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

7 views (last 30 days)
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 3 Feb 2017
@Grillteller: is there a particular reason why you need to use sym? If not, then you are learning some incredibly inefficient and difficult ways to write MATLAB code. Why not simply use fast and efficient array operations with vectorized code?
Grillteller on 6 Feb 2017
Hi Stephen, i need to set up a lot of terms with undefined variables (the term is longer than the example) and in a next step i have to differentiate the terms. After this step i substitute the variables with numbers. Perhaps there is a better way...
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

#### 1 Comment

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