How to use VARARGIN with two types of input?

I want to preface this by saying I'm a MechEng student & MatLab beginner, so I hope I've explained my problem clearly

I'm trying to create a function that calculates the equivalent rotational inertia of a gearbox for any number of shafts. The mathematical operation would be as follows:

((Rotational Inertia 1)*(Angular Velocity 1)^2 + (Rotational Inertia 2)*(Angular Velocity 2)^2 ...) up to N

How would I go about defining the function input? I've looked into varargin but I'm not sure how I would seperate between Rotational inertia inputs and angular velocities. Let me know if I need to clarify anything

[Stephen23]

The name MATLAB comes from "MATrix LABoratory". Because its magic power lies in using vectors, matrices, and arrays.

Whenever you find yourself writing pseudo-indices on the end of variable names like this:

+((Rotational Inertia 1)*(Angular Velocity 1)^2 + (Rotational Inertia 2)*(Angular Velocity 2)^2 ...) up to N
%                     ^                    ^                          ^                    ^  

that is a big sign that should be thinking in terms of vectors, matrices, and arrays, to use MATLAB effectively.

Answer 1

[Hassaan]

Example 1: Without using varargin

One array will be for the rotational inertias and the other for the corresponding angular velocities. Here's the function with some dummy values:

% I have called this function with dummy values for inertias and velocities
% Assume inertias for three shafts
inertias = [0.5, 1.2, 0.3]; % in kg*m^2

% Assume angular velocities for three shafts
velocities = [100, 150, 200]; % in rad/s

% Call the function
J_eq = equivalentRotationalInertia(inertias, velocities);

% Display the result
disp(['The equivalent rotational inertia is ', num2str(J_eq), ' kg*m^2']);

The equivalent rotational inertia is 44000 kg*m^2

function J_eq = equivalentRotationalInertia(inertias, velocities)
    % Check if the number of inertias and velocities are the same
    if length(inertias) ~= length(velocities)
        error('The number of inertias must be equal to the number of velocities.');
    end
    
    % Calculate the equivalent rotational inertia
    J_eq = sum(inertias .* velocities.^2);
end

In this example, inertias contains three dummy inertia values and velocities contains three dummy angular velocity values. These are passed to the equivalentRotationalInertia function to calculate the equivalent rotational inertia of the gearbox.

Example 2: With using varargin

Using varargin that can accept an arbitrary number of inertia and velocity pairs and calculate the equivalent rotational inertia.I will be use some dummy values as inputs when calling the function

% I have called this function with dummy values for inertias and velocities
% Assume dummy values for three pairs
I1 = 0.5; % in kg*m^2
w1 = 100; % in rad/s
I2 = 1.2; % in kg*m^2
w2 = 150; % in rad/s
I3 = 0.3; % in kg*m^2
w3 = 200; % in rad/s

% Call the function
J_eq_varargin = equivalentRotationalInertiaVarargin(I1, w1, I2, w2, I3, w3);

% Display the result
disp(['The equivalent rotational inertia using varargin is ', num2str(J_eq_varargin), ' kg*m^2']);

The equivalent rotational inertia using varargin is 44000 kg*m^2

function J_eq = equivalentRotationalInertiaVarargin(varargin)
    % Check if the number of arguments is even
    if mod(nargin, 2) ~= 0
        error('Inputs must be in pairs of inertia and velocity.');
    end
    
    % Initialize equivalent inertia
    J_eq = 0;
    
    % Loop through pairs of arguments
    for i = 1:2:length(varargin)
        inertia = varargin{i};
        velocity = varargin{i + 1};
        
        % Sum up the equivalent rotational inertia
        J_eq = J_eq + inertia * velocity^2;
    end
end

In this example, I1, w1, I2, w2, I3, w3 are dummy inertia and velocity values. They are passed in pairs to the equivalentRotationalInertiaVarargin function, which loops through each pair, calculates the product of inertia and the square of velocity, and sums them up to give the equivalent rotational inertia.

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If you find the solution helpful and it resolves your issue, it would be greatly appreciated if you could accept the answer. Also, leaving an upvote and a comment are also wonderful ways to provide feedback.

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[Stephen23]

With the note that storing lots of scalar numeric in a cell array and performing operations on them one-at-a-time is an inefficient use of MATLAB. Using vectors is much better.

[Hassaan]

@Stephen23 I agree vectorization is much better. My purpose was to provide one of the many possible ways to tackle the solution.

Answer 2

[Dyuman Joshi]

You do not need to use varargin here.

Store both set of values as a vectors of same dimension (i.e. both 1xN or Nx1) and define the function like this -

%Function definition
function out = calculateInertia(Rot_Inertia, Ang_Vel)
out = sum(Rot_Inertia.*Ang_Vel.^2);
end

See - Array vs Matrix Operations

[Joseph]

Thank You!