[Tutor] Extending a list within a list comprehension
Hi all,
I have solved my problem, but would like to know if what I accomplished
can be done with different data using list comprehensions.
the list I want to sort has the following format:
elements = [
(codigo, [ cant, importe, porc]),
(codigo, [ cant, importe, porc]),
...
]
Actual data is:
In [129]: elements[0:5]
Out[129]:
[('2712', [22.0, 3618.80999, 0.0032389476163069883]),
('2713', [19.0, 6551.81004, 0.0058640739309320719]),
('2710', [21.0, 2553.57999, 0.0022855336019435113]),
('2716', [19.0, 8215.27004, 0.0073529224203034461]),
('4305', [4.0, 348.37, 0.00031180199598565978])]
And I want to sort descending on 'importe', which is x[1][1] for x in
elements.
What I did was the following, following the Schwarzian Transform:
temporal = []
temporal = [ [x[1][1], (x[0], description[x[0]],
x[1][0], x[1][1], x[1][2] ) ] for x in elements ]
temporal.sort()
temporal.reverse() # sort descending
elements = [ x[1] for x in temporal ]
If the second element in each array passed as x is of variable length
(that is, it has a different element count than three, in this case),
the program needs to extend the list instead. Without list
comprehensions, and the added capability to utilize and sized list as a
second element, my code ended up looking like the following:
temporal = []
for x in elements:
lst = [x[0], description[x[0]]]
lst.extend(x[1])
temporal.append([x[1][1], lst])
temporal.sort()
temporal.reverse() # sort descending
elements = [ x[1] for x in temporal ]
Is there a way to use list comprehensions to append or extend the array
as needed by the second code listing?
Thanks.
Victor
___
Tutor maillist - Tutor@python.org
http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Extending a list within a list comprehension
On 12/04/06, Victor Bouffier [EMAIL PROTECTED] wrote: elements = [ (codigo, [ cant, importe, porc]), (codigo, [ cant, importe, porc]), ... ] And I want to sort descending on 'importe', which is x[1][1] for x in elements. In python 2.4, you could achieve this by saying: elements.sort(key=lambda x: x[1][1]) In earlier versions of python, you can do: elements.sort(lambda x, y: cmp(x[1][1], y[1][1])) (but this is less efficient than key= in 2.4) There's also the decorate-sort-undecorate idiom: dec = [(x[1][1], x) for x in elements] dec.sort() elements = [x[1] for x in dec] -- John. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Extending a list within a list comprehension
Hi Victor, I've gotta say that I much prefer the second version here. temporal = [] temporal = [ [x[1][1], (x[0], description[x[0]], x[1][0], x[1][1], x[1][2] ) ] for x in elements ] temporal.sort() temporal.reverse() # sort descending elements = [ x[1] for x in temporal ] temporal = [] for x in elements: lst = [x[0], description[x[0]]] lst.extend(x[1]) temporal.append([x[1][1], lst]) temporal.sort() temporal.reverse() # sort descending elements = [ x[1] for x in temporal ] That looks a lot easier to rtead (and debug) and will I suspect be easier to maintain. Copmprehensions are great but unfortunately can rapidly become incomprehensible! Is there a way to use list comprehensions to append or extend the array as needed by the second code listing? There may be but I wouldn't try. I might however look at using a simple list or dictionary of objects based on a class... It might be easier. Alan G ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Extending a list within a list comprehension
On Tue, 2006-04-11 at 23:42 +0100, Alan Gauld wrote: Hi Victor, I've gotta say that I much prefer the second version here. temporal = [] temporal = [ [x[1][1], (x[0], description[x[0]], x[1][0], x[1][1], x[1][2] ) ] for x in elements ] temporal.sort() temporal.reverse() # sort descending elements = [ x[1] for x in temporal ] temporal = [] for x in elements: lst = [x[0], description[x[0]]] lst.extend(x[1]) temporal.append([x[1][1], lst]) temporal.sort() temporal.reverse() # sort descending elements = [ x[1] for x in temporal ] That looks a lot easier to rtead (and debug) and will I suspect be easier to maintain. Copmprehensions are great but unfortunately can rapidly become incomprehensible! Is there a way to use list comprehensions to append or extend the array as needed by the second code listing? There may be but I wouldn't try. I might however look at using a simple list or dictionary of objects based on a class... It might be easier. Alan G Hi Alan, I believe you are right. The most pythonic way is not always the most perlish way ;-) (no offense intended to Perl-comers, but Perl does tend to promote nesting of expressions). It is much easier to read and maintain, which is what I will need to do eventually. Thanks for the feedback. Victor ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Extending a list within a list comprehension
Victor Bouffier wrote: If the second element in each array passed as x is of variable length (that is, it has a different element count than three, in this case), the program needs to extend the list instead. Without list comprehensions, and the added capability to utilize and sized list as a second element, my code ended up looking like the following: temporal = [] for x in elements: lst = [x[0], description[x[0]]] lst.extend(x[1]) temporal.append([x[1][1], lst]) temporal.sort() temporal.reverse() # sort descending elements = [ x[1] for x in temporal ] Is there a way to use list comprehensions to append or extend the array as needed by the second code listing? I think you are looking for temporal = [ [x[0], description[x[0]]] + x[1] for x in elements ] but I would make two steps, one for the sort using just [ (x[0], x) for x in elements ] then when you pick the data back out you can format it how you like. Kent PS to John: the original solution is using DSU, aka Schwarzian Transform ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Extending a list within a list comprehension
On Tue, 2006-04-11 at 22:17 -0400, Kent Johnson wrote: Victor Bouffier wrote: If the second element in each array passed as x is of variable length (that is, it has a different element count than three, in this case), the program needs to extend the list instead. Without list comprehensions, and the added capability to utilize and sized list as a second element, my code ended up looking like the following: temporal = [] for x in elements: lst = [x[0], description[x[0]]] lst.extend(x[1]) temporal.append([x[1][1], lst]) temporal.sort() temporal.reverse() # sort descending elements = [ x[1] for x in temporal ] Is there a way to use list comprehensions to append or extend the array as needed by the second code listing? I think you are looking for temporal = [ [x[0], description[x[0]]] + x[1] for x in elements ] Hi Kent, I try this one and get the following error: TypeError: list objects are unhashable I figured it is because of the x[0] element being used as a dict key. Can you explain further where this error comes from? Are you getting it too? but I would make two steps, one for the sort using just [ (x[0], x) for x in elements ] You are right. This is cleaner and not a big deal to do in two steps. However for the issue at hand, it still keeps x as a two element list: [codigo, [ cant, importe, porc]], which I would like to have extended: [codigo, cant, importe, porc] It is not a big deal. That is why I finally went with the longer version Alan suggested. Before the sort I get a two element list, the second element being the list I finally want as output. then when you pick the data back out you can format it how you like. Right. After the sort I just get that second element in another list comprehension lines = [ x[1] for x in temp ] Thanks for your help. Victor Kent PS to John: the original solution is using DSU, aka Schwarzian Transform ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
