** Python Support Thread ** - Page 23 - Computer Technical Help

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06-23-2012 , 01:05 AM

#551

unluckyboy

banned

Join Date: May 2012 Posts: 163

i am trying tree.xpath("//{http://odds.sbrforum.com}side-name"), and root.xpath("//{http://odds.sbrforum.com}side-name"), tree.xpath("//{http://odds.sbrforum.com}:side-name"), and root.xpath("//side-name")

none work

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06-23-2012 , 01:08 AM

#552

unluckyboy

banned

Join Date: May 2012 Posts: 163

when i try to grab some random element and do tree.getpath(random_element) it gives me bull**** paths like /*/*[1]

what the hell is that

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06-23-2012 , 05:10 AM

#553

Alex Wice

aka Double Ice

Join Date: Jun 2007 Posts: 5,950

Quote:

Originally Posted by unluckyboy

here is a programming problem i am having a problem with. i have data for 1300 football games. for each game i have the spread, and what the result was compared to the spread. it is set up as a list of lists now [(game1_spread, game1_result_compared_to_spread), (game2_spread, game2_result_compared_to_spread), etc.]

i want to take this information and make a printout with these two pieces of data:

spread: game1_result_compared_to_spread==0/total_games_with_this_spread
spread2: game1_result_compared_to_spread==0/total_games_with_this_spread
etc. for each spread

how can i get this?

I might not have understood your question btw.

Code:

def avgspread(sp,lt):
   #given a spread, and a list of games [ (spread,result),... ]
   # , return the average result compared to spread
   ans,ct = 0,0
   for i in lt:
       if i[0] == sp:
         ans += i[1]
         ct += 1
   return ans/float(ct)

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06-30-2012 , 12:24 PM

#554

fluorescenthippo

Pooh-Bah

Join Date: Apr 2005 Posts: 5,849

taking the MIT python course and came across this:

self.viruses = [x for x in newViruses]

can someone please explain to me how this is better than a typical for loop. I'm also uncertain as to what this does

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06-30-2012 , 12:45 PM

#555

Xhad

Carpal \'Tunnel

Join Date: Jul 2005 Posts: 10,962

That's called a list comprehension. In this case it's just copying the list. The equivalent as a for would be:

Code:

self.viruses = []
for x in newViruses:
    self.viruses.append(x)

The main advantage is code length and readability once you understand it; see my response to this post

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07-01-2012 , 07:16 PM

#556

fluorescenthippo

Pooh-Bah

Join Date: Apr 2005 Posts: 5,849

what do __lt__ and __eq__ do? I keep getting examples when i google but not an explanation. basically im confused as to how a class can return more than one thing using these fuctions.

Quote

07-02-2012 , 04:22 AM

#557

Xhad

Carpal \'Tunnel

Join Date: Jul 2005 Posts: 10,962

How much do you know about classes? Those are for operator overloading ('<' and '==', respectively).

Quote

07-07-2012 , 04:48 PM

#558

pokergrader

Carpal \'Tunnel

Join Date: Apr 2005 Posts: 9,757

Quote:

Originally Posted by Xhad

That's called a list comprehension. In this case it's just copying the list. The equivalent as a for would be:

Code:

self.viruses = []
for x in newViruses:
    self.viruses.append(x)

The main advantage is code length and readability once you understand it; see my response to this post

List comprehensions are also faster (and in some cases, much faster). Python is a slow language, and when you write out a for loop python actually has to continually run over the loop to execute it, which is slow. List comprehensions, since they encapsulate the operation you are trying to do more comprehensively, allow python to optimize the execution.

For example in the below code, I'm manually copying a list with 1000 items and then doing the same operation with a list comprehension. The list comprehension is about 2.5x faster to do the exact same operation.

Code:

import timeit

things = [0] * 1000

def manual_copy():
    copy = []
    for thing in things:
        copy.append(thing)

def list_comprehension():
    copy = [thing for thing in things]

t = timeit.Timer('manual_copy()', 'from __main__ import manual_copy')
print 'Manual copy:', t.timeit(number=100000)

t = timeit.Timer(
    'list_comprehension()', 'from __main__ import list_comprehension')
print 'List comprehension:', t.timeit(number=100000)

Output (time in seconds, lower is faster):

Code:

Manual copy: 16.9621839523
List comprehension: 6.79605603218

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07-07-2012 , 05:41 PM

#559

clowntable

Carpal \'Tunnel

Join Date: Jun 2006 Posts: 45,557

Same benchmark with PyPy please

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07-07-2012 , 05:55 PM

#560

pokergrader

Carpal \'Tunnel

Join Date: Apr 2005 Posts: 9,757

Same code in pypy 1.9

Output in seconds, lower is better:

Code:

Manual copy: 5.08202004433
List comprehension: 1.11676502228

It is just much easier for the python interpreter to optimize a list comprehension than it is to optimize a for loop, regardless of which one you are using

Of course one of the reasons for this is list comprehensions are much less flexible than for loops. But the general rule is that if you are doing something simple that can be a list comprehension, it should be a list comprehension. However, if you try to cram something too complicated into a list comprehension it can get really gnarly and unreadable, and at that point you should probably just use a for loop for readability. There was a python style rule where I used to work that if your list comprehension was longer than 2 lines (80 character width), then it was too complicated and needed to be refactored somehow.

Last edited by pokergrader; 07-07-2012 at 06:01 PM.

Quote

07-07-2012 , 06:26 PM

#561

kerowo

lolcat

Join Date: Nov 2005 Posts: 37,183

Code:

Manual copy: 13.7768809795
List comprehension: 3.7635409832

Woot! Retina Display MBP is quick!

Quote

07-08-2012 , 08:37 AM

#562

clowntable

Carpal \'Tunnel

Join Date: Jun 2006 Posts: 45,557

List comprehensions that aren't super complicated are a lot easier to read than loops as well imo

Quote

07-08-2012 , 08:26 PM

#563

daveT

S.A.G.E. Master

Join Date: Jun 2005 Posts: 23,955

Quote:

Originally Posted by kerowo

Code:

Manual copy: 13.7768809795
List comprehension: 3.7635409832

Woot! Retina Display MBP is quick!

$530 i5 Lenovo:

Manual copy: 10.962864498256714
List comprehension: 3.4187096686791882

Last edited by daveT; 07-08-2012 at 08:27 PM. Reason: yeah, trolling ftw!

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07-08-2012 , 08:32 PM

#564

kerowo

lolcat

Join Date: Nov 2005 Posts: 37,183

What's that got, a 5x5 resolution screen...

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07-11-2012 , 04:05 PM

#565

Mariogs37

adept

Join Date: May 2012 Posts: 825

So I'm getting a syntax error here for month in the third line...not sure why:

month = int(input("Please give me an integer for the month: "))

day = int(input("Please give me an integer for the day of the month: "))

# Now we test that the input date is during the 2011-2012 academic year and
# is valid (e.g. 1/37 is not a valid date).

If month == 9 and day == 4:
print("This date is not during the academic year")

Elif month == 1 or month == 3 or month == 5 or month == 7 or month == 8 or month == 10 or month == 12 and day < 1 or day > 31
print("This date is invalid")

Elif month == 4 or month == 6 or month == 9 or month == 11 and day < 1 or day > 30
print("This date is invalid")

Anyone know why?

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07-11-2012 , 04:28 PM

#566

Xhad

Carpal \'Tunnel

Join Date: Jul 2005 Posts: 10,962

are you actually capitalizing if and elif? that isnt' allowed

Quote

07-11-2012 , 05:27 PM

#567

Mariogs37

adept

Join Date: May 2012 Posts: 825

Ah, I'm ******ed. Also, is there an efficient way to do this:

Elif month == 1 or month == 3 or month == 5 or month == 7 or month == 8 or month == 10 or month == 12 and day < 1 or day > 31
print("This date is invalid")

What I'm trying to get it to do is:

If month == 1 and day < 1 OR month ==1 and day > 31 OR month == 3 and day < 1...

Know what I mean?

Thanks again for the help.

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07-11-2012 , 06:43 PM

#568

Neko

Pooh-Bah

Join Date: Jul 2005 Posts: 4,000

Quote:

Originally Posted by Mariogs37

Code:

If month in (3, 5, 7,) and (day <	1 or 31 <	day):
				foo

Quote

07-11-2012 , 08:16 PM

#569

Neko

Pooh-Bah

Join Date: Jul 2005 Posts: 4,000

Quote:

Originally Posted by Neko

Code:

If month in (3, 5, 7,) and (day < 1 or 31 < day):
    foo

Sent that from my phone not sure why it inserted tabs instead of spaces :/

Quote

07-11-2012 , 11:01 PM

#570

Mariogs37

adept

Join Date: May 2012 Posts: 825

perfect, thanks a lot man. one last thing: is there a good place to go over these exercises? my book only has super basic ones...

thanks again.

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07-12-2012 , 08:59 PM

#571

fluorescenthippo

Pooh-Bah

Join Date: Apr 2005 Posts: 5,849

So I'm getting a good feel for python now. I would still like to be more familiar with all the popular packages and modules. What should I be getting familiar with? sys,os,itertools,etc...