How to find out if a curve passes through zero and goes from positive to negative value?
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Hi.I have divided this curve:
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/330866/image.bmp)
into 8 region where the points of 1st 1/8 portion is in x=1,2nd portion in x=2 etc like this:
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/330869/image.bmp)
Now I want to find out if there is a code by which for x=1,2,3,4,5,6,7,8 if there is zero crossing and points changing from positive to negative value.Hope you understand.Thank you in advance.
My code is this:
j=diff(I);
figure
plot(j);
n = 8 ;
d=j';
a=length(j);
% check whether data needed to be appended to reshape into n
b = n * ceil(a / n)-a ;
%
d = [d NaN(1,b)] ;
iwant = reshape(d,[],n);
% plot
figure
plot(iwant','b*')
3 Comments
Answers (1)
Image Analyst
on 12 Jul 2020
A single value cannot "change" sign or "cross the axis". Chances are that none of the y values where x = 0, 2, 4, ... are EXACTLY zero. So you need to look at x and find which ones are above the axis and which are below. You can then determine where the value went from - to + or vice versa.
y = rand(1, 20) - 0.5
s = sign(y)
% Find first index of the two where the crossing occurs.
positiveGoingIndexes = strfind(s, [-1, 1])
negativeGoingIndexes = strfind(s, [1, -1])
If you need more help, please attach "I" (poor name by the way) in a mat file with the paper clip icon.
10 Comments
Image Analyst
on 14 Jul 2020
Where did you show that error? I looked above and am not seeing it.
And I ran your latest attached code and it ran without error, though it did not have strfind() in it anywhere so it doesn't look like you took my suggestion.
And I still think looking for zero crossings of the derivative is not needed.
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