spline interpolation error , index in line 1

c0 = 47000000 ; 
e0 = 6110000 ; 
dt = 0.5 ; 
tmax = 5;
t = 0:dt:tmax ;
gamma = 0.08; 
u = 0.05; 
v = 0.05; 
e = 0.02; 
a = 1; 
b = 1; 
a1 = 4;
b1 = 9; 
phi = gamma+u+v+e;
xi = u + v;

e_t = e0 + b.*t - 0.5.*(xi.*a-e.*b).*t.^2;
e_t_1 = e0 + b1.*t - 0.5.*(xi.*a1-e.*b1).*t.^2;

spline = spline(t,e_t);
spline1 = spline(t,e_t_1); 

plot(t, spline) 
hold on 
plot(t,e_t_1)

hi i'm fairly new on matlab and would like to interpolate my results to get an accurate graph. Im getting an error which says " Index in position 1 is invalid. Array indices must be positive integers or logical values. Error in line 28 spline = spline(t,e_t); ". Would you be able to help me solve this issue? Also would it be more beneficial to use lagrange interpolation or spline? Thanks!

Answer 1

[Walter Roberson]

spline = spline(t,e_t);

After that line spline refers to an array not the function. Use a different variable name.

[Karolina Nowobilska]

Thank you, once I did that It was able to work, however I got another error saying "Error using plot There is no form property on the Line class."

So I changed the code to say:

plot(t, k,'r')

to which I got another error saying "Invalid data argument."

Is there anyway you can also help me with that?

[Walter Roberson]

variable = spline(t,e_t);

That form of spline() returns a piecewise polynomial. In order to plot it, you need to use ppval() indicating the places you wish to plot at.

In your case you could probably use

variable = spline(t,e_t, t);

which will return the interpolated values.