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 = ...
deltaX(a,b,x,y)
deltaY(g,d,x,y)
I don't see where you defined deltaX or deltaY...
n=eval(input("Number of runs:")
r = 0
count=0
yList = [0]
while r <= n:
r = r + 1
count = count + 1
yList.append(dx + dx)
zList= [0]
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')
pylab.show()
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.
HTH,
Wayne
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