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Re: Drowning in a teacup?

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Fillmore wrote:

>
> notorious pass by reference vs pass by value biting me in the backside 
> here. Proceeding in order.
>
> I need to scan a list of strings. If one of the elements matches the 
> beginning of a search keyword, that element needs to snap to the front 
> of the list.
> I achieved that this way:
>
>
>     for i in range(len(mylist)):
>          if(mylist[i].startswith(key)):
>              mylist = [mylist[i]] + mylist[:i] + mylist[i+1:]
>
> Since I need this code in multiple places, I placed it inside a function
>
> def bringOrderStringToFront(mylist, key):
>
>      for i in range(len(mylist)):
>          if(mylist[i].startswith(key)):
>              mylist = [mylist[i]] + mylist[:i] + mylist[i+1:]
>
> and called it this way:
>
>   if orderstring:
>       bringOrderStringToFront(Tokens, orderstring)
>
> right?
> Nope, wrong! contrary to what I thought I had understood about how 
> parameters are passed in Python, the function is acting on a copy(!) and 
> my original list is unchanged.
>

Nope, that's not your problem.  Your problem is the line:

>              mylist = [mylist[i]] + mylist[:i] + mylist[i+1:]

Which should be read as: "Take mylist[i], all of mylist before i,
and all of mylist after i and create a new list from it. Store that
new list in a variable called mylist, overwriting the previous value."

If instead you were using the .insert and .remove methods, you'd be
manipulating the existing list instead.  But you don't want to do that,
because those methods are O(N) on the list length; manipulating the
middle of lists is slow.

That said, you've got some logic problems too; which come from the fact
that your iteration is shooting at a moving target.

How about:
  newlist = (
    [x for x in mylist if x.startswith(key)] +
    [x for x in mylist if not x.startswith(key)]
  )
  return newlist

Or if you really insist on mutating the original list (which seems less
clean to me, but you do you), then:
  newlist = blahblahblah
  mylist[:] = newlist

-- 
Rob Gaddi, Highland Technology -- www.highlandtechnology.com

Email address domain is currently out of order.  See above to fix.

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Thread

Drowning in a teacup? Fillmore <fillmore_remove@hotmail.com> - 2016-04-01 16:27 -0400
  Re: Drowning in a teacup? Rob Gaddi <rgaddi@highlandtechnology.invalid> - 2016-04-01 20:39 +0000
    Re: Drowning in a teacup? Ian Kelly <ian.g.kelly@gmail.com> - 2016-04-01 14:52 -0600
  Re: Drowning in a teacup? MRAB <python@mrabarnett.plus.com> - 2016-04-01 21:45 +0100
  Re: Drowning in a teacup? Michael Selik <michael.selik@gmail.com> - 2016-04-01 20:46 +0000
  Re: Drowning in a teacup? Steven D'Aprano <steve@pearwood.info> - 2016-04-02 09:46 +1100
  Re: Drowning in a teacup? Fillmore <fillmore_remove@hotmail.com> - 2016-04-02 00:03 -0400
  Re: Drowning in a teacup? Vito De Tullio <vito.detullio@gmail.com> - 2016-04-02 07:45 +0200
  Re: Drowning in a teacup? Michael Selik <michael.selik@gmail.com> - 2016-04-02 05:51 +0000
  Re: Drowning in a teacup? Vito De Tullio <vito.detullio@gmail.com> - 2016-04-02 08:26 +0200
  Re: Drowning in a teacup? Peter Otten <__peter__@web.de> - 2016-04-02 12:31 +0200
  Re: Drowning in a teacup? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-04-02 12:53 +0100
  Re: Drowning in a teacup? Michael Selik <michael.selik@gmail.com> - 2016-04-02 14:36 +0000
  Re: Drowning in a teacup? Ned Batchelder <ned@nedbatchelder.com> - 2016-04-02 12:28 -0700
    Re: Drowning in a teacup? Random832 <random832@fastmail.com> - 2016-04-02 15:54 -0400
      Re: Drowning in a teacup? Steven D'Aprano <steve@pearwood.info> - 2016-04-03 12:33 +1000
    Re: Drowning in a teacup? Ethan Furman <ethan@stoneleaf.us> - 2016-04-02 16:15 -0700
    Re: Drowning in a teacup? Random832 <random832@fastmail.com> - 2016-04-02 19:19 -0400

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