Why do I get the error "Array indices must either be real positive integers or logical"?

I have the following code (below). Why do I get the error "Array indices must either be real positive integers or logicals"?

clear all
clc
syms f(x) m b 
Df = diff(f(x),x);
D2f = diff(f(x),x,2);
y = diff(f(x),x,3)-m^2*diff(f(x),x,1)+(1/2)*f(x)*((b/(2*m))*exp(m*x)-(b/(2*m))*exp(-m*x))
cond1 = f(0)== 0;
cond2 = Df(0)== 0;
cond3 = D2f(0)== 1;
conds = [cond1 cond2 cond3];
fsol(x) = dsolve(y, conds)

Answer 1

[Ameer Hamza]

Change the following lines in your code as follow

Df = diff(f,x);
D2f = diff(f,x,2);

i.e. remove the (x). It will remove the error but still dsolve() is not able to find an analytical solution. You might need a numerical solver e.g. ode45().