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how to separate a list into two lists?

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  how to separate a list into two lists? smith jack <thinke365@gmail.com> - 2011-08-07 01:07 +0800
    Re: how to separate a list into two lists? bud <only@fleshwound.org> - 2011-08-06 18:13 +0000
      Re: how to separate a list into two lists? Gelonida N <gelonida@gmail.com> - 2011-08-06 22:00 +0200
        Re: how to separate a list into two lists? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-08-07 10:07 +1000
          Re: how to separate a list into two lists? Gelonida N <gelonida@gmail.com> - 2011-08-07 19:11 +0200
    Re: how to separate a list into two lists? Tim Roberts <timr@probo.com> - 2011-08-06 17:58 -0700
      Re: how to separate a list into two lists? Chris Angelico <rosuav@gmail.com> - 2011-08-07 02:05 +0100
        Re: how to separate a list into two lists? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-08-07 11:35 +1000
    Re: how to separate a list into two lists? Paul Rubin <no.email@nospam.invalid> - 2011-08-06 22:29 -0700

#10976 — how to separate a list into two lists?

if a list L is composed with tuple consists of two elements, that is
L = [(a1, b1), (a2, b2) ... (an, bn)]

is there any simple way to divide this list into two separate lists , such that
L1 = [a1, a2... an]
L2=[b1,b2 ... bn]

i do not want to use loop, any methods to make this done?

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#10980

On Sun, 07 Aug 2011 01:07:00 +0800, smith jack wrote:

> if a list L is composed with tuple consists of two elements, that is L =
> [(a1, b1), (a2, b2) ... (an, bn)]
> 
> is there any simple way to divide this list into two separate lists ,
> such that L1 = [a1, a2... an]
> L2=[b1,b2 ... bn]
> 
> i do not want to use loop, any methods to make this done?


(x,y) = [ [z[i] for z in L] for i in range(len(L[0]))]

x
: ['a1', 'a2', 'an']

y
: ['b1', 'b2', 'bn']

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#10987

On 08/06/2011 08:13 PM, bud wrote:
> On Sun, 07 Aug 2011 01:07:00 +0800, smith jack wrote:
> 
>> if a list L is composed with tuple consists of two elements, that is L =
>> [(a1, b1), (a2, b2) ... (an, bn)]
>>
>> is there any simple way to divide this list into two separate lists ,
>> such that L1 = [a1, a2... an]
>> L2=[b1,b2 ... bn]
>>
>> i do not want to use loop, any methods to make this done?
> 
> 
> (x,y) = [ [z[i] for z in L] for i in range(len(L[0]))]
> 
> x
> : ['a1', 'a2', 'an']
> 
> y
> : ['b1', 'b2', 'bn']
> 
> 
Asuming you are not an alias of Jack Smith and assuming you did not see
Jack's thread asking the same question:

x,y = unzip(*L)

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#10991

Gelonida N wrote:

> Asuming you [Bud] are not an alias of Jack Smith and assuming you did 
> not see Jack's thread asking the same question:

That's a strange thing to say when Bud *answered* Jack's question.
 

> x,y = unzip(*L)

What's unzip? It doesn't exist in any version of Python between 1.5 and 3.3
that I have.



-- 
Steven

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#11010

On 08/07/2011 02:07 AM, Steven D'Aprano wrote:
> Gelonida N wrote:
> 
>> Asuming you [Bud] are not an alias of Jack Smith and assuming you did 
>> not see Jack's thread asking the same question:
> 
> That's a strange thing to say when Bud *answered* Jack's question.
>  
> 
>> x,y = unzip(*L)
> 
> What's unzip? It doesn't exist in any version of Python between 1.5 and 3.3
> that I have.
> 


Arg typo:

I meant of course zip


> 
> 

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#10995

smith jack <thinke365@gmail.com> wrote:
>
>if a list L is composed with tuple consists of two elements, that is
>L = [(a1, b1), (a2, b2) ... (an, bn)]
>
>is there any simple way to divide this list into two separate lists , such that
>L1 = [a1, a2... an]
>L2=[b1,b2 ... bn]
>
>i do not want to use loop, any methods to make this done?

There will always be a loop.  It might not be written with a "for"
statement, but there will always be a loop.

  L1 = [k[0] for k in L]
  L2 = [k[1] for k in L]

I did momentarily consider the following slimy solution:
  L1 = dict(L).keys()
  L2 = dict(L).values()
but that reorders the tuples.  They still correspond, but in a different
order.
-- 
Tim Roberts, timr@probo.com
Providenza & Boekelheide, Inc.

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#10996

On Sun, Aug 7, 2011 at 1:58 AM, Tim Roberts <timr@probo.com> wrote:
> I did momentarily consider the following slimy solution:
>  L1 = dict(L).keys()
>  L2 = dict(L).values()
> but that reorders the tuples.  They still correspond, but in a different
> order.
>

Which can be overcome with collections.OrderedDict. But what's dict(L)
going to do? It's going to loop over L, more than once in fact.

I guess the real question is: Why do you wish to avoid a loop?

ChrisA

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#10997

Chris Angelico wrote:

> On Sun, Aug 7, 2011 at 1:58 AM, Tim Roberts <timr@probo.com> wrote:
>> I did momentarily consider the following slimy solution:
>> L1 = dict(L).keys()
>> L2 = dict(L).values()
>> but that reorders the tuples.  They still correspond, but in a different
>> order.
>>
> 
> Which can be overcome with collections.OrderedDict. But what's dict(L)
> going to do? It's going to loop over L, more than once in fact.
> 
> I guess the real question is: Why do you wish to avoid a loop?

I think what the Original Poster actually meant was he wanted to avoid
*writing out an explicit loop*. That is, he wants a one-liner, so he
doesn't have to think about the details of iterating over the list.

When we write:

a = sum(a_sequence)

aren't we doing the same thing really?


-- 
Steven

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#11000

smith jack <thinke365@gmail.com> writes:
> if a list L is composed with tuple consists of two elements, that is
> L = [(a1, b1), (a2, b2) ... (an, bn)]
>
> is there any simple way to divide this list into two separate lists , such that
> L1 = [a1, a2... an]
> L2=[b1,b2 ... bn]
>
> i do not want to use loop, any methods to make this done?

That is called unzipping and there is a sneaky idiom for it:

 L1,L2 = zip(*L)

Your homework assignment is figuring out how that works ;-).

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