how to find value of y from a graph with a known x value

Hi There

I already have another question up on this subject but i think i may be over-complicating what i'm trying to ask. In the file i have attached, i have a graph that i have plotted from the data in the worksheet. what i am trying to do is find for values of x, not limited to the data i have, find the corresponding y values. after some reading i think i would have to use interp1, but that came with some issues when i tried to implement it (The grid vector must contain unique points' error and then 'Vectors must be the same lengths') and also use index. would i have to interpolate first to get numbers in incrementys of 1 or 2 etc then use index=find?

thanks, Ross

Answer 1

[Chad Greene]

Hi Ross,

The data you provide make a multi-valued function. There are several possibly y values for any given x value, so interp1 would not be a valid way to get an answer. Even if you flip the x and y, it's still multi-valued.

Depending on your application, you might be able to think of each x value and each y value as functions of distance along the path. The distance along the path would be something like

d = [0;cumsum(hypot(diff(x),diff(y)))];

Then if you want to get the points xi,yi at some distance di, it would be

xi = interp1(d,x,di); 
yi = interp1(d,y,di);

[Ross Hanna]

im relatively new to matlab, and trying to execute this is problematic. if i import my data e.g. Force and Velocity, then type in your code, it comes up as "undefined function or variable 'di'". What should i set di equal to? if i set it equal to say 400N it says

Error using griddedInterpolant The grid vectors must contain unique points.

Error in interp1 (line 161) F = griddedInterpolant(X,V,method);

do i have to set it equal to a point in the data i have?

what i am trying to do is write a code that i can give it any value from 1-1400 for example and it will give me the y values associated with them and then i can average them. i looked at your other answer using the scatstat1 function but i dont know if i have to download a file that has it in for example?

Thanks for your reply