How to convert text file into a stiff differential equation matrix

I have the following in a .txt file:

dP/dt=-Pm+bHP
dH/dt=Hr-aHP

There are 2 elements in vector N (P and H), I want to be able to take this text from the text file and essentially be able to do the folllowing automatically:

function dNdt= PredPrayDynamics_ODE(t,N)
dNdt = [-Pm+bHP;...
    Hr-aHP];
end

how should I approch this? I know how to open the .txt file, I just don't know how to convert the cell array to this type of method

Answer 1

[Sameer]

Hi Abrar,

From my understanding you want to convert the given .txt file into a MATLAB function that defines the system of ODEs.

To achieve this, you need to parse the equations to extract the parameters and terms, then compose and write this processed information into a new MATLAB function file.

Below is the code for it:

% Define the filename of the text file containing the equations

filename = 'file_name.txt';


% Open the file and read the contents

fileID = fopen(filename, 'r');

if fileID == -1
    
    error('File not found or permission denied');
    
end

fileContents = fscanf(fileID, '%c');

fclose(fileID);



% Split the contents by lines (assuming each equation is on a new line)

equations = strsplit(fileContents, '\n');



% Initialize an empty cell array for the ODE system

odeSystem = {};



% Loop through each equation and parse it

for i = 1:length(equations)
    
    eq = strtrim(equations{i}); % Remove leading/trailing whitespace
    
    if isempty(eq)
        
        continue; % Skip empty lines
        
    end
    
    
    
    % Replace 'P' with 'N(1)' and 'H' with 'N(2)'
    
    eq = strrep(eq, 'P', 'N(1)');
    
    eq = strrep(eq, 'H', 'N(2)');
    
    
    
    % Remove 'dP/dt=' and 'dH/dt='
    
    eq = regexprep(eq, 'd[PN]\(?\d?\)?\/dt=', '');
    
    
    
    % Ensure multiplication is explicit
    
    eq = regexprep(eq, '(N\(\d\))([a-zA-Z])', '$1*$2');
    
    eq = regexprep(eq, '([a-zA-Z])(N\(\d\))', '$1*$2');
    
    
    
    % Add the parsed equation to the ODE system
    
    odeSystem{end+1} = eq;
    
end



% Construct the function definition

functionDef = 'function dNdt = PredPrayDynamics_ODE(t, N)';

% Join the equations with a semicolon and a newline character

functionBody = strjoin(odeSystem, ';...\n    ');

% Construct the body with the joined equations

functionBody = sprintf('dNdt = [%s];', functionBody);

functionEnd = 'end';



% Combine the function definition, body, and end to form the complete function

completeFunction = sprintf('%s\n%s\n%s', functionDef, functionBody, functionEnd);



% Display the function

disp(completeFunction);



% Write the function to a .m file

funcFilename = 'PredPrayDynamics_ODE.m';

funcFileID = fopen(funcFilename, 'w');

if funcFileID == -1
    
    error('Could not create output function file');
    
end

fprintf(funcFileID, '%s', completeFunction);

fclose(funcFileID);

Please refer to the links below to learn more about parsing equations, regular expressions and string operations that were used in the above code.

www.mathworks.com/help/releases/R2021a/matlab/matlab_prog/regular-expressions.html

www.mathworks.com/help/releases/R2021a/matlab/ref/string.html

I hope this helps!

Sameer