# Extract every second value of a vector and write into a new vector

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Grillteller on 3 Feb 2017
Edited: Grillteller on 6 Feb 2017
Hi, another trivial question i guess - i'm a beginner at Matlab.
MATLAB code
Xi = sym('Xi',[1,n2]);
for i=1:1:6
BGx(i,1)=L1*Xi(i);
BGy(i,1)=L5*Xi(i);
end
I want to write the results into a new vector. I tried vertcat and some for loops but it didn't work out.
MATLAB code
BG=vertcat(BGx; BGy);
Here i get a result like (L1*Xi1, L1*Xi2, L1*Xi3, ...., L5*Xi1, ...., L5*Xi6) but i want to get (L1*Xi1, L5*Xi1, L1*Xi2, L5*Xi2, ...., L5*Xi6). Is there a trick when using vertcat or do i need to write a loop. I have the problem that BG has another dimension than the 2 vectors BGx and BGy. Thanks in advance
Stephen Cobeldick on 6 Feb 2017
@Grillteller: if you need mathematically exact algebraic outputs then sym is fine. If you are doing calculations with numbers then a numeric solution might do the job, and be faster and simpler to code than using sym. For example it is easy to use gradient, or fit a spline and differentiate that.

Niels on 3 Feb 2017
result = zeros(12,0);
for i=1:1:6
result((i-1)*2+1)=BGx(i,1);
result(i*2)=BGy(i,1);
end
Grillteller on 6 Feb 2017
When i try this, i get an error code:
The following error occurred converting from sym to double: DOUBLE cannot convert the input expression into a double array.
Edit: I found the solution - A "sym" was missing before "zeros".
result = sym(zeros(12,0));
for i=1:1:6
result((i-1)*2+1)=BGx(i,1);
result(i*2)=BGy(i,1);
end

Stephen Cobeldick on 3 Feb 2017
Edited: Stephen Cobeldick on 3 Feb 2017
This is MATLAB so why use an ugly loop? MATLAB is much better than that! The code is simpler without it (I also got rid of sym: you don't say why you need it, and numeric operations will be much faster and more efficient):
>> X = [1,2,3,4];
>> L1 = 90;
>> L5 = 10;
>> Z = [L1+X;L5+X];
>> Z = Z(:)'
Z =
91 11 92 12 93 13 94 14