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Hi something is wrong with my for loop i have no idea what to do

% write a program that will explore population dynamics

% The population dynamics of a community can be described by:

% dp/dt = G(Pmax - P(t))*P(t)

% Use Euler's Method to determine the population (") as a function of time (#)

% Plot the population (") as a function of time (#) from t = 0 to t = 20 years

% Use the following values for constants in the differential equation:

% pmax = 10,000

% G = 0.00005

% Use an initial condition of p = 800 when t=0

%% set perameters

pmax = 10000

G = 0.00005

tmin = 0

tmax = 20

Nx = 50

t = linspace(tmin, tmax, Nx)

dt = t(1)-t(0)

p = zeros(1,Nx)

p(0) = 800

%% calculate p values using euler method

p(1) = G*(pmax - p(0))*p(0)*dt+p(0)

for ip = 1:Nx

p(ip+1) = G*(pmax - p(t(ip))*p(t(ip))*dt+p(t(ip))

end

Geoff Hayes
on 21 Sep 2020

Kelsey - there are a couple of coding mistakes with the above. Before the code can execute, there is an error

Invalid expression. When calling a function or indexing a variable, use parentheses. Otherwise, check for

mismatched delimiters.

with the line

p(ip+1) = G*(pmax - p(t(ip))*p(t(ip))*dt+p(t(ip))

becaue there is a missing closing bracket. The above line should be

p(ip+1) = G*(pmax - p(t(ip))*p(t(ip))*dt+p(t(ip)));

Next, there is the error

Array indices must be positive integers or logical values.

with the line

dt = t(1)-t(0)

As the error message indicates, positive integers or logical values must be used as indices. 0 is not a positive integer, so you if you want to find the delta, then just do

dt = t(2)-t(1)

Similarly, you must change

p(0) = 800

%% calculate p values using euler method

p(1) = G*(pmax - p(0))*p(0)*dt+p(0)

for ip = 1:Nx

to

p(1) = 800

%% calculate p values using euler method

p(2) = G*(pmax - p(1))*p(1)*dt+p(1)

for ip = 2:Nx

The final problem will be with

p(ip+1) = G*(pmax - p(t(ip))*p(t(ip))*dt+p(t(ip)));

and the error

Array indices must be positive integers or logical values.

because of p(t(ip)) where you are using the vaues of t - which are not integers - as indices into p. What is this code trying to do here?

James Tursa
on 21 Sep 2020

Edited: James Tursa
on 21 Sep 2020

p(ip) is the value of p at time t(ip). p isn't a function that you are passing time into like you are doing with p(t(ip)). So get rid of that t(ip) subscripting and simply use ip subscripting instead. E.g.,

p(ip+1) = G*(pmax - p(ip))*p(ip)*dt + p(ip);

Also, the initial condition is p = 800, and the initial value is p(1), so replace that p(1) line with simply

p(1) = 800;

and get rid of the p(0) = 800 line since 0 is not a valid subscript.

Which also means that the dt calculation should be this:

dt = t(2) - t(1);

James Tursa
on 21 Sep 2020

James Tursa
on 22 Sep 2020

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