interp1 problem

Question

Hannah 2011-5-7

Hi, I am trying to use the interp1 function. I keep getting a NaN response. This is what I have tried:

EDU>> rho=[1.4133,1.3587,1.2614,1.1769,1.1032];

EDU>> T=[250,260,280,300,320];

EDU>> interp1(rho,T,275)

ans =

NaN

It is very simple, but I am trying to work my way up. Thank you

Answer 1

Teja Muppirala 2011-5-7

Are you trying to find the value of rho given that T=275?

rho=[1.4133,1.3587,1.2614,1.1769,1.1032];
T=[250,260,280,300,320];
interp1(T,rho,275)

Hannah 2011-5-7

Yes, I want to interpolate rho for a range of T from 275 - 320.

Teja Muppirala 2011-5-7

Then you can do:

Ti = 275:320;

rho_i = interp1(T,rho,Ti)

You basically had your rho and T backwards.

Your code:

interp1(rho,T,275)

computes "When rho is 275, what is T?"

You get a NaN because 275 is way outside the data for rho, and by default it does not extrapolate outside the original data range.