# Why isn't my matrix being filled?

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Coby Juarez on 16 Oct 2021
Edited: C B on 16 Oct 2021
The intention of this funtion is to be able to give an amount of days and populations for Immune, Sick, and Well people and the function should ouput a matrix with the new population values. Sadly, the function only works for the first two days with the following columns being all zeros. Any tips for me? I'm a beginner
function PopAfterT(t,I0,S0,W0)
populations=zeros(3,t)
populations(:,1)=[I0;S0;W0]
I=zeros(1,t)
I(1,1)=I0
S=zeros(1,t)
S(1,1)=S0
W=zeros(1,t)
W(1,1)=W0
for day=(2:t)
populations(1,day)=I(1,(day-1))+(S(1,(day-1))*.12)-(I(1,day-1)*.01)
populations(2,day)=S(1,(day-1))+(W(1,(day-1))*.08)-(S(1,(day-1))*.04)-(S(1,(day-1))*.02)-(S(1,(day-1))*.12)
populations(3,day)=W(1,(day-1))+(W(1,(day-1))*.06)+(S(1,(day-1))*.02)-(W(1,(day-1))*.02)-(W(1,(day-1))*.08)+(I(1,day-1)*.03)
end
John D'Errico on 16 Oct 2021
Do you understand that if you do not return a variable from that function, then nothing changes in your workspace?
Coby Juarez on 16 Oct 2021
@John D'Errico I thought I did but your question makes me feel like I do not. Can you elaborate and help me fix it please?

C B on 16 Oct 2021
Edited: C B on 16 Oct 2021
@Coby Juarez Its because you are using old values of I S W but you are not updating them in loop.
Please have a look at below code where i have updated I S W i hope answer is correct.
function populations = PopAfterT(t,I0,S0,W0)
populations=zeros(3,t);
populations(:,1)=[I0;S0;W0];
I=zeros(1,t);
I(1,1)=I0;
S=zeros(1,t);
S(1,1)=S0;
W=zeros(1,t);
W(1,1)=W0;
for day=(2:t)
I(day)=I(1,(day-1))+(S(1,(day-1))*.12)-(I(1,day-1)*.01);
S(day)=S(1,(day-1))+(W(1,(day-1))*.08)-(S(1,(day-1))*.04)-(S(1,(day-1))*.02)-(S(1,(day-1))*.12);
W(day)=W(1,(day-1))+(W(1,(day-1))*.06)+(S(1,(day-1))*.02)-(W(1,(day-1))*.02)-(W(1,(day-1))*.08)+(I(1,day-1)*.03);
populations(1,day)=I(day);
populations(2,day)=S(day);
populations(3,day)=W(day);
end
Output = PopAfterT(10,4,5,6)
Output =
4.0000 4.5600 5.0640 5.5214 5.9402 6.3267 6.6864 7.0238 7.3428 7.6465
5.0000 4.5800 4.2340 3.9494 3.7159 3.5249 3.3693 3.2434 3.1424 3.0623
6.0000 5.9800 5.9692 5.9670 5.9730 5.9866 6.0074 6.0351 6.0693 6.1096

DGM on 16 Oct 2021
Edited: DGM on 16 Oct 2021
When you define a function, you need to describe its input arguments and output arguments.
This function definition has no inputs and no outputs
function mybeep()
beep
end
This function definition adds two numbers and returns a single result.
c = a+b;
end
I'm assuming that the output is supposed to be the populations variable, so
function populations = PopAfterT(t,I0,S0,W0)
% ...
end

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