What is wrong with my code?

% It finds the velocity using RK 4 method
clc; % Clears the screen
clear all;
h = input('Insert the value of the step size: '); % step size
t = 0:h:20; % Calculates till v(20)
v = zeros(1,length(t)); 
v(1) = input('Insert the value of the initial velocity: '); % initial condition
F_xy = @(v,t) 9.81 - ((0.28/85) * (v^2));
for i=1:(length(t)-1) % calculation loop
    k_1 = F_xy(t(i),v(i));
    k_2 = F_xy(t(i)+0.5*h,v(i)+0.5*h*k_1);
    k_3 = F_xy((t(i)+0.5*h),(v(i)+0.5*h*k_2));
    k_4 = F_xy((t(i)+h),(v(i)+k_3*h));

    v(i+1) = v(i) + (1/6)*(k_1+2*k_2+2*k_3+k_4)*h % main equation
end

Answer 1

[Cris LaPierre]

Nothing. It's doing exactly what you've told it to do.

You specified a stepsize of 0 (variable h).

This causes t to be an empty vetor.

t=0:0:20

t =

  1×0 empty double row vector

This causes your for loop to never run, since 1:(length(t)-1) simplifies to 1:-1.

[Ibrahim Butt]

Thank you for helping me.

Answer 2

[Alan Stevens]

Your code works if you put in sensible numbers for step size and velocity.

Zero is not sensible!

[Ibrahim Butt]

Thank you for helping me.