Comprehension with two variables - explanation needed

Ivan Evstegneev webmailgroups at gmail.com
Sun Nov 23 15:56:55 CET 2014


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.

 

Then I tried to apply those brackets to "j", like this: 

 

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

 

In this case I get, pretty predictable result, like this: [[num], [num],
[num],...,[num]], and  this is still not what I'm looking for..

 

So I moved further, and succeeded to solve my problem in a different manner:

 

>>> noprimes=[]

>>> for i in range(2,8):

noprimes.append(list(range(i*2,50,i)))

 

Which yields, in fact, the matrix I wanted from the beginning:

 

noprimes == [[4, 6, 8, 10, 12,.., 48], [6, 9, 12, 15, 18, ., 42, 45,
48],..,[numbers]]

Yep, this the "classical" way for me, on the other hand I have the feeling,
that this is not, the ultimate solution. ^_^

 

So my questions is still persists:

 

1)      Is it possible to realize such a thing by applying changes only to
initial comprehension? If yes, it would be nice to see the solution.

2)      How actually bad is my solution? I mean, after all, there are nested
functions call in my code,  so how much the speed is affected(if any)? 

 

Thanks a lot in advance,

 

Ivan.

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