Indexing irregular, constant width "stripes" of a 2-d array
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I want to index an array in asynchronous stripes of constant width. Perhaps best described by an example:
bob = magic(7);
w = 2;
c = [1 3 6];
r = [1 4 3];
for stripe = 1:length(c)
res(stripe,:) = bob(r(stripe), c(stripe)+(0:w-1));
end
bob =
30 39 48 1 10 19 28
38 47 7 9 18 27 29
46 6 8 17 26 35 37
5 14 16 25 34 36 45
13 15 24 33 42 44 4
21 23 32 41 43 3 12
22 31 40 49 2 11 20
res =
30 39
16 25
35 37
Can the experts recommend a way to generally express this that is compact, readable and fast?
1 个评论
KSSV
2016-11-18
Not clear, you want to extract elements with a common difference/ width? Check res, the difference is 9,9,2.
回答(1 个)
Walter Roberson
2016-11-18
Anything much different than that stops being as readable.
You could think in terms of calculating the linear indices of the sources using sub2ind. Once you have that then you can see how the entire source could be vectorized, and if the destination does not exist (or is being completely overwritten) you could construct the destination with reshape() instead of sub2ind for that.
You could then take the step of bringing the sub2ind inline. For 2d arrays it is ((row_number-1)*number_of_rows+column_number)
That would increase your speed. But it is less readable.
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