How to multiply a matrix to another cell matrix

Hello

I have a 3*3 matrix such as

[1 0 0;

8 23 1;

5 7 10]

On the other hand, I have matrix 2000*1 containing 2000 cell arrays which are 3*4 matrices such as

[ [3*4];

[3*4];

[3*4];

.

.

]

I want to multiplythis 3*3 matrix to each of 3*4 matrices in the big matrix and obtain a final 2000*1 matrix. I have seen the code @(x)x.*matrix in MATLAB forum.

However, it gives out a function handle..I need to use the numeric output. What should i do?

[David Hill]

You need to explain more. How are you getting a 2000x1 matrix? What do you mean when you say multiply? 3x3 * 3x4 = 3x4 matrix, you cannot perform element-wise multiplication on different sized matrices.

Answer 1

[Stephan]

Use cellfun:

A = magic(3)

A = 3×3

8 1 6 3 5 7 4 9 2

C = {rand(3,4); rand(3,4); rand(3,4); rand(3,4); rand(3,4)}

C = 5×1 cell array
    {3×4 double}
    {3×4 double}
    {3×4 double}
    {3×4 double}
    {3×4 double}

result = cellfun(@(x)mtimes(A,x),C,'UniformOutput',false)

result = 5×1 cell array
    {3×4 double}
    {3×4 double}
    {3×4 double}
    {3×4 double}
    {3×4 double}

C{1}

ans = 3×4

0.7660 0.7423 0.4707 0.8602 0.4490 0.2799 0.8553 0.2608 0.4265 0.9783 0.5613 0.8271

result{1}

ans = 3×4

9.1360 12.0880 7.9885 12.1046 7.5281 10.4742 9.6180 9.6741 7.9578 7.4446 10.7034 7.4423

[Pooneh Shah Malekpoor]

Great, thanks!

[Pooneh Shah Malekpoor]

BTW, Is there any faster approach?

[Stephen23]

"Is there any faster approach?"

Use a loop (with preallocation).

Answer 2

[Jan]

Create a simple loop:

A = rand(3, 3);
for iC = 1:numel(C)   % Where C is your cell
  C{iC} = A * C{iC};
end

The approach with cellfun looks smart, but it is slower:

C = cellfun(@(c) A*c, C, 'UniformOutput', false);

By the way, .* is the elementwise operation. You need *, because the matrix and the cell elements do not have the same number of elements.

[Stephan]

@Jan will this be faster than cellfun for a big number of operations?

[Stephen23]

"will this be faster than cellfun for a big number of operations? "

Yes.

Try it.