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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: save dictionary to a file without brackets.
Date: Thu, 09 Aug 2012 17:46:29 -0400
References: <930ab3d8-4ab9-446d-9970-ee811eb70a44@googlegroups.com> <F1B463BB-19A6-4DB1-99B3-929CCBFB5920@gmail.com> <50241F14.2060209@tim.thechases.com> <36EA3847-6713-4C12-B47B-9B5E10325F00@gmail.com> <502429C3.5000600@tim.thechases.com>
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On 8/9/2012 5:21 PM, Tim Chase wrote:
> On 08/09/12 15:41, Roman Vashkevich wrote:
>> 10.08.2012, =D0=B2 0:35, Tim Chase =D0=BD=D0=B0=D0=BF=D0=B8=D1=81=D0=B0=
=D0=BB(=D0=B0):
>>> On 08/09/12 15:22, Roman Vashkevich wrote:
>>>>> {(4, 5): 1, (5, 4): 1, (4, 4): 2, (2, 3): 1, (4, 3): 2}
>>>>> and i want to print to a file without the brackets comas and semico=
lon in order to obtain something like this?
>>>>> 4 5 1
>>>>> 5 4 1
>>>>> 4 4 2
>>>>> 2 3 1
>>>>> 4 3 2
>>>>
>>>> for key in dict:
>>>> 	print key[0], key[1], dict[key]
>>>
>>> This might read more cleanly with tuple unpacking:
>>>
>>>   for (edge1, edge2), cost in d.iteritems(): # or .items()
>>>     print edge1, edge2, cost
>>>
>>> (I'm making the assumption that this is a edge/cost graph...use
>>> appropriate names according to what they actually mean)
>>
>> dict.items() is a list - linear access time whereas with 'for
>> key in dict:' access time is constant:
>> http://python.net/~goodger/projects/pycon/2007/idiomatic/handout.html#=
use-in-where-possible-1
>
> That link doesn't actually discuss dict.{iter}items()
>
> Both are O(N) because you have to touch each item in the dict--you
> can't iterate over N entries in less than O(N) time.  For small
> data-sets, building the list and then iterating over it may be
> faster faster; for larger data-sets, the cost of building the list
> overshadows the (minor) overhead of a generator.  Either way, the
> iterate-and-fetch-the-associated-value of .items() & .iteritems()
> can (should?) be optimized in Python's internals to the point I
> wouldn't think twice about using the more readable version.

In 3.x, .keys, .values, and .items are set-like read-only views=20
specifically designed for iteration. So in 3.x they are THE way to do so =

for whichever alternative is appropriate. Iterating by keys and then=20
looking up values instead of yielding the values at the same time is=20
extra work.

--=20
Terry Jan Reedy