Re: filter the nth note

[Jean Abou Samra]

Le 19/06/2022 à 10:58, Stefan Thomas a écrit :
> Dear community,
> I would like to do something like the following, but automatically, 
> without typing all the music again:
> allNotes= { c'8 g' e' g' d' g' f' g' c' g' e' g'   }
> someNotes = { c' d' c' }
> In other words: I would like to filter out every 4th note of allNotes, 
> so that I could do in combination with changePitch (see 
> https://github.com/gilles-th/arranger.ly/blob/master/changePitch.ly) 
> the following:
> patI = { c8 r r4 }
> someNotes =  { \cPI { \someNotes s } }
> Thanks for Your help,
> Stefan

I'd do

\version "2.22.2"

filterNth =
#(define-music-function (n music) (index? ly:music?)
   (music-filter
    (let ((i -1))
      (lambda (m)
        (set! i (1+ i))
        (zero? (modulo i n))))
    music)
   music)

\filterNth 4 \relative { c'8 e g e c e g e d f g f d f g f }

Note that if you use this with \relative, it must be outside the 
\relative body.

Best,
Jean