Comprehension with two variables - explanation needed

Ivan Evstegneev webmailgroups at gmail.com
Sun Nov 23 16:37:37 CET 2014


>Is this what you want?
>
>>>> [[j for j in range(i*2, 50, i)] for i in range(2,8)]
>[[4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40,
42, 44, 46, 48], [6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 45, 48],
[8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48], [10, >15, 20, 25, 30, 35, 40,
45], [12, 18, 24, 30, 36, 42, 48], [14, 21, 28, 35, 42, 49]]

>[I am not sure what you are trying -- just making local changes to the ZF
to make it work]

>https://mail.python.org/mailman/listinfo/python-list

Yes, awesome. Thanks.

But it breaks all the picture that I've built in my head about comps till
now...

As I know from books and googling, the comps main idea looks approximately
like this:

[target <--main loop<--nested loop/s (and maybe some conditions)]    Am I
right? 

But your code looks somehow inverted to me o_0

Like: 

[[target with nested  loop] <--- main loop with initial values for target]

On the other hand, if I'm comparing to my tryouts:

noprimes = [[j for i in range(2, 8)] for j in range(i*2, 50, i)]

It looks same but the variables are not separated, so it definitely  was not
defined....

Sincerely,

Ivan


     




-----Original Message-----
From: Python-list
[mailto:python-list-bounces+webmailgroups=gmail.com at python.org] On Behalf Of
Rustom Mody
Sent: Sunday, November 23, 2014 17:09
To: python-list at python.org
Subject: Re: Comprehension with two variables - explanation needed

On Sunday, November 23, 2014 8:28:16 PM UTC+5:30, Ivan Evstegneev wrote:
> Hello guys,
>  
> I would like to ask you for some explanations on comprehensions. 
> (Don't be scared, it just some particular example ^_^)
>  
> I found this little "find prime number" example over the internet:
>  
> >>> noprimes = [j for i in range(2, 8) for j in range(i*2, 50, i)] 
> >>> primes = [x for x in range(2, 50) if x not in noprimes] print 
> >>> primes
> [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47]
>  
>  
> It looked  pretty clear to me, till the time I decided to make some 
> changes to it %)))
>  
> Ok this is the case: 
>  
> As you can see the first line's comprehension produces a list, that
includes all the multiplications of "i" (that range(2,8) ) in range (i*2,
50, i).
> So  I get this pattern: noprimes  = =  [4, 6, 8.....48, 6, 9,12.... ].
> The change that I tried to apply to this comprehension would lead to 
> the following pattern: noprimes = = [[4, 6,8...48], [6, 9, 12....], 
> ....,[some numbers]]
>  
> But despite my struggling on it for the whole day, I haven't got to 
> the desirable result(using comprehensions only)
>  
> Please, don't get me wrong, I'm not a lazy one (just a beginner). 
>  
> In my case, I  made some googling, and found some patterns for matrices,
that look this one(for example):
>  
> Matrix = [[0 for x in range(5)] for x in range(5)]
>  
> But when I tried to use square brackets with my code, it yields an error
about definition of "i", for instance:
>  
> File "<pyshell#30>", line 1, in <module> noprimes = [[j for i in 
> range(2, 8)] for j in range(i*2, 50, i)]
> NameError: name 'i' is not defined.

Is this what you want?

>>> [[j for j in range(i*2, 50, i)] for i in range(2,8)]
[[4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40,
42, 44, 46, 48], [6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 45, 48],
[8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48], [10, 15, 20, 25, 30, 35, 40,
45], [12, 18, 24, 30, 36, 42, 48], [14, 21, 28, 35, 42, 49]]

[I am not sure what you are trying -- just making local changes to the ZF to
make it work]
--
https://mail.python.org/mailman/listinfo/python-list




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