Or, you could write your own.

With the extra information from the comments that you're maxing out memory, this seems inapplicable for your situation; the numpy solution is the way to go. However, I'll leave this up for reference.

edited Apr 4, 2011 at 15:13

answered Apr 4, 2011 at 15:01

GreenMatt

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