How do I vectorise a for loop that reorders 4 bytes of an array with each iteration

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Scorp 2016-9-7

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https://ww2.mathworks.cn/matlabcentral/answers/302160-how-do-i-vectorise-a-for-loop-that-reorders-4-bytes-of-an-array-with-each-iteration

评论： Scorp 2016-9-8

A = uint8(Data); %Assume the data comes in multiples of 4 bytes
B = [];
for byteNo = 1 :4: length(A)-3 % changed 4  to 3 to fix the typo that was flagged by Walter Roberson
   B = cat(2,B,[A(byteNo+2),A(byteNo+3),A(byteNo),A(byteNo+1)]);
end

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Walter Roberson 2016-9-7

I suspect the termination should be length(A)-3 rather than -4 . Consider that if A was length 8, then there would be room for two swaps of length 4, but your upper bound would be byteNo = 4 which is before the second lower byteNo needed for the swap, which has to start at byteNo = 5.

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Answer 1

Walter Roberson 2016-9-7

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If the vector is not a multiple of length 4, discard mod(length,4) values from the end of it. Take the resulting vector and reshape() it into 4 rows. Reorder the rows. Reshape the result into a vector.