MATLAB: Sequence within a sequence

sequencing loops

Hi I want to be able to create a sequence as such

j=1111,2222,3333,4444

It is for use in a loop where j will jump up by 1 for every forth value of i.

thanks

Best Answer

Another way (you need Signal Processing Toolbox)

   x = 1:4;
   y = upsample(x,4);
   h = ones(4,1);
   y = filter(h,1,y);

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MATLAB: How to multiply a vector with each column of a matrix most efficiently

The first question has been addressed, so I'll just throw out a couple of points about the second question.

The newer the version you're running, the more benefit you'll see from JIT enhancements, so loops aren't as slow and generally evil as they used to be.
Methods 2 & 4 both require a lot more memory and related memory operations. Assuming the variables are large, that's a lot of overhead on top of the calculations. (Also remember that MATLAB requires contiguous memory addresses for arrays.)
Method 3 not only incurs the same memory-monkeying penalty, but also needs more calculations.

MATLAB: How to create a block diagonal sparse matrix from a paged array in an efficient way

You can create such a matrix using the "repmat", "reshape" and "transpose" operations, by means of the following code:

% Sample paged matrix
m=3;n=2;p=4;
A = rand(m,n,p);
% The following create the indices via repmat, reshape and transpose operations
I2 = repmat((1:m)',1,n); 
I3 = repmat(I2,1,1,p); 
If = repmat((0:m:(p-1)*m),m*n,1); I = I3 + reshape(If,m,n,p);
J2 = repmat((1:n),m,1);
J3 = repmat(J2,1,1,p); 
Jf = repmat((0:n:(p-1)*n)',1,m*n)'; J = J3 + reshape(Jf,m,n,p); 
%Create the sparse matrix
S = sparse(I(:),J(:),A(:))

If you are using MATLAB R2016b or a more recent release, this code can be simplified by means of implicit expansion

m=3;n=2;p=4;
A = rand(m,n,p);
I = repmat((1:m)', 1, n) + permute(m*(0:p-1), [3 1 2]);
J = repmat(1:n, m, 1) + permute(n*(0:p-1), [3 1 2]);
S = sparse(I(:), J(:), A(:))

Best Answer

Related Solutions

MATLAB: How to multiply a vector with each column of a matrix most efficiently

MATLAB: How to create a block diagonal sparse matrix from a paged array in an efficient way

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