Matrix Dimensions Must agree

Hi, currently I'm trying to create a function that with the given inputs will output various graphs of the system, however I'm running into problem with the matrix dimensions on line 49 not working (the S function). I was wondering if a fresh set of eyes could help me see where I'm going wrong with it. Also note this is a function that is to be used in another script with a plotting function. Any and all help very much appreciated.

function [t,S,i_arr,Upeak,Lpeak] =calculation(m,time,tsd,k,eta)

%%%%%%%%%% Write 4-5 lines (comments about what this function does and sends back)
%%%%%%%%%%
%%%%%%%%%%
%%%%%%%%%%

% This function needs to take the following as input values:
% (1) m (mass)
% (2) time
% (3) tsd
% (4) k (stiffness)
% (5) eta
tsd = 0.01;
m = 2.5;
k = 175; %stiffness
time = [0 4];
eta = [0.015 0.09 1]
eta2 = [0.02 1 0.08 2 1.5 0];
t=(0:tsd:4);
A = 5; 

w0= sqrt(k/m) ; % Determine w0

% Build three empty arrays
i_arr=[]; % Saves all etas less than 1
Upeak=[]; % Saves all upper peaks 
Lpeak=[]; % Saves all lower peaks
%%
for i=1:length(eta2) % Run index 'i' from 1 to the length of 'eta' array
    count  = 1; % Initialize both counters to 1
    count1 = 1; % 
    
    if i < 1 %check if the current iteration (i) of eta is less than 1
        % Set first row and the 2*i-1 column for both Upeak and Lpeak to 0
        Upeak(1,2*i-1)=0;
        Lpeak(1,2*i-1)=0;
        Upeak(1,2*i)=A;
        Lpeak(1,2*i)=-A;
        % Add this iteration of eta to i_arr
        i_arr=[i_arr i];
    end
    
    wd = w0.*(sqrt(1-eta.^2)) ;% Fill in for wd
    
%%    
    for j=1:length(t) 
        
        S(i,j) = exp((-i).*w0.*t).*(A.*cos(wd.*j)) ;
        if i > 1 && j > 3
            
            right = S(i,j)==exp(-i*w0.*t)*A*cos(wd.*j);
            middle = S(i,j)==exp(-i*w0.*t)*A*cos(wd.*(j-1));
            left =S(i,j)==exp(-i*w0.*t)*A*cos(wd.*(j-2));
            
             
            %%
            if middle > left && middle > right% Middle value is greater than both left AND right
                Upeak(count+1,2*i-1)= j; %the current value of time
                Upeak(count+1,2*i)=S(i,j-1) ; %S(<current i>, <current j - 1>)
                count=count + 1 ; %add 1 to count  
            end
            
           
            % Check if this is lower peak value
            if middle < left && middle < right % Middle value is less than both left AND right    
                Lpeak(count+1,2*i-1)=j ; %the current value of time
                Lpeak(count+1,2*i)=S(i,j-1) ; %S(<current i>, <current j - 1>)
                count1= count1 + 1; %add 1 to count1
            end
        end
        
    end
    
end

%%
if i == length(eta) % Check if the length of eta is 3 (for part (a))
    
    % Output a table, with proper headings, of the "Upeak" array
    fprintf(' Upeak Data Points \n')
    fprintf('%3.3f \n', Upeak(i,j));
else 
    
    % Output a table, with proper headings, of the "Lpeak" array
    fprintf(' Lpeak Data Points \n')
    fprintf('%3.3f \n', Lpeak(i,j));
end


end

Answer 1

[Walter Roberson]

t=(0:tsd:4);

t is a vector

    for j=1:length(t) 

taking length(t) again implies t is a vector.

                S(i,j) = exp((-i).*w0.*t).*(A.*cos(wd.*j)) ;

t is a vector and you use all of t, so the right hand side is a vector.

Considering that you loop j to length(t) you should probably be using t(j)

Answer 2

[Tristen Hernandez]

        S(i,j) = exp((-eta(i)).*w0.*t(j)).*(A.*cos(wd.*t(j))) ;

My S looks like this now and I'm getting the error of:

        Unable to perform assignment because the indices on the left side are not
compatible with the size of the right side.

Error in calculation (line 49)
        S(i,j) = exp((-eta(i)).*w0.*t(j)).*(A.*cos(wd.*t(j))) ;

Which I'm presuming probably has to deal with how I'm mulitplying the various elements?

[Walter Roberson]

eta = [0.015 0.09 1]

eta is a vector

    wd = w0.*(sqrt(1-eta.^2)) ;% Fill in for wd

so wd is a vector the same size as eta. (Note: it is being calculated inside a loop even though w0 and eta are not changed inside the loop.)

            S(i,j) = exp((-eta(i)).*w0.*t(j)).*(A.*cos(wd.*t(j))) ;

wd is a vector so the right hand side is a vector.

Since wd is the same size as eta, perhaps you should be indexing wd by the same thing you index eta by.