performance of diff on a logical array
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Tom DeLonge
el 27 de Feb. de 2018
Comentada: Tom DeLonge
el 9 de Mzo. de 2018
I have a 5000x5000 logical array. I want to find the indices where the logical array has a rising edge, e.g. whenever the array goes from 0 to 1 within a row. Until now I did this as follows:
rising_edges = diff(A) == 1;
However, I notice that this is slow. I am pretty certain that this is slow because diff(A) has to allocate new memory for a 5000x5000 double array.
I am wondering: is there a more elegant and faster way of doing this? I'd also be happy with just the subscripts instead of a full logical index array.
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Walter Roberson
el 9 de Mzo. de 2018
rising_edges = A(:,2:end)) & ~A(:,1:end-1);
this operates on the entire array at the same time. You can use the two-output version of find() on the result, prioritized by row instead of by column. You can put the two outputs together and sortrows() or there are various other ways like findgroups()
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lokender Rawat
el 9 de Mzo. de 2018
You can simply declare an array "mat" and do the following, you do not need to create another new double array here. It will find the index from the logical array for each row and put it in "index" variable.
[matRow,matCol]=size(mat)
for i=1:matRow
index=find(mat(i,2:end)-mat(i,1:end-1)==1)
end
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Walter Roberson
el 9 de Mzo. de 2018
This will overwrite the entire variable index each time through the loop.
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