Array indices must be positive integers or logical values.

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cruxsign on 19 Apr 2021

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https://au.mathworks.com/matlabcentral/answers/806937-array-indices-must-be-positive-integers-or-logical-values

Edited: cruxsign on 19 Apr 2021

Accepted Answer: Chad Greene

Open in MATLAB Online

I don't know what I'm doing wrong! I'd appreciate any help! I'm using MATLAB 2019b. It keeps giving me this error

Array indices must be positive integers or logical values.

Here is my code:

  n = 32;  	    % choose a number which is a exponent of 2
    Cp = 0.24; % cal/g C
    K = 0.007; % cal/(cm s C)
    rho = 2.7; % g/cm3
    kapp = K/(Cp*rho); % cm2/s
    d = 5*100; % cm
    start_temp = 1000;
    t = 1*24*60*60;  % (days)
    T=zeros(1:numel(n));
    
    for j = 1:n/2
       T(j) = start_temp;
    end
    
    for j = n/2+1:n
       T(j) = -start_temp;
    end
    
    figure(1);
    
    x = linspace(0,d,length(T));
    plot(x,T,'^')
    hold on
    % plot(T,n)
    xlabel('distance (units)')
    ylabel('Temperture (units)')
    % Title('Heated Dike, Lab #3, #...days')
    
    %%%%%% TAKE FAST FOURIER TRANSFORM (FFT) of your Temp function %%%%
    % (You are now working in frequency (or in this case - wave number) space
    % (This will output a coefficient in alternating sine,cos for each point) 
    
    %%%%%% here is my function
    
%             function f = dfft(y)
%             % First use standard Matlab routine to find Fourier transform of y.
%             % z contains the complex coefficeints of the Fourier exponential series.
%              z=fft(y); 
%              n = length(y);
%              half=n/2;
%             % This section takes the exponential series coefficients and gives the     
%             % coefficients of the Fourier Sine and Cosine series.
%             % f(1) = constant term = average value of the function over period sampled
%             % f(2) = 
%              for i = 1:2:n
%               j=(i+1)/2;
%               f(i)=real(z(j))/half;
%               f(i+1)=-imag(z(j))/half;
%              end  
%              j=n/2 +1;
%              f(2)=real(z(j))/half; 
   
    y=dfft(T);
    
    %%%%% GET FOURIER COEFFICIENTS (Bo, An, Bn) %%%%%%
    
    COEFF(1) = y(1)/2;
    COEFF(2) = y(2)/2;
    for j = 3:2:n		% increment from 3 to n by 2's
       COEFF(j) = y(j);
       COEFF(j+1) = y(j+1);
    end
    
    %%%%%%  SOLVE FOR TEMPERATURE CHANGE using solution to heatflow eqn %%%%%
    %   Y = exp(1i*pi)
    % 
    for j = 3:2:n
       NEWT(j) = y(j).*exp(-kapp*t(2*pi*j/(2*d))^2)*(sin((pi*j)));
       NEWT(j+1) = y(j+1).*exp(-kapp*t(2*pi*j/(2*d))^2)*(sin((pi*j)));
    end
    
    NEWT = T(2)*exp(-kapp*t(2*pi*j/(2*d))^2)*(sin((pi*j)));