ZERO crossing with certain conditions

Question

Bran 2015-4-13

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/202745-zero-crossing-with-certain-conditions

关闭： MATLAB Answer Bot 2021-8-20

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?