downsample fuction error, I want to know how to design the downsampling filter

x = [1 2 3 4 5 6 7 8 9 10];
y = downsample(x,3)

y =

       9    27    45    63    81

I want to know how to design the downsampling filter. I run the code on matlab2015b and the answer is wrong. so I consult the downsampling function. but I don't know how to design the filter. the following is the code of the downsample function provided by matlab:

% Downsampling procedure.
%
% Arguments:
%   grayscale I image
%   downsampling filter 'filter', should be a 1D separable filter.
%   'border_mode' should be 'circular', 'symmetric', or 'replicate'. See 'imfilter'.
%
% If image width W is odd, then the resulting image will have width (W-1)/2+1,
% Same for height.
%
% [e-mail], August 2007
%

function R = downsample(I, filter)

border_mode = 'symmetric';

% low pass, convolve with separable filter
R = imfilter(I,filter,border_mode);     %horizontal
R = imfilter(R,filter',border_mode);    %vertical

% decimate
r = size(I,1);
c = size(I,2);
R = R(1:2:r, 1:2:c, :);

Answer 1

[Honglei Chen]

This is a third party program and is not provided by MathWorks. I don't know what your goal is but the function is doing what exactly it's asked to do. From the comment, the function is meant to be used with an image, not a 1D signal. HTH