System of nonlinear differential equations

I modified the code from this page: https://www.mathworks.com/matlabcentral/answers/384298-how-to-solve-and-plot-system-of-nonlinear-differential-equations

Can anyone explain why the following code is incorrect? It attempts to graph the solution to the differential equation y' = y'' which is ce^x.

kp = 3;
kpm = 30;
kmp = 25;
klm = 15;
kl = 1;
f = @(t,y) [(y(2))];
tspan = [0, 25];
xinit = [0.01];
ode45(f, tspan, xinit)
legend('p(t)', 'm(t)', 'l(t)')

[darova]

Please explain how did you write your equation? Why

f = @(t,y) [(y(2))];

[Aleem Andrew]

As in the code from the linked page that line equates the first derivative of y to the second derivative.

Answer 1

[Walter Roberson]

Your xinit is a scalar, so the second parameter received by f will be a scalar; it is not possible to index the second element of a scalar.

If you are working with second derivatives then your xinit should be providing boundary conditions for the first derivative and second derivative, and your f should return a column vector with the same number of elements that xinit has.