MATLAB: Find index of missing data point/ extrapolatation

extrapolationmissing data

I want fo find time and index when current is 90% of peak value during rise and fall.
Its easy to find rise but during fall, current drop quickly and there is no data point/index available at 90% of peak value.
How can I get index and time at 90% fall/drop value? probably by extrapolation or data fiting. please help.
Thanks
A1= load('data.txt');
T=A1(:,1); %Time
I=A1(:,2); %Current
[Imx,iImx]=max(I); %locate peak value and index in current
i90=find(I>=Imx*0.9,1,'first'); % First 90% of Peak during rise.
ii90=find(I>=Imx*0.9,1,'last'); % Last/second 90% of peak during fall
T90R=T(i90);
T90F=T(ii90);
plot(T,I,'.'); hold on
plot(T(i90),I(i90),'bs','LineWidth',2 ,'MarkerSize',6)
plot(T(i90),I(i90),'bs','LineWidth',2 ,'MarkerSize',6)
hold off

Best Answer

Another option: 
A1= load('data.txt'); 
T=A1(:,1); %Time
I=A1(:,2); %Current
[Imx,iImx]=max(I); %locate peak value and index in current
i90 = find(diff(sign(I-0.7*Imx)));                                      % Finds Both Crossing Indices (Does Not Need To Be Exact)
for k = 1:numel(i90)
    idx = i90(k)+[-2 2];
    I90(k) = interp1(I(idx), T(idx), 0.9*Imx, 'linear','extrap');       % Interpolate To Find More Precise Times
end
figure
plot(T,I)
hold on
plot(I90, 0.9*Imx*[1 1],'bs')
hold off
grid
xlabel('Time')
ylabel('Current')
producing:
Note that this will not find the indices, because there are no indices corresponding exactly to the 90% values. It will find the approximate corresponding times (within the precision limits of the data) for both of those points.
EDIT — (24 Jul 2020 at 12:53)
Corrected typographical error.
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