Calculating a time duration from the graph

Hello

I am interested in finding out the time between two points in a graph (x-axis is the time, y-axis is the quantity of interest). For testing my logic, I'm using a sinusoidal wave to see if I get the correct responses. In the sinusoidal wave defined below, I'm interested in calculating the starting and ending x-coordinate when the y-axis value is less than 0.4.

t= [0:0.1:2*pi];
a=sin(t);
figure;
h=plot(t,a);

for i=1:max(t)
  y=get(h,'ydata');
  if a < 0.4
      start=get(h,'xdata');
      if a > 0.4
      fin=get(h,'xdata');
  end
end

This is an infinite loop, which doesn't give any output. I'm not really familiar with matlab and am not able to figure out how to get the start and fin values above.

Any help is appreciated.

Thanks!

Answer 1

[KSSV]

t= [0:0.1:2*pi];
a=sin(t);
figure;
h=plot(t,a);
% Get y data
y=get(h,'ydata');
% get x data
x=get(h,'xdata');
% find indices less then 0.4 in y 
idx = find(y<0.4) ;
% get values of y less then 0.4
y1 = y(idx) ;
% get time for respective y values
x1 = x(idx) ;
% min time of 0.4
xmin = min(x1)
% max time of 0.4
xmax = max(x)

[Mridul Garg]

Thank you so much for this. However, I want to generalize this function, in the sense that if there are two instances of the curve going below 0.4 and coming up above 0.4, I would want the start and end indices of both instances?

[KSSV]

Can be done.....doc find..

Answer 2

[Aleksandra Ksiezyk]

hallo,

I came across this post because I am looking for something similar.

do you know how one can calculate the length of the x axis (or a curve) for a given condition? e.g. idx = find (y <0.4 & y> 0.3). I have a plot describing the temperature of time (24h) and I would like to calculate how much time during the day the temperature is from 17.5 to 18.5 C?

cheers,

Ola