沿第3维度平均每4个条目

Question

How can I average every 4 data points along the 3rd dimension of a matrix?

My matrix is 245x85x1460 (lonxlatxhour). Since the 3rd dimension is 6 hourly data and I want daily data, I want to average every 4 data points (ie 1:4, 5:8, etc) and end up with a matrix of size 245x85x365.

Answer 1

Using reshape :

R = rand(245,85,1460); % Some random data, same dimensions as your example
szR = size(R);
A = squeeze(mean(reshape(R,[szR(1:2) 4 szR(3)/4]),3)); % Mean across 3rd dimension

The squeeze function is needed to push the result back down to a three-dimensional array. A(:,:,1) from above should be equivalent to mean(R(:,:,1:4),3) , and so on.

Answer 2

assuming that your data are stored in the variable old_data and you need a result in new_data, this sample code can be used.

new_data=zeros(245,85,365);
for k=0,364
  new_data(:,:, k+1) = sum(old_data(:,:,4*k+1:4*k+4), 3)/4
end

Answer 3

It's best to use arrayfun for this kind of stuff. Lets assume your original data is in matrix .

Create a 3-dimensional index vector for arrayfun to use:

index3d=zeros(1,1,size(matrix,3)/4);
index3d(1,1,:)=(1:size(matrix,3)/4);

Use arrayfun , unfortunately we must specify UniformOutput to be false which leads to a cell array as a result.

resultcell=arrayfun(@(x) mean(matrix(:,:,4*x-3:4*x), 3), index3d,'UniformOutput',false);

Convert cell array to 3-dimensional matrix:

result=cell2mat(resultcell);