Reverse the order of an array?

Reverse the order of an array?

[Steve Talley]

Hi there,

How can I easily reverse the order of an array in zsh?  I was hoping
that something like

    osvers=($osvers[{$#osvers..1}])
    
would work, but no luck.

Any help would be greatly appreciated.

Thanks,

Steve

Re: Reverse the order of an array?

[Bart Schaefer]

On Fri, 8 Feb 2002, Steve Talley wrote:

> How can I easily reverse the order of an array in zsh?

Depends what you mean by "easily."

> I was hoping that something like
>
>     osvers=($osvers[{$#osvers..1}])
>
> would work, but no luck.

Array "slices" in zsh only have start and end points, and are always in
ascending order by numeric position in the array.  So you can't do this
purely with parameter expansion in the general case.

This works:

for ((i=$#osvers; i>0; --i)) { osvers[i*2]=($osvers[1]); shift osvers }

Re: Reverse the order of an array?

[Steve Talley]

Thanks Bart.  The best that I had come up with was:

    indexes=({$#osvers..1})
    tmp=('$osvers['$^indexes']')
    osvers=${(e)tmp}

Do you see any way (other than skipping the whole thing :) to simplify
this?

Thanks,

Steve

Bart Schaefer wrote:

> On Fri, 8 Feb 2002, Steve Talley wrote:
> 
> > How can I easily reverse the order of an array in zsh?
> 
> Depends what you mean by "easily."
> 
> > I was hoping that something like
> >
> >     osvers=($osvers[{$#osvers..1}])
> >
> > would work, but no luck.
> 
> Array "slices" in zsh only have start and end points, and are always in
> ascending order by numeric position in the array.  So you can't do this
> purely with parameter expansion in the general case.
> 
> This works:
> 
> for ((i=$#osvers; i>0; --i)) { osvers[i*2]=($osvers[1]); shift osvers }

Re: Reverse the order of an array?

[Bart Schaefer]

On Fri, 8 Feb 2002, Steve Talley wrote:

> Thanks Bart.  The best that I had come up with was:
>
>     indexes=({$#osvers..1})
>     tmp=('$osvers['$^indexes']')
>     osvers=${(e)tmp}
>
> Do you see any way (other than skipping the whole thing :) to simplify
> this?

Hmm, I should have thought of that.

	eval osvers\=\( \"\$osvers\[{$#osvers..1}\]\" \)

Re: Reverse the order of an array?

[Steve Talley]

Perfect!  I knew there was a better solution.

Thanks Bart!

Steve

Bart Schaefer wrote:

> On Fri, 8 Feb 2002, Steve Talley wrote:
> 
> > Thanks Bart.  The best that I had come up with was:
> >
> >     indexes=({$#osvers..1})
> >     tmp=('$osvers['$^indexes']')
> >     osvers=${(e)tmp}
> >
> > Do you see any way (other than skipping the whole thing :) to simplify
> > this?
> 
> Hmm, I should have thought of that.
> 
> 	eval osvers\=\( \"\$osvers\[{$#osvers..1}\]\" \)

Re: Reverse the order of an array?

[Oliver Kiddle]

 --- Bart Schaefer <schaefer@brasslantern.com> wrote: 
> 
> Hmm, I should have thought of that.
> 
> 	eval osvers\=\( \"\$osvers\[{$#osvers..1}\]\" \)
> 

Not very intuitive that is it.

I recently wanted to loop over an array's elements in reverse and
though I could do it with a for loop counting the indices backwards I
did think that there ought to be a better way.

Allowing array slices to go backwards, is a possibility though I'd not
be suprised if implementing it caused other things to break. The
easiest might be a parameter expension flag (r and R are gone so we'd
need a letter. ^ perhaps as we used that for the reversed prompt
state). Does anyone else have any views on whether we should add
something for this and if so how?

Oliver

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Re: Reverse the order of an array?

[Bart Schaefer]

On Mon, 11 Feb 2002, Oliver Kiddle wrote:

> Allowing array slices to go backwards, is a possibility though I'd not
> be suprised if implementing it caused other things to break.

Right now, using a backwards array slice references an imaginary "empty
element" to the left of the left end of the slice; assigning to that
imaginary element makes it real.  E.g.:

zsh% x=(a b c d e)
zsh% x[4,2]=(y)
zsh% echo $x
a b c y d e

This was motivated by desiring to have a syntax for inserting elements
into the array; "forwards" array slices only allow for replacement.

> The easiest might be a parameter expension flag (r and R are gone so
> we'd need a letter.

We could also use a subscripting flag (though that doesn't help with r/R).

> ^ perhaps as we used that for the reversed prompt state).

No, I don't like that.  ${(^)^x} is just too confusing.

How about if (oa) means "sort in array index order" and (Oa) means "sort
in reverse array index order"?  There's precedent with (oi) and (Oi) for
case-insensitive sorting.  Obviously (oa) is equivalent to the default,
but so what?