How to replace values in a very large array

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Jonathan Weber on 3 Aug 2022
Commented: Jonathan Weber on 4 Aug 2022
Hi
I have an array idx_Laser which is very large (~1 billion values). Whenever a certain condition is fulfilled for another array tdcRelTime I want to replace the values in my idx_Laser array.
The trivial solution would probably be:
for xx = 1:length(tdcRelTime)
if tdcRelTime(xx) < 1.5e10
idx_Laser(idx_Laser==xx) = 0;
end
end
However, as both arrays are quite big, this takes extremely long.
Is there a solution for that problem that is computionally more efficient?
I thought about iterating only through a part of the idx_Laser array, as this is sorted by the size of the values, but I couldnt come up with a good solution.
3 CommentsShow 1 older commentHide 1 older comment
Jonathan Weber on 4 Aug 2022
Hey :)
Thanks for the comment.
Yes I can hold the arrays in memory. But
idx_Laser(ix) = 0;
is not really what I was searching for.
The Problem is in the line
idx_Laser(idx_Laser==xx) = 0;
Because sweeping through that array is what takes so long.
I will take a look at these tall arrays though.
Jonathan Weber on 4 Aug 2022
What I came up with is a bit confused, but the performance is not so bad.
As I mentioned the array idx_Laser is sorted and the longest bin has "noElements" values.
Maybe someone has a better idea:
for zz = 1:length(tdcRelTime)
if tdcRelTime(zz) < 1.5e10
idx = find(idx_Laser == zz, 1);
for xx = idx:idx + noElements
if idx_Laser(xx) == zz
idx_Laser(xx) = NaN;
end
end
end
end

Mohammad Sami on 4 Aug 2022
You can modify what @dpb suggested as follows.
xx=find(tdcRelTime < 1.5e10);
idx_Laser(ismember(idx_Laser,xx)) = 0;
Jonathan Weber on 4 Aug 2022
yes, thats much faster than my approach...
Thanks to both of you!

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