Array indices must be positive integers or logical values

I am trying to model a step input to a damped spring-mass system. Code is pretty simple as pasted below:

%% Parameters
k = 70e3;
mq = 20;
c_crit = 2360;
c = 2294.38;
z = c/c_crit;

n_freq = sqrt(k/mq);
d_freq = n_freq*sqrt(1-z^2);

F0 = 412.02;
%%
Ts = 1/1000;
t = [0:Ts:0.3-Ts];

Q = F0/n_freq^2;
R = exp(-z*n_freq.*t);
S = (z*n_freq)/d_freq;

x = Q(1-R(cos(d_freq.*t) + S*sin(d_freq.*t)));

I'm getting the error stated due to the function x. I've not much experience with MATLAB and I have no idea what the problem is. Any tips would be appreciated!

[Torsten]

Two multiplication signs are missing in the definition of x.

Answer 1

[Image Analyst]

A thorough discussion of the error is in the FAQ:

https://matlab.fandom.com/wiki/FAQ#.22Subscript_indices_must_either_be_real_positive_integers_or_logicals..22

Q is an array and you are trying to get an element that is whatever the value of this expression is:

1-R(cos(d_freq.*t) + S*sin(d_freq.*t))

and I bet that is not an integer like 1,2,3,etc. It can't be zero. It can't be negative (there is no -1 row of a matrix). It can't be a fraction. There is no 2.4th row of a matrix - it must be either 2 or 3, not 2.4.