lists as an efficient implementation of large two-dimensional arrays(!)

Gerald Britton gerald.britton at gmail.com
Tue Feb 2 21:36:45 CET 2010


Did you try it with an array object using the array module?

On Tue, Feb 2, 2010 at 3:14 PM, Mitchell L Model <MLMDev at comcast.net> wrote:
> An instructive lesson in YAGNI ("you aren't going to need it"), premature
> optimization, and not making assumptions about Python data structure
> implementations.
>
> I need a 1000 x 1000 two-dimensional array of objects. (Since they are
> instances of application classes it appears that the array module is
> useless; likewise, since I am using Python 3.1, so among other things, I
> can't use numpy or its relatives.) The usage pattern is that the array is
> first completely filled with objects. Later, objects are sometimes accessed
> individually by row and column and often the entire array is iterated over.
>
> Worried (unnecessarily, as it turns out) by the prospect of 1,000,000
> element list I started by constructing a dictionary with the keys 1 through
> 1000, each of which had as its value another dictionary with the keys 1
> through 1000. Actual values were the values of the second level dictionary.
>
> Using numbers to fill the array to minimize the effect of creating my more
> complex objects, and running Python 3.1.1 on an 8-core Mac Pro with 8Gb
> memory, I tried the following
>
> #create and fill the array:
> t1 = time.time()
> d2 = {}
> for j in range(1000):
>    d2[j] = dict()
>    for k in range(1000):
>        d2[j][k] = k
> print( round(time.time() - t1, 2))
>
> 0.41
>
> # access each element of the array:
> t1 = time.time()
> for j in range(1000):
>    for k in range(1000):
>        elt = d2[j][k]
> print( round(time.time() - t1, 2))
>
> 0.55
>
>  My program was too slow, so I started investigating whether I could improve
> on the two-level dictionary, which got used a lot. To get another baseline I
> tried a pure 1,000,000-element list, expecting the times to be be
> horrendous, but look!
>
> # fill a list using append
> t1 = time.time()
> lst = []
> for n in range(1000000):
>    lst.append(n)
> print( round(time.time() - t1, 2))
>
> 0.26
>
> # access every element of a list
> t1 = time.time()
> for n in range(1000000):
>    elt = lst[n]
> print( round(time.time() - t1, 2))
>
> 0.25
>
> What a shock! I could save half the execution time and all my clever work
> and awkward double-layer dictionary expressions by just using a list!
>
> Even better, look what happens using a comprehension to create the list
> instead of a loop with list.append:
>
> t1 = time.time()
> lst = [n for n in range(1000000)]
> print( round(time.time() - t1, 2))
>
> 0.11
>
> Half again to create the list.
>
> Iterating over the whole list is easier and faster than iterating over the
> double-level dictionary, in particular because it doesn't involve a
> two-level loop. But what about individual access given a row and a column?
>
> t1 = time.time()
> for j in range(1000):
>    for k in range(1000):
>        elt = lst[j * 1000 + k]
> print( round(time.time() - t1, 2))
>
> 0.45
>
> This is the same as for the dictionary.
>
> I tried a two-level list and a few other things but still haven't found
> anything that works better than a single long list -- just like 2-D arrays
> are coded in old-style languages, with indices computed as offsets from the
> beginning of the linear sequence of all the values. What's amazing is that
> creating and accessing 1,000,000-element list in Python is so efficient. The
> usual moral obtains: start simple, analyze problems (functional or
> performance) as they arise, decide whether they are worth the cost of
> change, then change in very limited ways. And of course abstract and
> modularize so that, in my case, for example, none of the program's code
> would be affected by the change from a two-level dictionary representation
> to using a single long list.
>
> I realize there are many directions an analysis like this can follow, and
> many factors affecting it, including patterns of use. I just wanted to
> demonstrate the basics for a situation that I just encountered. In
> particular, if the array was sparse, rather than completely full, the
> two-level dictionary implementation would be the natural representation.
> --
> http://mail.python.org/mailman/listinfo/python-list
>



-- 
Gerald Britton



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