Vectorizing a nasted loop
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hello,
I'm using a simple nasted loop for assignment:
for i=1:Chunks %loop for number of chunks
for n=1:Window
DataChunk(n,i) = Raw.RAW(idx); %for each itteration - take the right raw data into the right chunk
TimeChunk(n,i) = Raw.Time(idx);
idx = idx+1; % increment the indes
end
end
this loop takes forever for a very big amount of data,
anyome can please help me vectorize it ?
2 个评论
Jos (10584)
2019-5-1
Did you preallocate the two output arrays? That should speed things up considerably.
Add this before the loop:
Datachunk = zeros(Window, Chunk)
Oded T
2019-5-2
采纳的回答
Rik
2019-5-1
This should work:
idx=reshape(1:(Chunks*Window),Window,Chunks);
DataChunk=Raw.RAW(idx);
TimeChunk=Raw.Time(idx);
The output should be the correct size, as it usually retains the shape of the index. If this is not the case, you can always use reshape to adjust the arrays to suit your needs.
If idx didn't start at 1 in your code, you can add the old value after the reshape.
4 个评论
Rik
2019-5-2
Which part do you want me to explain? The indexing, or the way I created the indexing array?
Guillaume
2019-5-2
There's nothing mysterious about it. It's simply reshaping a vector/matrix in the desired shape (in your case a Window * Chunks array). The idx business is just in case your original matrix/vector is larger than the desired result, and simply crops the original array/matrix to the required number of elements.
If the original array/matrix has already the required number of elements, then you don't need to bother with idx. See my answer.
更多回答（1 个）
Guillaume
2019-5-1
DataChunk = reshape(Raw.RAW, Window, Chunks);
TimeChunk = reshape(Raw.Time, Window, Chunks);
That's assuming that RAW and Time are structure fields or object properties and not object methods.
5 个评论
Rik
2019-5-1
It also assumes the size of both is Window*Chunks (and my solution assumes they are at least that size, but so does the original code).
Guillaume
2019-5-2
Yes, of course. There is a lot of information missing in the question.
To Oded, if numel(Raw.RAW) > Window*Chunks, you should use Rik's solution. If numel(Raw.RAW) == Window*Chunks, you should use mine as it's simpler in that case.
Guillaume
2019-5-2
I think we need a bit more context to understand your question. It seems you want to divide some sampled data into chunks. Why? What are you going to do with it?
Reshaping the samples into a matrix has some advantages as matrix are easier to work with, but it forces you to have the same number of samples in each chunk, which by the sound of it may be a problem. What are you doing with the matrix afterward?
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