vim - Easy way to change to parent dir of a buffer? -

if buffer list looks this:

 :ls   1 %a   "application/powermodeserver/test/testtimer.cpp" line 1   2      "application/powermodeserver/test/fakeclient.cpp" line 0   3      "application/powermodeserver/test/fakevp.cpp" line 0   7      "application/powermodeserver/private/imsgsender.h" line 0   9      "application/powermodeserver/private/udpsocket.h" line 0  17      "application/powermodeserver/src/powerfsm.cpp" line 0  18      "application/powermodeserver/src/lua/src/lfunc.h" line 0  19      "application/powermodeserver/src/lua/src/lmem.h" line 0  20      "application/powermodeserver/src/lua/src/ltable.h" line 0  41      "application/powermodeserver/src/powermodeserver.cpp" line 0  42      "application/powermodeserver/src/udpsocket.cpp" line 0  43      "application/powermodeserver/src/timer.cpp" line 0 

what's easiest way change vim's current working directory parent directory of, say, buffer 19? i'm used finger-mumbling way there using:

:cd a<tab>p<tab>s<tab>l<tab>s<tab>

requires quite few keystrokes---especially if completions ambiguous. i'd more concise, like:

:cd ~19

recommendations?

edit: added mods suggested zyx here since don't have sufficient points edit jeet's answer directly:

function! cdbufworkingdir(target)     if empty(a:target)         let targetdir = expand("%:p:h")     else         if a:target =~# '^\~\d'             let targetdir = fnamemodify(bufname(str2nr(a:target[1:])), ":p:h")         else             let targetdir = a:target         endif     endif     execute "cd ".fnameescape(targetdir)     echo targetdir endfunction command! -nargs=? cdbuf :call cdbufworkingdir(<q-args>) 

i try in "~/.vimrc" (updated in accordance op's specs):

function! cdbufworkingdir(...)     if a:0 == 0         let l:targetdir = expand("%:p:h")     else         if a:1[0] == "~"             let l:targetdir = fnamemodify( bufname(str2nr(a:1[1:])), ":p:h" )         else             let l:targetdir = a:1         endif     endif     execute "cd ". l:targetdir     echo l:targetdir endfunction command! -nargs=* cdbuf :call cdbufworkingdir(<q-args>) 

then issuing command buffer number preceded "~" argument (e.g., :cdbuf ~3) switch working directory buffer's working directory. if argument not preceded "~", treated directory path directly change. while command without argument switch working directory current buffer's working directory. robustness, should add idiot-checking codes (e.g., happens when multiple arguments given?), or handle special cases (e.g., happens if string or buffer name passed, or file symlink).