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

%% Euler framåt

clc

clear all

close all

%parametrar (samma som ovan)

tMax = 20;

timeSpan = [0 tMax];

S0 = 1.0;

I0 = 0.0;

R0 = 0.0;

dt = 0.01;

time_vector = 0:dt:tMax;

nIterations = length(time_vector);

tau = 0.6;

h = 0.5;

rho = 0.8;

r = 0.2;

beta = (h*exp(-h*tau))/(1-exp(-h*tau));

%Euler

S = zeros(size(time_vector));

I = zeros(size(time_vector));

R = zeros(size(time_vector));

S(1) = S0;

I(1) = I0;

R(1) = R0;

%vi utnyttjar att: nIterations = length(time_vector) = numel(S)

for i=1:nIterations-1

S(i+1)= S(i)-dt.*(h.*S+rho.*I+beta.*R);

I(i+1)= I(i)+dt.*(h.*S-rho.*I-r.*I);

R(i+1)= R(i)+dt.*(r.*I-beta.*R);

end

%plotta euler framåt

figure(2)

plot(0:tMax,S)

hold on

plot(0:tMax,I)

hold on

plot(0:tMax,R)

Answer 1

[Star Strider]

Subscript the vectors in the loop that calculates the new values:

for i=1:nIterations-1
    S(i+1)= S(i)-dt.*(h.*S(i)+rho.*I(i)+beta.*R(i));
    I(i+1)= I(i)+dt.*(h.*S(i)-rho.*I(i)-r.*I(i));
    R(i+1)= R(i)+dt.*(r.*I(i)-beta.*R(i));
end

Then make appropriate changes to the plotting routine:

%plotta euler framåt
t = linspace(0, tMax, numel(S));
figure(2)
plot(t,S)
hold on 
plot(t,I)
plot(t,R)  
hold off
grid
legend('S(t)','I(t)','R(t)')

.

[Chameleon]

I'm sorry but I'm not quite sure what you mean by subscripting the vectors?

[Star Strider]

‘Subscripting’ means referrring to them as individual elements, so ‘S(i)’ instead of ‘S’, and for the others as well, in all 3 equations.

[Chameleon]

I changed to

S(i) = zeros(size(time_vector));
I(i) = zeros(size(time_vector));
R(i) = zeros(size(time_vector));

But now I get the error:

Array indices must be positive

integers or logical values.

Error in Malaria (line 68)

S(i) = zeros(size(time_vector));

Maybe I changed the wrong thing?

[Star Strider]

Please just leave your code as you originally posted it! It worked correctly with my minor changes.

[Chameleon]

Thank you so much for your help! I really appreciate it :)

[Star Strider]

As always, my pleasure!