Expanding Matrix

Hi all,

I have a couple of array of varying size and would like to expand it to 100 x 360 matrix.

So it has to expand in 2 dimension (row and column)

As an example;

1 2 3

4 5 6

7 8 9

to

1 1 1 2 2 2 3 3 3

1 1 1 2 2 2 3 3 3

1 1 1 2 2 2 3 3 3

4 4 4 5 5 5 6 6 6

4 4 4 5 5 5 6 6 6

4 4 4 5 5 5 6 6 6

7 7 7 8 8 8 9 9 9

7 7 7 8 8 8 9 9 9

7 7 7 8 8 8 9 9 9

I have tried some of the solution offered in the forum but nothing work so far.

I really hope that there are matlab expert out there that can help me with this problem. Been stuck with it for some weeks already

Regards, Mo

Answer 1

[the cyclist]

Yet another:

 bigA = kron(a,ones(3))

I think Matt's comment in Paulo's answer is basically an obfuscation of this, right?

[Matt Fig]

Basically!

[mo pejo]

I like this answer the most. Although i need to understand the kron function more but this is the simplest.

Thanks a lot cyclist.

Answer 2

[Matt Fig]

I wrote the EXPAND function to do just this, in the general case, and do it efficiently.

a = [1 2 3; 4 5 6; 7 8 9];
b = expand(a,[3,3])
b =
   1     1     1     2     2     2     3     3     3
   1     1     1     2     2     2     3     3     3
   1     1     1     2     2     2     3     3     3
   4     4     4     5     5     5     6     6     6
   4     4     4     5     5     5     6     6     6
   4     4     4     5     5     5     6     6     6
   7     7     7     8     8     8     9     9     9
   7     7     7     8     8     8     9     9     9
   7     7     7     8     8     8     9     9     9

[mo pejo]

For your suggestion, i have this error message.

??? Undefined function or method 'expand' for input arguments of type 'double'.

maybe i should mention that i am using matlab 2010a.

doesn't really know whether it makes any difference.

[Matt Fig]

But did you download the function? That would be the first step. Then move the file to your current directory in MATLAB.

Answer 3

[the cyclist]

Here is one way to do it:

a = [1 2 3; 4 5 6; 7 8 9];

[nx ny] = size(a);

bigFactor = 3;

bigA = zeros(bigFactor*nx,bigFactor*ny);

for ix = 1:nx
for iy = 1:ny
    bigA(1+bigFactor*(ix-1):bigFactor*ix,1+bigFactor*(iy-1):bigFactor*iy) = a(ix,iy);
end
end

This will "expand" each element to a 3x3 array, which is what you example does. If you want something more general, you might need to define a "bigFactorX" and a "bigFactorY".

There are probably more efficient ways to do this, but I hope this way is at least clear, and if your arrays are not too huge, maybe the efficiency is not your biggest concern.

[mo pejo]

This answer is usable but as you said need to include bigfactorx and y for better manipulation.

Answer 4

[Paulo Silva]

yet another way

a=[1 2 3
4 5 6
7 8 9];
c=cell(size(a));
for b=1:numel(a)
c{b}=ones(3,3)*a(b);
end
cell2mat(c)

[Matt Fig]

Just for fun, a one-line version:

cell2mat(cellfun(@times,repmat({ones(3)},size(a)),num2cell(a),'Un',0))

[Matt Fig]

Or, perhaps more efficient:

cell2mat(cellfun(@(x,y) times(ones(3),x),num2cell(a),'Un',0))

[Paulo Silva]

that's great :) I really have to learn how to use cellfun :)

Answer 5

[Jos (10584)]

No need for loops, cell2mat or cellfun. Here is the simple approach using indexing:

% data
  A = [1 2 3 ; 4 5 6] 
  ef = [4 2]  % expand factor (row and column)

% engine
  [nr,nc] = size(A) ;
  ri = ceil((1:(ef(1)*nr))/ef(1)) ;
  ci = ceil((1:(ef(2)*nc))/ef(2)) ;
  B = A(ri,ci) 
% ... and as a one-liner for fun:
  B1 = A(ceil((1:(ef(1)*size(A,1)))/ef(1)), ceil((1:(ef(2)*size(A,2)))/ef(2))) ;

% test
  B2 = kron(A,ones(ef)) ;
  isequal(B, B1, B2) % yep!