Finding Corresponding x axis values of a constant curve on Y-axis

Hello,

I have a curve similar to shown below in the image.

I plan to find the 'x' values at which the function 'y' is maximum

I have denoted 'y' by 'cylpressure'

When i write the following,

[y,x] = max(cylpressure);

It returns me values

y = 400000

x= 118

But i need the values of 'x' when y starts to get constant(i.e. at x = 118, which i got from above code) and when y starts dwelling down(i.e at x = 180)

What changes must be done?

Answer 1

[KSSV]

Let (x,y) be your data. Where y is cylpressure Dont use

[y,x] = max(cylpressure);

Rather, use:

[val,idx] = max(cylpressure);

In the baove val gives you maximum value of y and idx gives you index of the maximum value. You can get the respective value of x using x(idx).

If you know the value of x and you want to get corresponding y value use interp1.

yi = interp1(x,y,xi) ; 

[Sandip Ghatge]

Thanks for the answer.

I have implemented this, but still it gives me the single value of x corresponding to y i.e it gives x = 118

What i was looking for is to get both values of x corresponding to max y

i.e i am looking for x = 118 and x = 180 as an answer

Can you please guide me through?

[KSSV]

max gives you only one value. You can read about findpeaks. Or sort the given array in descending order and pcick the first two values. Read about sort.

[Sandip Ghatge]

Thank you, i guess 'sort' will work. I was aware about findpeaks but i dont have a signal processing toolbox hence did not use it.

[Sarah Nasra]

how we can do the opposite? if i know the y value and want to extract the x value?