Re: Filtering array on index

[Daniel Shahaf <d.s@daniel.shahaf.name>]

Mikael Magnusson wrote on Thu, 25 Oct 2018 20:17 +0200:
> On 10/25/18, Jesper Nygårds <jesper.nygards@gmail.com> wrote:
> > I have an array which is the result of using zparseopts on a specification
> > that makes it possible to specify several filters with a -v flag. The
> > resulting array might look like the following:
> >
> > myarr=(-v filter1 -v filter2)
> >
> > I want to get rid of the "-v" elements, but only when they are the flag,
> > and not the argument. In other words, I would the like the array (-v
> > filter1 -v -v) to result in (filter1 -v).
> >
> > I had previously used:
> >
> > ${myarr:#-v}}
> >
> > which worked well, as I could accept the fact that -v was not possible to
> > use as the option argument.
> >
> > However, now I need to make sure that a -v argument is preserved, and I
> > thus need a compact way of specifying that I want to keep every other
> > element in the array.
> >
> > The most compact way I have found is:
> > for dash arg in $myarr; do myarr[(r)$dash]=()
> >
> > This works, but I was wondering if there's some more elegant solution,
> > preferrably without using iteration?
> 
> The above example doesn't work if you need to preserve the order (it
> will just remove the first $dash it finds).
> 
> % a=(-v filter1 -v -v -v middle -v -v -v final)
> % for dash arg in $a; do a[(r)$dash]=(); done; pl $a
> filter1
> middle
> -v
> -v
> final
> 
> middle "should" be between the two remaining -v.
> 
> This works, but is still a loop obviously:
> % for i in {$(($#a/2))..1}; do a[i*2-1]=(); done; pl $a
> filter1
> -v
> middle
> -v
> final
> 
> (Note that it is necessary to iterate backwards as the indices will
> change otherwise)

Another option:

myarr=(-v filter1 -v filter2)
() { local i j; myarr=(); for i j; do myarr+=($j) ; done } "${myarr[@]}"

In zparseopts context we can probably assume the arguments to the -v
option don't contain literal NUL bytes, so perhaps something like this:

% myarr=( -v 'foo foo' -v 'bar bar' -v '' )
% printf -v x '\0k1%s\0k2%s\0v' "$myarr[@]"
% x=${x//$'\0k1-v\0k2'}
% x=${x%$'\0v'}
% () { typeset -p argv } "${(@ps.\0v.)x}"
typeset -g -a argv=( 'foo foo' 'bar bar' '' )
% 

Is there a way to do this in O(N) time?  (The array-based solutions are
quadratic complexity due to indexing/appending being O(N).  I'm not sure
about the printf.)

Cheers,

Daniel