[Tutor] question about list

[Liam Clarke]

This is fantastic - 

a = [1,2,3,4,5,6]
idx = [1,2,4]
idx.sort()
idx.reverse()
for i in idx:
   del a[i]

It's so simple once you know the answer. Like a cryptic crossword, you
slap your forehead and say 'of course.'

Cheers Bill.

Liam Clarke

On Thu, 11 Nov 2004 22:01:43 -0500, Bill Mill <bill.mill at gmail.com> wrote:
> Liam,
> 
> Sure. Sorry I was kinda short, I was in a rush earlier. Now, I got time.
> 
> In [1]: a = [1,2,3,4,5,6]
> 
> In [2]: idx = [1,2,4]
> 
> Now, what we want to do is delete the first, second and fourth
> elements (we'll let 1 be the 'zeroth' element, for convenience) of
> 'a'. I've renamed 'b' to 'idx', meaning index. When we delete the
> first element of 'a', we have a problem - what does a look like now?
> 
> In [3]: del a[idx[0]]
> 
> In [4]: a
> Out[4]: [1, 3, 4, 5, 6]
> 
> We've removed the first element, but the second and fourth elements
> have now become the first and third elements. Thus, we need to
> subtract one from every element of 'idx'. I accomplished this with a
> list comprehension:
> 
> In [5]: idx = [elt-1 for elt in idx]
> 
> In [6]: idx
> Out[6]: [0, 1, 3]
> 
> To read list comprehensions, read from the right. The first clause is
> "for elt in idx" which works exactly like a for loop. The second
> clause is "elt-1", which tells python to subtract one from the current
> element. So, overall, the list comprehension tells python "make a list
> composed of elt-1 for each elt in idx".
> Does that make sense? Try playing around with them  in the interpreter
> for a while. If you still don't understand what's going on (they're
> hard), send a new message to the list and I'll try to give a more
> detailed explanation of what's going on.
> 
> Now, to continue, when we do:
> 
> In [7]: del a[idx[1]]
> 
> In [8]: a
> Out[8]: [1, 4, 5, 6]
> 
> We delete the correct element. However, again, the third element has
> become the second element, so we have to update idx, and so on until
> we're done.
> 
> Now, looking at this with more time has given me a better idea. If we
> start removing entries from the back, instead of the front, the order
> of the elements we want to remove won't change. Thus, this code:
> 
> a = [1,2,3,4,5,6]
> idx = [1,2,4]
> idx.sort()
> idx.reverse()
> for i in idx:
>     del a[i]
> 
> Which works much more nicely, and very likely more efficiently, since
> it doesn't have to iterate through the entire idx list every time it
> deletes an element of a.
> 
> Does it make sense now? If not, feel free to drop me more questions.
> I've been doing it for a while, so I tend to miss newbie mistakes.
> 
> 
> 
> Peace
> Bill Mill
> 
> On Fri, 12 Nov 2004 15:26:14 +1300, Liam Clarke <cyresse at gmail.com> wrote:
> > Heh,
> >
> > If you don't mind, could you explain how that works?
> >
> > Especially the b=[element..] part.
> >
> > Day by day, I learn that I have so much more to learn. Sheesh.
> >
> > Thanks,
> >
> > Liam Clarke
> >
> >
> > On Thu, 11 Nov 2004 19:59:09 -0500, Bill Mill <bill.mill at gmail.com> wrote:
> > > Lin,
> > >
> > > b.sort()
> > > for i in b:
> > >     del a[i]
> > >     b = [element-1 for element in b]
> > >
> > > Peace
> > > Bill Mill
> > >
> > >
> > >
> > >
> > > On Fri, 12 Nov 2004 07:24:50 +0800, Lin Jin <jinlin555 at msn.com> wrote:
> > > > > i am new to python.i have a question about list.if i have two list:
> > > > > a=[a,b,c,d,e,f,g]
> > > > > b=[1,2,4]
> > > > > and i want to remove the element of a using b,that is i want
> > > > a=[a,d,f,g],my
> > > > > code is like this:
> > > > > >>>for i in b:
> > > > > >>>    del a[i]
> > > > >
> > > > > but it is not working, it can't remove the correct items.what should i do
> > > >
> > > > > to make it correct?thx
> > > >
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> > > >
> > > _______________________________________________
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> > > http://mail.python.org/mailman/listinfo/tutor
> > >
> >
> > --
> > 'There is only one basic human right, and that is to do as you damn well please.
> > And with it comes the only basic human duty, to take the consequences.
> > _______________________________________________
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> > http://mail.python.org/mailman/listinfo/tutor
> >
> 


-- 
'There is only one basic human right, and that is to do as you damn well please.
And with it comes the only basic human duty, to take the consequences.