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How is this list comprehension evaluated?

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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

#54226 — How is this list comprehension evaluated?

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!

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#54227

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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#54229

On 09/16/2013 07:43 AM, Arturo B wrote:
> 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:

It helps to convert it to a conventional for loop to see how it works:

a = [1, 2, 3, 4]
n = len(a)

resultj = []

for j in range(n):
    resulti = []

    for i in range(n):
        resulti.append(a[i-j])

    resultj.append(resulti)

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#54257

In article <eae87c72-f62d-4815-bb69-ca862ff78f1e@googlegroups.com>,
 Arturo B <a7xrturodev@gmail.com> wrote:

> 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-redu
> ced-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)]

You can re-write any list comprehension as a for loop.  In this case you 
have to un-wrap this one layer at a time.  First step:

    a = [1, 2, 3, 4]
    n = len(a)
    temp1 = []
    for j in range(n):
        temp2 = [a[i - j] for i in range(n)]
        temp1.append(item)

then, unwrap the next layer:

    a = [1, 2, 3, 4]
    n = len(a)
    temp1 = []
    for j in range(n):
        temp2 = []
        for i in range(n):
             temp3 = a[i - j]
             temp2.append(temp3)
        temp1.append(item)

Does that make it any easier to understand?

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