Skip to main content

Python: groupbysorted

Updated: It turns out that I was wrong about itertools.groupby. It works exactly the same as this code, so you should use it instead.

This is a variation of itertools.groupby.

The itertools.groupby iterator assumes that the input is not sorted but will fit in memory. This iterator has the same API, but assumes the opposite.

Updated:
__docformat__ = "restructuredtext"


class peekable:

"""Make an iterator peekable.

This is implemented with an eye toward simplicity. On the downside,
you can't do things like peek more than one item ahead in the
iterator. On the bright side, it doesn't require anything from
itertools, etc., so it's less likely to encounter strange bugs,
which occassionally do happen.

Example usage::

>>> numbers = peekable(range(6))
>>> numbers.next()
0
>>> numbers.next()
1
>>> numbers.peek()
2
>>> numbers.next()
2
>>> numbers.next()
3
>>> for i in numbers:
... print i
...
4
5

"""

_None = () # Perhaps None is a valid value.

def __init__(self, iterable):
self._iterable = iter(iterable)
self._buf = self._None

def __iter__(self):
return self

def _is_empty(self):
return self._buf is self._None

def peek(self):
"""Peek at the next element.

This may raise StopIteration.

"""
if self._is_empty():
self._buf = self._iterable.next()
return self._buf

def next(self):
if self._is_empty():
return self._iterable.next()
ret = self._buf
self._buf = self._None
return ret


def groupbysorted(iterable, keyfunc=None):

"""This is a variation of itertools.groupby.

The itertools.groupby iterator assumes that the input is not sorted
but will fit in memory. This iterator has the same API, but assumes
the opposite.

Example usage::

>>> for (key, subiter) in groupbysorted(
... ((1, 1), (1, 2), (2, 1), (2, 3), (2, 9)),
... keyfunc=lambda row: row[0]):
... print "New key:", key
... for x in subiter:
... print "Row:", x
...
New key: 1
Row: (1, 1)
Row: (1, 2)
New key: 2
Row: (2, 1)
Row: (2, 3)
Row: (2, 9)

This requires the peekable class. See my comment here_.

Note, you must completely iterate over each subiter or groupbysorted will
get confused.

.. _here:
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/304373

"""

iterable = peekable(iterable)

if not keyfunc:
def keyfunc(x):
return x

def peekkey():
return keyfunc(iterable.peek())

def subiter():
while True:
if peekkey() != currkey:
break
yield iterable.next()

while True:
currkey = peekkey()
yield (currkey, subiter())

Comments

jjinux said…
Updated: It turns out that I was wrong about itertools.groupby. It works exactly the same as this code, so you should use it instead.

Popular posts from this blog

Ubuntu 20.04 on a 2015 15" MacBook Pro

I decided to give Ubuntu 20.04 a try on my 2015 15" MacBook Pro. I didn't actually install it; I just live booted from a USB thumb drive which was enough to try out everything I wanted. In summary, it's not perfect, and issues with my camera would prevent me from switching, but given the right hardware, I think it's a really viable option. The first thing I wanted to try was what would happen if I plugged in a non-HiDPI screen given that my laptop has a HiDPI screen. Without sub-pixel scaling, whatever scale rate I picked for one screen would apply to the other. However, once I turned on sub-pixel scaling, I was able to pick different scale rates for the internal and external displays. That looked ok. I tried plugging in and unplugging multiple times, and it didn't crash. I doubt it'd work with my Thunderbolt display at work, but it worked fine for my HDMI displays at home. I even plugged it into my TV, and it stuck to the 100% scaling I picked for the othe

ERNOS: Erlang Networked Operating System

I've been reading Dreaming in Code lately, and I really like it. If you're not a dreamer, you may safely skip the rest of this post ;) In Chapter 10, "Engineers and Artists", Alan Kay, John Backus, and Jaron Lanier really got me thinking. I've also been thinking a lot about Minix 3 , Erlang , and the original Lisp machine . The ideas are beginning to synthesize into something cohesive--more than just the sum of their parts. Now, I'm sure that many of these ideas have already been envisioned within Tunes.org , LLVM , Microsoft's Singularity project, or in some other place that I haven't managed to discover or fully read, but I'm going to blog them anyway. Rather than wax philosophical, let me just dump out some ideas: Start with Minix 3. It's a new microkernel, and it's meant for real use, unlike the original Minix. "This new OS is extremely small, with the part that runs in kernel mode under 4000 lines of executable code.&quo

Haskell or Erlang?

I've coded in both Erlang and Haskell. Erlang is practical, efficient, and useful. It's got a wonderful niche in the distributed world, and it has some real success stories such as CouchDB and jabber.org. Haskell is elegant and beautiful. It's been successful in various programming language competitions. I have some experience in both, but I'm thinking it's time to really commit to learning one of them on a professional level. They both have good books out now, and it's probably time I read one of those books cover to cover. My question is which? Back in 2000, Perl had established a real niche for systems administration, CGI, and text processing. The syntax wasn't exactly beautiful (unless you're into that sort of thing), but it was popular and mature. Python hadn't really become popular, nor did it really have a strong niche (at least as far as I could see). I went with Python because of its elegance, but since then, I've coded both p