How to get matlab to find a point on a graph where there is a sudden change?

I have a graph of 'data point number' against 'distance'.

Around data point number 2500 damage is introduced and the graph has a sudden spike.

Is there some way of getting matlab to detect this sudden change?

Answer 1

[Michael Haderlein]

You can define a threshold value either for y or for diff(y). Assume you define failure as point when y>0.2:

plot(x,y), hold all
ind=find(y>0.2,1,'first');
plot(x(ind:end),y(ind:end),'rx')
fprintf(1,'Failure at x = %d\n',ind);

If you want to define the threshold on diff(y), do the same but use ind=find(diff(y)>0.2,1,'first'); instead.

[Joe Smith]

Could you clarify what '%d/n' does?

[Image Analyst]

%d means to print the value of ind assuming that ind is an integer. \n adds a line feed (new line) after it's been printed.