First point smaller than threshold

I have written the following code in order to find the first point of an array which is smaller than a threshold and calculate, afterwards, its distance from a certain point:

xfb1 = 7*10^7;
xfb2 = 6*10^7;
for i = 1:length(pks3)  %find the point which is the start of QRS   
    k = pks3(i):-1:length(xfb);
    first = k(k<xfb1); % find all k such that k is smaller than xfb1
    if ~isempty(first)
        first = first(1); %store the first number which is smaller than xfb1
    else
        disp('No value of k is smaller than xfb1.')
    end
    numPointsQ = 1:length(locs_RwaveB(i):first);
    mean_numPointsQ = mean(numPointsQ);
    Qstart = locs_RwaveB(i)*mean_numPointsQ;
end

The data come from electrogram recordings and the problem is that even though a have 250,000 samples in total from a recording, I get a massive amount of points in the end (10^12) which cannot be possible. I think it has to do with the for loop, as I ask it to run 10 times starting from 10 specific pks3 points each time, but for each array k I create I want only one first point to be detected. Could someone please tell me what's wrong with the for loop?

Thanks

Answer 1

[Steven Lord]

Take a look at the n input described in the documentation for the find function.

[Konstantinos]

I'm sorry but that's not helpful enough for my case. Thanks though for the answer.