Unable to perform assignment because the left and right sides have a different number of elements.

Please help me out.

% Model parameters
beta = 0.07; % daily rate of infection
gamma = 0.12; % rate of recovery
delta = 0.02; % rate of immunity loss
N = 15*10^3; % Total population N = S + I + R
I0 = 10; % initial number of infected
T = 100; % period of 300 days
B = 1000; % 
k = 10^5; % 
ub = 63*10^-6; % daily rate of birthsper thousand
uc = 68.49*10^-6; % rate of deaths related to cholera
ud = 24.66*10^-6; % rate of deaths unrelated to cholrea
E = 10.0; % cells per mL water per day
dt = 1/4; % time interval of 6 hours (1/4 of a day)
fprintf('Value of parameter R0 is %.2f',N*beta/gamma)
% Calculate the model
[S,I,R,B_] = sirb_model(beta,gamma,delta,ub,uc,ud,E,B,k,N,I0,T,dt);
% Plots that display the epidemic outbreak
tt = 0:dt:T-dt;
% Curve
plot(tt,S,'b',tt,I,'r',tt,R,'g','LineWidth',2); grid on;
xlabel('Days'); ylabel('Number of individuals');
legend('S','I','R');

plot(tt,B,'b','LineWidth',2); grid on;
xlabel('Days'); ylabel('Number of Viruses');
legend('B');

% Map
plot(I(1:(T/dt)-1),I(2:T/dt),"LineWidth",1,"Color",'r');
hold on; grid on;
plot(I(2),I(1),'ob','MarkerSize',4);
xlabel('Infected at time t'); ylabel('Infected at time t+1');
hold off;

Function that calculates the model evolution over a time period T.
function [S,I,R,B_] = sir_model(beta,gamma,delta,ub,uc,ud,E,B,k,N,I0,T,dt)
    S = zeros(1,T/dt);
    S(1) = N;
    I = zeros(1,T/dt);
    I(1) = I0;
    R = zeros(1,T/dt);
    B_ = zeros(1,T/dt);
    for tt = 1:(T/dt)-1
        % Equations of the model
        dS = ((-beta*(B/(B+k))*S(tt) + ub*(S(tt)+I(tt)+R(tt)) - ud*S) * dt)';
        dI = (beta*(B/(B+k))*S(tt) - gamma*I(tt) - (uc+ud))* dt;
        dR = (gamma*I(tt) - ud*R(tt)) * dt;
        dB = (E*I - delta*B) * dt;
        S(tt+1) = S(tt) + dS;   %%%% The error occurs around here 
        I(tt+1) = I(tt) + dI;
        R(tt+1) = R(tt) + dR;
        B_(tt+1) = B_(tt) + dB;
    end
end

Answer 1

[Walter Roberson]

    S = zeros(1,T/dt);
    S(1) = N;

S is a vector.

dS = ((-beta*(B/(B+k))*S(tt) + ub*(S(tt)+I(tt)+R(tt)) - ud*S) * dt)';
                                                           ^

You are using all of S in the marked place, so dS is not a scalar.

    I = zeros(1,T/dt);
    I(1) = I0;

I is a vector.

        dB = (E*I - delta*B) * dt;
                ^

You are using all of I in the marked place, so dB is not a scalar.

[Nana Kwaku Okai-Mensah]

I just changed that and that error is gone.

However, now I'm getting this error:

% % Index exceeds the number of array elements (1).
% % 
% % Error in LordHlpUs>sir_model (line 55)
% %         dS = ((-beta*(B(tt)/(B(tt)+k))*S(tt) + ub*(S(tt)+I(tt)+R(tt)) - ud*S(tt)) * dt)';


[S,I,R,B_] = sir_model(beta,gamma,delta,ub,uc,ud,E,B,k,N,I0,T,dt);

Function that calculates the model evolution over a time period T.
function [S,I,R,B_] = sir_model(beta,gamma,delta,ub,uc,ud,E,B,k,N,I0,T,dt)
    % if delta = 0 we assume a model without immunity loss
    S = zeros(1,T/dt);
    S(1) = N;
    I = zeros(1,T/dt);
    I(1) = I0;
    R = zeros(1,T/dt);
    B_ = zeros(1,T/dt);
    B_(1) = B;
    for tt = 1:(T/dt)-1
        % Equations of the model
        dS = ((-beta*(B(tt)/(B(tt)+k))*S(tt) + ub*(S(tt)+I(tt)+R(tt)) - ud*S(tt)) * dt)';
        dI = (beta*(B(tt)/(B(tt)+k))*S(tt) - gamma*I(tt) - (uc+ud)*I(tt))* dt;
        dR = (gamma*I(tt) - ud*R(tt)) * dt;
        dB = (E*I(tt) - delta*B(tt)) * dt;
        S(tt+1) = S(tt) + dS;
        I(tt+1) = I(tt) + dI;
        R(tt+1) = R(tt) + dR;
        B_(tt+1) = B_(tt) + dB;
    end
end

% what's wrong with my indexing?

[Walter Roberson]

Your input B is a scalar.

You might be confusing B and B_ (which is a vector)

Answer 2

[the cyclist]

It looks like inside your function, in these lines

dS = ((-beta*(B/(B+k))*S(tt) + ub*(S(tt)+I(tt)+R(tt)) - ud*S) * dt)';
dI = (beta*(B/(B+k))*S(tt) - gamma*I(tt) - (uc+ud))* dt;
dR = (gamma*I(tt) - ud*R(tt)) * dt;
dB = (E*I - delta*B) * dt;

you probably intended to use S(tt) in the first line, and I(tt) in the last line, instead of just S and I.

[Nana Kwaku Okai-Mensah]

Thanks. It really helped.