find first time where data crosses 1

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Benjamin
Benjamin le 11 Avr 2019
Commenté : Mark Sherstan le 11 Avr 2019
I have data stored in variables r and f. At some point the variable f goes above 1. How can I return the value of r the FIRST time f goes above 1? Interpolation of the r-value to find where f goes above 1 may be needed to be more precise. Any help would be greatly appreciated!

Réponses (2)

Mark Sherstan
Mark Sherstan le 11 Avr 2019
This example should help you with your data set
x = -2*pi:pi/12:2*pi;
y = 2*sin(x);
idx = find(y>1);
solution = idx(1)
fprintf("Solution at index %0.0f, x = %0.2f, y = %0.2f\n",solution,x(solution),y(solution))
figure(1)
hold on
plot(x,y)
plot(x(solution),y(solution),'*r')
  2 commentaires
Benjamin
Benjamin le 11 Avr 2019
Modifié(e) : Benjamin le 11 Avr 2019
Does this actually interpolate though? I have this:
ix = find(y > 1, 1, 'first');
ix_r(count) = r(:,ix);
ix_r = ix_r';
but the problem is that it just returns r for when f exceeds the threshold of 1. How can I interpolate to find exactly where it would cross 1? then interpolate r
Mark Sherstan
Mark Sherstan le 11 Avr 2019
You can use the built in function interp1 as you have a region of interest.

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Star Strider
Star Strider le 11 Avr 2019
I created ‘zci’ to do just that:
zci = @(v) find(v(:).*circshift(v(:), [-1 0]) <= 0); % Returns Approximate Zero-Crossing Indices Of Argument Vector
Then either use the interp1 function to do the interpolation, or create your own linear interpolation function:
x = 0:0.2:5; % Create Data
y = x.^2 - rand(size(x))*5; % Create Data
zci = @(v) find(v(:).*circshift(v(:), [-1 0]) <= 0); % Returns Approximate Zero-Crossing Indices Of Argument Vector
yzi = zci(y-1);
lintrp = [[x(yzi(1));x(yzi(1)+1)] ones(2,1)] \ [y(yzi(1));y(yzi(1)+1)] % Linear Interpolation
xint = (1-lintrp(2))/lintrp(1); % y=1 X-Value
figure
plot(x, y, xint, 1, '+')
grid
Experiment to get the result you want.

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