Unable to perform assignment because the size of the left side is 1-by-1 and the size of the right side is 1-by-2.

I'm writing a stochastic simulation of predator-prey dynamics. To simulate it properly I had to put both the calculation of the number of prey and the number of predator individuals in one function, but it created issues with assignment. I am confused as to why, as I assigned an i-th element of a vector to a specific value of population, like I did in my previous codes and it has been working properly. Any ideas as to why it is happening? The only relevant solution I've been able to find is allocating the size of the array and the vectors inside it and filling it with zeros, which I already done and it is not helping. The code is as below and the errors are displayed for like 107 and 108, which allocate V or C to Results(1,i) or Results(2,i).

%C- predator individuals
% V- prey individuals
% um- maximum growth rate of predator
% VV- threshold prey concentration (predator growth rate = 0 )
% hh- constant
% dm- max mortality rate ?
% hd- constant
% r- prey specific growth rate
% h-  half saturation constant
% IM- maximum predator ingestion rate
%delta_t - timestep
%constants
um = 1.4275; VV = 12.5; hh = 49.7; dm = 0.48; hd = 49.7; %C parameters
r = 0.4; h = 261; IM = 28.5; %V parameters
delta_t=0.2;
%arrays of values
params_C=[um VV hh dm hd];
params_V=[r h IM]; %lists containing population dynamics parameters

%access to functions via handles
fh=localfunctions; %store all functions

PopulationVariables = fh{5}(2,50,delta_t); %stored in vector not an array, can change if needed
C = PopulationVariables(1,1);
V = PopulationVariables(1,2);

C_vector = fh{6}(C,V,delta_t,params_C);
V_vector = fh{6}(C,V,delta_t,params_V);

Results_array = [C_vector; V_vector];

Birth_Rate_C = fh{1}(C_vector,V_vector,um,VV,hh,dm,hd);
Birth_Rate_V = fh{2}(V_vector,r);
Death_Rate_C = fh{3}(C_vector,V_vector,dm,hd);
Death_Rate_V = fh{4}(C_vector,V_vector,r,h,IM);

%plot
figure
left_color = [0 0 1];
right_color = [1 0 0];
set(figure,'defaultAxesColorOrder',[left_color;right_color]);

yyaxis right
plot(V_vector,'-r.');
ylabel('Number of prey individuals');

yyaxis left
plot(C_vector,'-b.');
ylabel('Number of predator individuals');

legend('Prey','Predator');
hold on; grid on;
hold off;

%birth rate of predator
function [brc] = BirthRate_C(C,V,um,VV,hh,dm,hd)
    brc=((C*um.*(V-VV))./(hh+V-VV))+((-dm.*V)/(hd+V)+dm).*C;
end

%birth rate of prey
function [brv] =  BirthRate_V(V,r)
    brv=r*V;
end

%death rate of predator 
function [drc] = DeathRate_C(C,V,dm,hd)
    drc=((-dm*V)./(hd+V)+dm).*C;
end

%death rate of prey
function [drv] = DeathRate_V(C,V,r,h,IM)
    drv=(r.*V.^2)./h +(IM.*C.*V)./(h+V);
end

function [Population_variables] = timestep(C,V,~,~)
um = 1.4275; VV = 12.5; hh = 49.7;  dm = 0.48;  
hd = 9.35; delta_t = 0.2;
r = 0.4;    h = 261;    IM=28.5;

lambda_V = BirthRate_V(V,r)*delta_t;
pd_V = poissrnd(lambda_V);
V = V + pd_V;
V_survive = exp((-DeathRate_V(C,V,r,h,IM)*delta_t)/V);
V = binornd(V,V_survive);

lambda_C = BirthRate_C(C,V,um,VV,hh,dm,hd)*delta_t;
pd_C = poissrnd(lambda_C);
C = C + pd_C;
C_survive = exp((-DeathRate_C(C,V,dm,hd)*delta_t)/C);
C = binornd(C,C_survive);

Population_variables=[C,V];
end

function [Results] = simulation(C,V,delta_t,~)

um = 1.4275; VV = 12.5; hh = 49.7; dm = 0.48; hd = 49.7;
r = 0.4; h = 261; IM = 28.5;

Results = zeros(2,100);

params_V=[r h IM];
params_C=[um VV hh dm hd];
    for i=1:1:100
        V = timestep(C,V,delta_t,params_V);
        C = timestep(C,V,delta_t,params_C);
        Results(1,i) = V;
        Results(2,i) = C;   
    end
end

[Avinash]

just insert the :

clc, clearvars, format compact, close all

at scripts 1st line and then check

still facing same issue then I am sorry.

Answer 1

[Adam]

Your timestep function returns this:

Population_variables=[C,V];

Yet you call it twice as:

V = timestep(C,V,delta_t,params_V);
C = timestep(C,V,delta_t,params_C);

and attempt to assign the results of each of these to something which is clearly scalar (Results(1,i)).

So the error message gives exactly what is wrong. Your returned variable has concatenated both C and V and hence is of size [1,2] and cannot be assigned to a scalar.

I'm not sure exactly what your purpose is and exactly what you want to assign to what so I can't really tell how to correct it.

[Charles]

The timestep function is responsible for stochasticity and introduces random distribution. I am trying to create a vector of V and C values, each storing 100 entires, that represents change of each population with time. So the results of the timestep function for specific initial values of V or C and their corresponding parameters.

From what you told me, am I right to believe that if I split the timestep function into two separate ones, eg timestep_V and timestep_C that could solve the issue?

I've written a similar code for logistic growth and it works perfectly, I'll post it below to better show what I'm trying to do:

b=2;    d=1;    m=0.02;
params=[b d m];
delta_t=0.2;
fh=localfunctions; %store all local functions
n=fh{3}(10,delta_t);
n_vector = fh{4}(n,delta_t,params);
Birth_Rate = fh{1}(n_vector,b);
Death_Rate = fh{2}(n,d,m);
Carrying_Capacity=(b-d)/m;
plot(n_vector,'-r.');
ylabel('Number of individuals');
xlabel('Time(days)');
function [br]=BirthRate(n,b)
    br=b*n;
end

function [dr]= DeathRate(n,d,m)
    dr=n*d+m*(n^2);
end

function [n]= timestep(n,delta_t,~)%params is specified in the function code
    %delta_t is the time step
    %params is a list containing population dynamic parameters,
    %mean number of births is br*delta_t, relised number is Poisson 
    %distributed with this mean
    disp(n)
    b=2;    d=1;    m=0.01;
    lambda= BirthRate(n,b)*delta_t;
    pd = poissrnd(lambda);
    n =n + pd;
    disp(n) 
    % deaths
    %for a survival process with probability per unit time of 
    %dying H, the probability of surviving delta_t is:
    %ps- probability of surviving=exp(-H*delta_t)
    %pod=dr/n
    p_survive=exp((-DeathRate(n, d, m)*delta_t)/n); %check if brackets are corrects
    disp(p_survive)
    n=binornd(n,p_survive); %binomial random numbers
end
%code to run the simulation, maybe not necessary as a seperate function?    
function [n_vec] = simulation(n,delta_t,params) %n and params are specified in function, dont have to be added
  %n=10; %initial population size
  b=2;    d=1;    m=0.01;
  params=[b d m];
  n_vec=zeros(1,500);
  for i = 1:1:500  %match i final and n_vec final 
      n=timestep(n,delta_t, params);
      n_vec(i)=n;%adds the new(current) population size to the vector data
  end
end

Now in my current code I want both V and C vectors and to display them both on one graph.

[Charles]

Hi again, just wanted to let you know that splitting the timestep function into 2 and making small adjustments in the main function has worked. I created an additional array of all parameters and used it as an input in the final simulation function, creating Results_Array. Then I assigned the first row of that array as the V_vector, the second as C_vector and plotted both of them. Thanks for your answer! :)

[aqsa]

Unable to perform assignment because the size of the left side is 1-by-3377160 and the size of the right side is 1-by-3060288.

[Walter Roberson]

We would need to see the code. I suggest that you create a new Question with it.

Answer 2

[mani kandan]

A=[0 0.004 0.008 0.012 0.016 0.020 0.024];

B=[0 0.005334 0.010666 0.016 0.021334 0.026668 0.032];

C=[0 0.004 0.008 0.012 0.016 0.020 0.024];

[m n]=size(A)

[m n]=size(B)

[m n]=size(C)

for i=1:length(A)

for j=1:length(B)

for k=1:length(C)

K(i,j,k)=A(1:n).*B(1:n).*C(1:n)

end

end

end

but i got an error like this while attempting matrix multiplication

Unable to perform assignment because the size of the left side is 1-by-1 and the size of the right side is 1-by-7.

Error in main (line 10)

K(i,j,k)=A(1:n).*B(1:n).*C(1:n)

anyone please clarify this. thanks by advance.

Answer 3

[Walter Roberson]

A(i) * B(j) * C(k)