How do I get the mean of always 4 columns

Hi,

I have a 527040 x 16 double array

I need the row mean of always 4 clomuns

So I need the row mean of column 1:4 5:8 9:12 13:16

So I want the mean([1:4],2) and mean([5:8],2)

but i dont want to wirte it down in that way. I need an more automatic way to do it

any ideas?

Answer 1

[madhan ravi]

AA=permute(reshape(A.',4,1,[]),[2,1,3]); % where A is size 527040 X 16
AAA=squeeze(mean(AA)); 
% if you want to reshape it as matrix then
% reshape(AAA,size(A,1),[]) %% edit removed transpose

[Antonio Melieni]

Its close to what I want!!!! As the result I wanted a 527040 x 4 array

What do I need to edit?

[madhan ravi]

reshape(AAA,size(A,1),[]) % at the end

[Antonio Melieni]

Element that I Want in {1,2} is in {2,1}

so close tho :)

[madhan ravi]

Just transpose!

[Matt J]

You will find this method less efficient (and more memory consuming) for larger data. Compare:

A=rand(527040,144);

tic;
AA=permute(reshape(A.',4,1,[]),[2,1,3]); % where A is size 527040 X 16
AAA=squeeze(mean(AA)); 
AAA=reshape(AAA,size(A,1),[]); % at the end
toc
%Elapsed time is 0.440566 seconds

tic;
result=sepblockfun(A,[1,4],'mean');
toc
%Elapsed time is 0.275432 seconds.

[madhan ravi]

Yes it‘s faster but I don‘t see why it would be memory consuming? If it all could be in one line.

[Antonio Melieni]

The transpose doesnt help.... I get an 4 X 527040 array then....

[Matt J]

Both the permute operation and the transpose operation allocate arrays the same size as A. sepblockfun avoids this.

[madhan ravi]

@Antonio:Matt‘s file exchange is Perfect why not use his solution then?

Answer 2

[Matt J]

One way is using sepblockfun, downloadable from here

https://www.mathworks.com/matlabcentral/fileexchange/48089-separable-block-wise-operations

Then you would do,

result = sepblockfun(yourArray, [1,4],'mean');

[Antonio Melieni]

Thanks, but thats a bit to complicated :)

[Catalytic]

@Antonio,

The only way I can Matt J's one line solution as being "too complicated" is that this is homework. So, you're not allowed to use third-party routines like sepblockfun as is, and it is a tedious matter for you to try to decipher what it is doing and adapt it to your assignment.

[Antonio Melieni]

@Matt

Thanks for your function and your help.

Will use your sultion :)