# Why am I getting this interp1 error?

32 views (last 30 days)
Shawn Alcorn on 3 Feb 2023
Answered: Bhanu Prakash on 13 Mar 2023 at 10:49
%% AEM 588 HW1 Question 2
clear all;
close all;
clc;
global A_const;
global B_const;
global C_const;
mu = 398600; %km^3/s^2
% % nu0 = 1e-3;
Isp = 100;
r0 = 2378 + 12800; %km
m0 = 8; %kg
T = 12/1000;
v0 = sqrt(mu/r0);
rho0 =1;
tau0 = 0;
nu0 = T/m0/9.81;
T0 = nu0*(mu*m0)/r0^2*1000.;
mdot = T0/Isp/9.81;
A_const = T0*r0^2/1000;
B_const = mu*m0;
C_const = mdot*sqrt(mu*r0^3);
tspan = [0 10000];
initialConditions = [1.;0.;1.];
%Solve ODE
[tau,rho] = ode45(@odefun,tspan,initialConditions);
t = tau;
r = rho(:,1);
rMoonNorm = ones(length(r),1).*(384400/r0);
b_line = polyfit(t,rMoonNorm,1);
y_line2 = polyval(b_line,t);
x_int = interp1((y_line2-r),t,0);
Error using matlab.internal.math.interp1
The sample points must be finite.

Error in interp1 (line 188)
VqLite = matlab.internal.math.interp1(X,V,method,method,Xqcol);
y_int = polyval(b_line,x_int);
figure(1); clf(1);
plot(t,r)
hold on
plot(t,rMoonNorm)
xlabel('Time, Non-Dimensional')
t = tau./sqrt(mu/r0^3);
r = rho(:,1).*r0;
rMoon = ones(length(r),1).*384400;
time_intercept = x_int/sqrt(mu/r0^3);
hours_intercept = time_intercept/3600;
days_intercept = hours_intercept/24;
Prop_mass = mdot*time_intercept;
fun = @(x) T0./(m0-mdot*x)
dV = integral(fun,0,time_intercept);
figure(2); clf(2);
plot(t,r)
hold on
plot(t,rMoon)
plot(x_int,y_int,'rx')
xlabel('Time (s)')
function dydt = odefun(t,y)
global A_const;
global B_const;
global C_const;
dydt = zeros(3,1);
dydt(1) = y(2);
dydt(2) = ((y(3)^2)-y(1))/(y(1)^3);
dydt(3) = (A_const/(B_const - C_const*t))*y(1);
end
##### 2 CommentsShowHide 1 older comment
KSSV on 3 Feb 2023
Check your tspan llimits.....You may reduce it? Also see are all the parameters used are in the same unit system.

Bhanu Prakash on 13 Mar 2023 at 10:49
Hi Shawn,
As per my understanding, you are trying to interpolate a 1-D function using "interp1" function and are facing some errors in it.
If you look at the code, there is a matrix "r" which is passed as an argument to the "interp1" function. The matrix "r" contains integers values along with "NaN" and "Inf (infinity)" values. These values are the reason for the error.
One workaround for this error is to replace the "NaN" and "Inf" values with finite values. I have tried this at my end and the code worked fine.
Thanks,
Bhanu Prakash.

### Categories

Find more on Signal Operations in Help Center and File Exchange

R2021a

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!