Re: [Tutor] making lists of prime numbers
On 15/09/11 03:01, c smith wrote: am I passing arguments from the command line correctly? Yes, although not converting to an int. this seems to count by twos (going through both 'if statements' maybe?) I thought the 'break' would send control back to the 'while' basically, is there an obvious problem here or is it just completely wrong? It might work once you iron out the bugs but it's horribly inefficient. It will take a very long time to run for large numbers. Did you try searching for prime number algorithms? There are several that will be much faster than this. -- Alan G Author of the Learn to Program web site http://www.alan-g.me.uk/ ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] making lists of prime numbers
On 09/14/2011 10:35 PM, Andre Engels wrote: On Thu, Sep 15, 2011 at 4:01 AM, c smithwrote: hi list, i am trying the MIT opencourseware assignments. one was to find the 1000th prime. since this isn't actually my homework, I modified the solution as I would like to collect lists of primes and non-primes up to N, also some log() ratio to one comparison. here is what I came up with on paper: #!/usr/bin/env python import sys, os, math def main(a): notprime = [] primelist = [2,3] nprime = sys.argv[1] numcheck = 4 while len(primelist)< nprime: for i in range(2,numcheck-1): if numcheck % i == 0: print numcheck,'is not prime' notprime.append(numcheck) numcheck += 1 break if i == numcheck and numcheck % i !=0: print numcheck, 'is prime' primelist.append(numcheck) numcheck += 1 break TwotoN = 0 for j in primelist: TwotoN += log(j) print 'sum of logs of primes from 2 to', nprime,'is',TwotoN print 'the ratio of this sum to 1 is' %f % (float(TwotoN)/nprime) if __name__=='__main__': main(sys.argv[1]) my questions would be: am I passing arguments from the command line correctly? this seems to count by twos (going through both 'if statements' maybe?) I thought the 'break' would send control back to the 'while' basically, is there an obvious problem here or is it just completely wrong? I'd say the obvious problem is your second if statement. Its guard will never be true. i goes through range(2,numcheck-1). The highest number in that range in numcheck-2, so i==numcheck will never be true. Even if you'd get i to equal numcheck somehow, numcheck % i would then ber equal to numcheck % numcheck, which equals 0, so numcheck % i != 0 would not be true. The obvious thing to do seems to be to get this code out of the for loop, without and if statement guarding it. Another thing that goes wrong is: nprime = sys.argv[1] You use nprime as an integer, but sys.argv[1] is a string. You should thus change this into nprime = int(sys.argv[1]) However, for the sake of reusability, it's better to have a function that gets the first n primes for n decided by the caller than to have a function that gets the first n primes for n always being the first argument, thus I would change it to: nprime = int(a) As Andre and Alan have said, there are some problems with your code. However, the basic concept is reasonable. I wouldn't worry much about efficiency, yet. Get it to work reliably, then figure how to tune it. And if you have a source control system (like git, svn, mercurial), check in each version that works, so you can always compare to what you currently have. Your specific questions: 1) you're passing the (one) argument from the command line reasonably, though I'd convert it to int before passing it, and I'd change the formal parameter name of it in your function to nprime. Then you don't need any additional assignment in the function. 2) it seems to count by twos because of the problems with your second if, as Andre has said. Start by fixing the second if statement. There are two ways to fix it. a) change the condition so it detects the end of the loop, which in your case is numcheck-2. Catch to that is that when you realize you can optimize the loop, you'll have to remember to change the if to match. b) move the code outside of the loop. However, you'd also have to make sure it *didn't* get executed if the first if statement did. That could be a real pain, except that Python has a nice "else" clause you can attach to a loop, that only gets executed if the loop completes normally. That else is exactly what you want, so it makes it straightforward. Once you see it generating the correct primes, you may notice that it sails right past the nprimes value. That'll get fixed when you add the int() function when calling main(). Comparing int to string usually doesn't do what you'd want. next problem you'll have is the log function, which is defined in math. So you want to call math.log() next problem is the misplaced quotes on the last print statement. You'll get an error when you hit that one. Finally, the wording of those two prints is all wrong, so you might want to fix it so it matches what the code does. HTH -- DaveA ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
[Tutor] What's the keyword for the Python creed?
You know, at the interactive prompt you enter some Monty Python word that I can't remember, and you get a small list of pithy pythonic advice such as "explicit is better than implicit", etc. Thanks, Dick Moores ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] What's the keyword for the Python creed?
On Thu, Sep 15, 2011 at 1:19 PM, Richard D. Moores wrote: > You know, at the interactive prompt you enter some Monty Python word > that I can't remember, and you get a small list of pithy pythonic > advice such as "explicit is better than implicit", etc. > import this You can also do... import antigravity ...for another Pythonic easter egg... Though I think this one requires Python 3? 2.7? Can't remember. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] What's the keyword for the Python creed?
I think you mean the Zen of Python: http://www.python.org/dev/peps/pep-0020/ The command is >>> import this -- Jordan On 09/15/2011 11:19 AM, Richard D. Moores wrote: > You know, at the interactive prompt you enter some Monty Python word > that I can't remember, and you get a small list of pithy pythonic > advice such as "explicit is better than implicit", etc. > > Thanks, > > Dick Moores > ___ > Tutor maillist - Tutor@python.org > To unsubscribe or change subscription options: > http://mail.python.org/mailman/listinfo/tutor ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] What's the keyword for the Python creed?
>import antigravity >...for another Pythonic easter egg... Though I think this one requires Python >3? 2.7? Can't remember. Works in 2.7 but not 2.6.4 Ramit Ramit Prasad | JPMorgan Chase Investment Bank | Currencies Technology 712 Main Street | Houston, TX 77002 work phone: 713 - 216 - 5423 This email is confidential and subject to important disclaimers and conditions including on offers for the purchase or sale of securities, accuracy and completeness of information, viruses, confidentiality, legal privilege, and legal entity disclaimers, available at http://www.jpmorgan.com/pages/disclosures/email. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] What's the keyword for the Python creed?
Thanks, all. Good to have that at hand. antigravity: any more? Dick ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
[Tutor] Please review my code
Hello All, While trying to learn Python I developed a small game, Text Text. It has rough edges and may not meet your class. However, by reviewing it you'll be helping me to advance. It was only tested on a Windows Box, but I see no reason why it would not work on Unix family. https://github.com/janus/Text-Twist I need you comments. -- *Regards, Emeka * ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
[Tutor] help importing pygame
For starters, I am a newish python programmer, far enough along to have my feet wet. Running Mac OSX Lion (10.7), and python 2.7.2. I am trying to expand my knowledge and use some packages outside of the base python library. Games are interesting so I decided I'd try pygame after reading many good things about it. Now I have been to the site and downloaded the install package for OSX Lion, and run it successfully. But when I try to run the example on the site (line by line chimp example) I get this error message: Traceback (most recent call last): File "/private/var/folders/j4/10dtxgh14hl6vdvzzhbbqzdcgn/T/Cleanup At Startup/untitled text 4-337832000.071", line 11, in import os, pygame File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pygame/__init__.py", line 95, in from pygame.base import * ImportError: dlopen(/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pygame/base.so, 2): no suitable image found. Did find: /Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pygame/base.so: no matching architecture in universal wrapper The program hasn't even made it anywhere else in the code, and I really don't understand what the problem is. Help would be much appreciated, thank you all ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
[Tutor] Strange zip syntax
I ran into this article (
http://blog.adku.com/2011/09/hodgepodge-of-python.html ) and found
myself temporarily stymied by one line it in:
zip(*[iter(a)]*2)
Used like this:
>>> a = ['a','1','b','2','c','3']
>>> zip(*[iter(a)]*2)
[('a', '1'), ('b', '2'), ('c', '3')]
While I'm unlikely to use such a construct (if I can't easily follow
it now, I or my successor likely won't follow it should it need to be
debugged sometime in the future), I found the education I got in
deciphering it was worth the effort. I'm sharing it here so others
can benefit from my puzzlement.
iter(a) returns a list iterator for a. See help(iter) for more.
[iter(a)] is a list containing one element, an iterator. This is
created only so we can do the below:
[iter(a)]*2 is a list containing two elements, each the SAME list iterator.
For simplicity, let's break this out for further analysis:
>>> b = iter(a)
>>> c = [b,b]
*[iter(a)]*2 flattens the list when passed into a function call.
Using our more verbose but simple syntax: *c. This only works when
passed to a function.
zip() creates tuples each holding the Nth elements from a number of
sequences. See help(zip) for more.
Thus, zip(a) or zip(a,a) would return:
>>> zip(a)
[('a',), ('1',), ('b',), ('2',), ('c',), ('3',)]
>>> zip(a,a)
[('a', 'a'), ('1', '1'), ('b', 'b'), ('2', '2'), ('c', 'c'), ('3', '3')]
What happens when we pass an iterator to zip? That's not mentioned in
the docstring blurb.
>>> zip(iter(a))
[('a',), ('1',), ('b',), ('2',), ('c',), ('3',)]
Answer: It works as intended.
Now we come to the magic of this little snippet.
zip(iter(a),iter(a)) wouldn't work, because each call to iter(a)
returns a DIFFERENT iterator.
>>> zip(iter(a), iter(a))
[('a', 'a'), ('1', '1'), ('b', 'b'), ('2', '2'), ('c', 'c'), ('3', '3')]
But by creating the list of two elements each of which is the SAME
iterator, as each is asked to iterate it advances the common element
indicator:
>>> zip(*c)
[('a', '1'), ('b', '2'), ('c', '3')]
Notice that the flattening is required, because zip needs to get
multiple arguments:
>>> b = iter(a) #our original iterator is spent, so we're assigning a new one
>>> c = [b,b]
>>> zip(c) #Not flattened, is just a single list, like a.
[(,), (,)]
>>> zip(b,b) # here it is two iterators sent to zip() (though they happen to
>>> be the SAME iterator)
[('a', '1'), ('b', '2'), ('c', '3')]
I hope some of you enjoy playing with this, and hopefully someone
learned something useful! While I'm not likely to use the listed
form, I can very well see myself saying:
>>> a = ['a','1','b','2','c','3'] #well, I can see myself using this with
>>> meaningful variable names
>>> b = iter(a)
>>> zip(b,b) # Group in sets of 2 elements
--
Brett Ritter / SwiftOne
swift...@swiftone.org
___
Tutor maillist - Tutor@python.org
To unsubscribe or change subscription options:
http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] What's the keyword for the Python creed?
On 2011/09/15 08:57 PM, Richard D. Moores wrote: Thanks, all. Good to have that at hand. antigravity: any more? Dick ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor from __future__ import braces :) -- Christian Witts Python Developer // ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
