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Re: How is this list comprehension evaluated?

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Op 16-09-13 15:43, Arturo B schreef:
> Hello, I'm making Python mini-projects and now I'm making a Latin Square
> 
> (Latin Square: http://en.wikipedia.org/wiki/Latin_square)
> 
> So, I started watching example code and I found this question on Stackoverflow: 
> 
> http://stackoverflow.com/questions/5313900/generating-cyclic-permutations-reduced-latin-squares-in-python
> 
> It uses a list comprenhension to generate the Latin Square, I'm am a newbie to Python, and  I've tried to figure out how this is evaluated:
> 
>     a = [1, 2, 3, 4]
>     n = len(a)
>     [[a[i - j] for i in range(n)] for j in range(n)]
> 
> I don't understand how the "i" and the "j" changes.
> On my way of thought it is evaluated like this:
> 
>     [[a[0 - 0] for 0 in range(4)] for 0 in range(4)]
>     [[a[1 - 1] for 1 in range(4)] for 1 in range(4)]
>     [[a[2 - 2] for 2 in range(4)] for 2 in range(4)]
>     [[a[3 - 3] for 3 in range(4)] for 3 in range(4)]
> 
> But I think I'm wrong... So, could you explain me as above? It would help me a lot.
> 
> Thanks for reading!

Just start your python interpreter and type the following

>>> [[(i,j) for i in range(3)] for j in range(3)]

That should give you a clue.

-- 
Antoon Pardon

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How is this list comprehension evaluated? Arturo B <a7xrturodev@gmail.com> - 2013-09-16 06:43 -0700
  Re: How is this list comprehension evaluated? Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2013-09-16 15:53 +0200
  Re: How is this list comprehension evaluated? Michael Torrie <torriem@gmail.com> - 2013-09-16 08:20 -0600
  Re: How is this list comprehension evaluated? Roy Smith <roy@panix.com> - 2013-09-16 20:04 -0400

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