IS_ABSOLUTE is path absolute

Windows, absolute paths must be at least 3 character long, starting with a root name followed by a slash

non-Windows, absolute paths must start with a slash

function y = is_absolute(p)
arguments
  p (1,:) char
end

% not Octave is_absolute_filename() because this is a stricter check for "c:" false

y = false;

L = strlength(p);
if ~L || (ispc && L < 3)
  return
end

if ispc
  y = strlength(stdlib.root_name(p)) && any(p(3) == ['/', '\']);
else
  y = p(1) == '/';
end

end

%!assert(is_absolute(''), false)
%!test
%! if ispc
%!   assert(is_absolute('C:\'))
%!   assert(is_absolute('C:/'))
%!   assert(!is_absolute('C:'))
%!   assert(!is_absolute('C'))
%! else
%!   assert(is_absolute('/'))
%!   assert(is_absolute('/usr'))
%!   assert(!is_absolute('usr'))
%! endif