How to combine multiple nx1 size of matrix into one matrix

As topic mentioned, i want to combine a matrix from many nx1 size matrix, i would like to have a script

Many Thnkas, Alex

[Azzi Abdelmalek]

Can you explain with an example?

[NG]

For example, I got matrix A, A=[1;2;3], and B, B=[4;5;6;7]. I want to combine them to be matrix C, C=[1;2;3;4;5;6;7], but now i got many matrix with differnet number of rows. How can i combine them ? Thanks!

[tran thang]

Hi! You can reference my code! Good lucky! >> a{1} = [1 1 1;1 1 1]; >> a{1}

ans =

     1     1     1
     1     1     1

>> a{2} = [2 2 2;2 2 2]; >> a{3} = [3 3 3]; >> a{4} = [4 4 4;4 4 4;4 4 4]; >> a{5} = [5 5 5;5 5 5]; >> A = vertcat(a{:})

A =

     1     1     1
     1     1     1
     2     2     2
     2     2     2
     3     3     3
     4     4     4
     4     4     4
     4     4     4
     5     5     5
     5     5     5

Answer 1

[Matt J]

could you plz exlpain more about vertcat(matrices{:}); by showing an example.

    >> matrices={[1,2;3 4],[5,6;7,8]};
    >> vertcat(matrices{:})

    ans =

         1     2
         3     4
         5     6
         7     8

[NG]

Sorry, i want to clarify that now i am using a loop to generate matrices out and then store them into a new matrix .Can i still use vertcat?

[Matt J]

Yes, store the i-th vector to matrices{i} in the loop and then when the loop is finished, use vertcat as above.

[NG]

However, Matlab appears this one (In an assignment A(:) = B, the number of elements in A and B must be the same.) , how can i fix it ?

[Matt J]

Sounds like you are not using cell array indexing. If you don't know what that means, you have some reading to do I'm afraid,

http://www.mathworks.com/help/matlab/matlab_prog/create-a-cell-array.html

http://www.mathworks.com/help/matlab/matlab_prog/access-data-in-a-cell-array.html

[NG]

Ok! Thanks !

[Michael Haderlein]

The first step to fix it is to show the code which is throwing this error ;-)

Answer 2

[Azzi Abdelmalek]

You mean concatenation

[A;B]

Read the documentation http://www.mathworks.com/help/matlab/math/creating-and-concatenating-matrices.html#f1-85019

Answer 3

[Joakim Magnusson]

Where do you have your matrices? If you can get them into cell arrays like this:

matrices = cell(1,2);
matrices{1} = A;
matrices{2} = B;
C = [];

Then you could do like this:

for i  = 1:size(matrices,2)
   C = [C ;cell2mat(matrices(i))]
end

I'm not sure how you want to combine your matrices but hope this was at least a little helpful.

[Michael Haderlein]

If A, B, ... are in a cell array, you wouldn't merge them by a loop but rather by cell2mat(matrices').

[Matt J]

cell2mat.m does use a loop. You could use vertcat, which is a builtin optimized function,

C=vertcat(matrices{:});

[NG]

Matt , could you plz exlpain more about vertcat(matrices{:}); by showing an example. Thank You!

[Michael Haderlein]

Thanks @Matt, I didn't use cell2mat a lot, so I didn't think about it too much. Your solution is very elegant.

Answer 4

[Joakim Magnusson]

Sorry, i want to clarify that now i am using a loop to generate matrices out and then store them into a new matrix .Can i still use vertcat?

NG, do you mean like this?

B = [];
for i = 1:10
   A = magic(4);
   A(:, 2:4) = [];
   B = vertcat(B, A);
end

[Matt J]

That would not be recommendable. With B growing inside the loop, you will be doing a lot of repeated memory re-allocation.

[Joakim Magnusson]

ah, okay!