ZERO crossing with certain conditions

I am trying to write an algorithm which will look for zero crossings (or in my case crossing across a specific value) in my data and then keep only those points that are intercepted by values above a certain threshold; Here is the code I have written so far

Crossings=0;
for i=1:(length(X)-1 )
if ((X(i)>=4 && X(i+1)<4) || (X(i)<=4 && X(i+1)>4))
Crossings=1+Crossings;
index(i) = i;
end
end
INDEX = find(index>0);
for i = 1:(length(INDEX)-1)
    a = INDEX(i);
    b = INDEX(i+1);
    for j = a:b
if X(j)>4
    INDEX2(i,:) = [INDEX(i) INDEX(i+1)];

end
    end
end

However, this code does not seem to be doing what I want it to do. It keep all the points instead of those with points above 4 inbetween. I have checked the dataset by plotting it out and I know that some of the values have values less than four between them. Where am I going wrong?

Answer 1

[pfb]

I'd go like

 Y = X-4;
 c = Y(1:(end-1)).*Y(2:end);

Now c(i) should be negative whenever X(i) and X(i+1) are "on different sides" of 4.

So, to find the crossings, all you need to do is

 i = find(c<0);

I'm not sure I understand what you want, though. In your example the threshold is 4?