fd1d_predator_prey, a MATLAB code which uses the finite difference method (FDM) to simulate predator-prey interactions in 1 spatial dimension and time, by Marcus Garvey.
The MATLAB code is mostly self explanatory, with the names of variables and parameters corresponding to the symbols used in the finite difference methods described in the paper.
The code employs the sparse matrix facilities of MATLAB when solving
the linear systems, which provides advantages in both matrix storage
and computation time. The code is
The linear systems are solved using MATLAB's built in function lu.m. We remark that a pure C or FORTRAN code is likely to be faster than our codes, but with the disadvantage of much greater complexity and length.
The user is prompted for all the necessary parameters, time and space-steps, and initial data. Due to a limitation in MATLAB, vector indices cannot be equal to zero; thus the nodal indices 0,...,J are shifted up one unit to give 1,...,(J+1) so x_{i}=(i-1)*h + a.
The program is structured as follows:
The initial data functions are entered by the user as a string, which can take several different formats. Functions are evaluated on an element by element basis, where x=(x_{1},...,x_{J+1}) is a vector of grid points, and so a "." must precede each arithmetic operation between vectors. The exception to this rule is when applying MATLAB's intrinsic functions where there is no ambiguity. Some arbitrary examples with an acceptable format include the following:
>> Enter initial prey function u0(x) 0.2*exp(-(x-100).^2) >> Enter initial predator function v0(x) 0.4*x./(1+x)or,
>> Enter initial prey function u0(x) 0.3+(x-1200).*(x-2800) >> Enter initial predator function v0(x) 0.4This last example shows that for a constant solution vector we need only enter a single number. It is also possible to enter functions that are piecewise defined by utilizing MATLAB's logical operators &, ('AND'), |, ('OR'), and ~ (`NOT'), applied to matrices. For example, on a domain Omega=[0,200], to choose an initial prey density that is equal to 0.4 for 90<=x_{i}<=110, and equal to 0.1 otherwise, the user inputs:
>> Enter initial prey function u0(x) 0.4*((x>90)&(x<110))+0.1*((x<=90)|(x>=110))
The computer code and data files described and made available on this web page are distributed under the GNU LGPL license.
fd1d_predator_prey is available in a FORTRAN90 version and a MATLAB version.
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Marcus Garvie