How to extract one element from array in function?

I want to extract one element from array, but it is difficult in function statement.

Please someone tell me the way.

function dtheta = odeFun(t, theta)
    g = 9.8;
    l = 1;
   
    Trq = randi([10 25],1,1);      % Generate random numbers from 10 to 25
    
    disp(Trq(1,1))            ←　I wanna extract one element here.
    
  
    % theta(1) = theta, theta(2) = dtheta
    
    dtheta = zeros(2, 1);
    dtheta(1) = theta(2);
    dtheta(2) = -g/l*theta(1)-Trq;
    
end

Answer 1

[LO]

It is not clear what you want the function to do. What is t? and theta?

If your aim is to apply a fixed transformation of theta and create 2 output variables (dtheta and Trq) you need to specify a "Trq output variable" in the function synthax together with dtheta.

test = odeFun(2,45);               % I've tried to test value pairs, in absence of any other hint

function [dtheta,Trq] = odeFun(t, theta)
    g = 9.8;
    l = 1;
   
    Trq = randi([10 25],1,1);      % Generate 1 random number from 10 to 25 (it generates only 1 Nr)
   
    dtheta = zeros(2, 1);
    dtheta(1) = theta(1);          % I've changed theta's index to 1, it did not work otherwise
    dtheta(2) = -g/l*theta(1)-Trq;
    
end

[ryunosuke tazawa]

I made that the torque is a random number from 10 to 25. I want to extract only one element from it. However, in the function, all the generated elements will be extracted.

I want fix the value of Trq.

[LO]

Well if you have a random number generated for an array of angles, you'll get an array of torques. If you want one output, just use 1 input. In the example above I get 1 output only. Do you mean to use a single randomly generated torque for an array of angles? Then you probably have to generate the torque out of the function and "feed" it to odefun as an input. In that case the same random torque will be used for all calculations

[ryunosuke tazawa]

I want to make the torque one value of angle and angular velocity.

As it is, all the elements of the generated torque are assigned to the pendulum states.

Instead, one torque value is needed when solving the equation of motion.

I want one torque value to be randomly selected each time it is executed.

[Walter Roberson]

You cannot use randomness inside a function you use with any of the ode*() routines -- or at least if you do, then the randomness cannot affect the computation.

If you want to do stocastic differential equations, see https://www.mathworks.com/help/finance/stochastic-differential-equation-sde-models.html or http://sdetoolbox.sourceforge.net/

[LO]

Trq = randi([10 25],1,1);      % this is your randomly generated torque
test = odeFun(2,45,Trq);       % this is your test measure using the torque value  

function [dtheta] = odeFun(t, theta, Trq)
    g = 9.8;
    l = 1;
    dtheta = zeros(2, 1);
    dtheta(1) = theta(1);      
    dtheta(2) = -g/l*theta(1)-Trq;
end

[Walter Roberson]

Using randomness outside of the odeFun and passing the value in, is something that is entirely valid.

If you were wanting to call this from ode45() then see http://www.mathworks.com/help/matlab/math/parameterizing-functions.html