Simpsons 1/3 Rule to solve integration

The enthalpy of a real gas is a function of pressure as described below. The data was taken for a real fluid. Estimate and report the enthalpy of the fluid at 400 K and 50 atm (evaluate the integral from 0.1 atm to 50 atm).

My Code:

% Use Simpsons 1/3 Rule to solve integration

% Lower Limit
a = 0.1; % Pressure in atm
  
% Upper Limit
b = 50; % Pressure in atm
  
% Number of Segments
n = 9;

% Declare the function
T = 400; % Temp. in K
V = [250 4.7 2.5 1.49 1.20 0.99 0.75 0.675 0.60]; % Volume at T = 400K
% dVdT = V(T = 350) - V(T = 450)/100
dVdT = [0.625 0.0113 0.005 0.002 0.0014 0.0013 0.001 0.0009 0.0008]; 
H = V-T.*dVdT;

% inline creates a function of string containing in H
f = inline(H);
  
% h is the segment size
h = (b - a)/n;
  
% X stores the summation of first and last segment
X = f(a) + f(b);
  
% variables Odd and Even to store summation of odd and even terms respectively
Odd = 0;
Even = 0;
for i = 1:2:n - 1
    xi = a + (i*h);
    Odd = Odd + f(xi);
end
for i = 2:2:n - 2
    xi = a+(i*h);
    Even = Even + f(xi);
end
  
% Formula to calculate numerical integration using Simpsons 1/3 Rule
I = (h/3)*(X + 4*Odd + 2*Even)

The Error My Code Produces:

Error using inline (line 51)

Input must be a character vector.

Error in HW6_P1 (line 23)

f = inline(H);

Used this to solve function

Answer 1

[Cris LaPierre]

As the error states, the input to inline must be a character vector (e.g. f = inline('sin(alpha*x)'))

Note that inline is not recommended. Use Anonymous Functions instead.

Answer 2

[Torsten]

Be careful here: the pressure values are not equally spaced.

Use

T = 400; % Temp. in K
P = [0.1 5 10 20 25 30 40 45 50];  % Pressure in atm
V = [250 4.7 2.5 1.49 1.20 0.99 0.75 0.675 0.60]; % Volume at T = 400K
% dVdT = V(T = 350) - V(T = 450)/100
dVdT = [0.625 0.0113 0.005 0.002 0.0014 0.0013 0.001 0.0009 0.0008]; 
H = V-T.*dVdT;
I = trapz(P,H)

to calculate enthalpy at T=400 K and P=50 atm.