how can i combine two vector into one vector by the follwoing pattern ( and not use "For" Loop which take a lot of time , since this vector is a Million of elements ):

example :

a1 [ 1,2,3,4,5,6......]
a2 [ x,y,z,w,v,b,n,....] 

a3 = [ 1,x,2,y,3,z,4,w,5,v,.....]

many thanks for the help

[Stephen23]

Are these two vectors of the same class?

[Andrei Briciu]

If

a1 and a2 are rows in a matrix a0,

then

a3=reshape(a0,1,numel(a0));

Answer 1

[Michael Haderlein]

One option

 a3=[a1; a2];
 a3=a3(:)';

This only works if a1, a2 are line vectors. Otherwise, take the transpose of each in the first line.

[John D'Errico]

+1. This is the common, and I think best, trick.

It requires one to understand how MATLAB stores elements in memory when creating an array. MATLAB stores the elements in column 1, then column 2, etc. So by creating an array with two rows, and then unrolling it into a vector, you get what you want.

The alternative is to use indexing, which can be useful in some cases too.

Suppose for example, you wished to interleave the elements of a1 and a2, but that a1 has one more element than does a2. So now the first and last elements in your result vector will come from a1. In that case, you cannot create an array with two rows without padding on a trash element to a2.

So you might do this:

a3((1:length(a1))*2 - 1) = a1;
a3((1:length(a2))*2) = a2;

See that I've created index vectors, which can be viewed as implicit loops. Internally, MATLAB will do the looping for you, but you need not use an explicit for loop, so the above scheme will still be fast.

[Ellyn Gray]

Thank you so much for this!! Very helpful explanations from you both

[sravankumar v]

can you please tell me how can i arrange the elements of three matrices alternatively in to another matrix using for loop?

Answer 2

[Orion]

Hi,

the command a3=[a1; a2]; will put the a2 vector at the end of the a1 vector and not alternate the value of a1 and a2.

to make the pattern asked, you should do something like :

a1 = 1:10; 
a2 = 21:30;
a3 = zeros(1,length(a1)*2);
a3(1:2:end-1) = a1;
a3(2:2:end)   = a2;

this will work very fast even for a million element vector.

[Michael Haderlein]

If you use both lines, then it will work. Of course it doesn't work if you only use the first line. No doubt, your solution will work as well.

[Orion]

Sorry, i misread your solution. it works fine.

[Chris Liu]

Thanks for the answer. So what's the case if and are matrix? When I do like that,

just change a3 = zeros(1,size(a1,2)*2);

it reports error: "unable to perform assignment because the left and right sides have a different number of elements." Thanks.

[Chris Liu]

sorry for my bad, I miss the zero matrix. It works fine under ur answer. thx