[Tutor] Lotka-Volterra Model Simulation Questions
wayne at waynewerner.com
Fri Sep 28 23:06:42 CEST 2012
On Fri, 28 Sep 2012, Jim Apto wrote:
> Hello folks,
> I'm relatively new to python, and was asked to program a lotka-volterra model (predator and prey
> relation) simulator. The program basically will basically have a menu that takes user input, collect
> data, and then create a graph. Currently i've been working on the simulator section; I can't seem to
> get the lists right. I've assigned the following variables and parameters to the model for the program:
I don't know anything about the Lotka-Volterra model, but I've definitely
got some recommendation
> x represents prey population
> y represents predator population
> dy/dt and dx/dt represents growth rate of the two populations over time
> t represents time
> a is the growth rate of prey
> b is the rate at which predators kill prey
> g is the death rate of predators
> d is the rate at which the predators population increases by consuming prey
> The equation:
> dx/dt = x(a-by)
> dy/dt = -y(g-dx)
> The code I have for this section is:
> def deltaX(a,b,x,y):
> dx = x*(a-b*y)
> def deltaY(g,d,x,y):
> dy = -y*(g-d*x)
> The simulation function is where I am having trouble.
> For the simulation function, I need to ask the user for the number of runs and then save it in a
> variable, create a list for prey and predator. For each run, i need to calculate the increment of
> change in prey and predator populations by calling the deltaX and deltaY functions, then save these in a
> variable, and then update the population information. The newly calculated populations then need to be
> added to the existing lists. After this is completed, a function for the graph is called.
> The following is my current simulation function:
> def simulation():
> a=eval(input("Growth rate of prey:"))
> b=eval(input("Rate at which predators eat prey:"))
> g=eval(input("Death rate of predators:"))
> d=eval(input("Rate at which predators increase by consuming prey:"))
> x=eval(input("Current prey population:"))
> y=eval(input("Current predator population:"))
Woah! Stop that right now - eval is an *incredibly* dangerous function. If
you want to convert these numbers to integer or float, there's the int()
and float() function. Additionally, single letter variables are really
horrible. You could do:
prey_growth_rate = float(input("Growth rate of prey: "))
predator_consumption_rate = #your code here
predator_death_rate = ...
predator_growth_rate = ...
initial_prey_population = ...
initial_predator_population = ...
I don't see where you defined deltaX or deltaY...
> n=eval(input("Number of runs:")
> r = 0
> yList = 
> while r <= n:
> r = r + 1
> count = count + 1
> yList.append(dx + dx)
> zList= 
> while r <= n:
> r = r + 1
> count = count +1
> zList.append(dy + dy)
> It seems terribly wrong. The following is my graph function:
> def drawCurve(yList,zList,n):
> x = pylab.arange(n)
> pylab.title("Foxes and Rabbits")
> pylab.ylabel("Number of predator (Foxes)")
> pylab.xlabel("\nNumber of prey (Rabbits)")
> pylab.plot(x, yList, 'b')
> pylab.plot(x, zList, 'r')
> pylab.legend(('Rabbits','Foxes'),loc='upper left')
> The issue i'm having is the logic in the lists. How can I create the simulation function using lists
> and make it perform the expected task of creating a graph? I can't seem to get the logic right.
Posting an image of what you expect, and what you got instead to imgur or
some other free hosting site would be a good thing to do.
When asking a question you should always post what you wanted to happen,
and what happened instead.
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