How to join two vectors

I have a vector x1 = [1; 0; -2] and an array R = [1 0; 2 -0.5; 6 0] and I want to create a vector Xg = [0 -0.5 1 0 -2 0].

The logic to create Xg is that it will be long enough to have all the elements in x1 and R and to find the elements in Xg, you first put the elements on the second column of R (in the lines specified by the first column of R) and, after putting all elements of R in Xg, complete Xg with the elements x1, in the order they appear.

Do you understand the problem? Any ideas how to do it?

Answer 1

[dpb]

>> x2(R(:,1))=R(:,2);
>> x2(~ismember([1:length(x2)],R(:,1)))=x1
x2 =
       0   -0.5000    1.0000         0   -2.0000         0

[dpb]

BTW, the above needs a preallocation to the maximum length or the second will truncate the former. Just so happens the '6' in the first data set predefines it with the first case to be the maximum needed.

Answer 2

[Roger Stafford]

 Xg(R(:,1)) = R(:,2);
 Xg(setdiff((1:size(x1,1)+size(R,1))',R(:,1))) = x1;