Indexing Problem with a For Loop

Hi, I'm trying to create a function that with given inputs gives an output of various things that will then be taken by another function to plot it. However, as it stands right now I'm having a problem with the calculations and for-loop indexing. I've tried various things and I'm still stuck. The error I'm recieving right now is:

Index in position 2 is invalid. Array indices must be positive integers
or logical values.

Error in calculation (line 58)
                Upeak(count+1,2*eta(i-1))= t(j); %the current value of
                time

Error in main (line 27)
[t,S,i_arr,Upeak,Lpeak]=calculation(m,time,tsd,k,eta);

The project has 3 parts with this being the major function that will be taken in by a plotting function. The code I'm currently working on is this:

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=2:length(eta)% Run index 'i' from 1 to the length of 'eta' array
    count  = 1; % Initialize both counters to 1
    count1 = 1; % 
    
    if eta(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 eta(i)];
    end
    
    wd = w0.*(sqrt(1-eta(i).^2)) ;% Fill in for wd
    
%%    
    for j=2:length(t) 
        
        S(i,j) = exp((-eta(i)).*w0.*t(j)).*(A.*cos(wd.*t(j))) ;
        if eta(i) < 1 && t(j) > 3
            
            right =  S(i,j)
            middle = S(i,j-1) 
            left = S(i,j-2) 
            
             
            if middle > left && middle > right% Middle value is greater than both left AND right
                Upeak(count+1,2*eta(i-1))= t(j); %the current value of time
                Upeak(count+1,2*eta(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    
                Lpeak(count+1,2*(i-1)) = t(j) ;
                Lpeak(count+1,2*(i))=S(i,j-1) ;
                count1= count1 + 1; 
            end
        end
        
    end
    
end

%%
if i == length(eta2) % 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('%3.3d \n', Upeak());
else 
    
    % Output a table, with proper headings, of the "Lpeak" array
    fprintf(' Lpeak Data Points \n')
    fprintf('%3.3d \n', Lpeak());
end


end

Earlier I was able to get it to run without error, however my plots were all empty and I didn't get my tables with the data printed either. So by tweaking a few things I've arrived at my current scenarior and honestly have no idea what to do.

Answer 1

[Tristen Hernandez]

I was just able to get it to run by transforming:

                Upeak(count+1,2*eta(i-1))= t(j); %the current value of time
                Upeak(count+1,2*eta(i))=S(i,j-1) ; %S(<current i>, <current j - 1>)
                count=count + 1 ; %add 1 to count  

into this:

                Upeak(count+1,2*i-1)= t(j); %the current value of time
                Upeak(count+1,2*i)=S(i,j-1) ; %S(<current i>, <current j ->)
                count=count + 1 ; %add 1 to count

However, it's still not plotting any of the data