# Creating a function with equation as an input

2 views (last 30 days)
Alirio De Araujo on 7 Apr 2021
Edited: Julius Muschaweck on 7 Apr 2021
HI, I am trying to create a function,with an equationas an input so that the equation runs within 4 loops while the values of the equation change with each iteration.
I want to use a function as the process nedds to be repeated for different equations multiiple times.
However matlab does not recognise variable p, i understand it hasnt been defined but im not sure how to input the equation so that it can be read and suitibly substituted in the equation.
I appreciate any assistance thank you
for Sigmainternal = 1:1
%sigma11 refers to 11:==> 1_: First U equation _1: fisrt term in equation.
sigma11 = stepPQ(N, ((p*p1/(p+p1-1))*((1-(-1)^(q+q1+1))/(q+q1+1))) );
end
function [sigma] = stepPQ(N,eqn)
sigma =zeros((N+1)^2,(N+1)^2);
for p=0 : N
for q = 0:N
for p1 = 0:N
for q1 = 0:N
temp = (eqn);
if isnan(temp) %check if divide by 0
temp = 0;
end
row = p*(N+1)+(q+1);
col = p1*(N+1)+(q1+1);
sigma(row,col) = temp ;
end
end
end
end
sigma = sigma + transpose(sigma);
end

Alan Stevens on 7 Apr 2021
Like this:
%sigma11 refers to 11:==> 1_: First U equation _1: fisrt term in equation.
N = 3; % arbitrary value
sigma11 = @(p,q,p1,q1) p*p1/(p+p1-1)*(1-(-1).^(q+q1+1))/(q+q1+1); % create as a function
sigma = stepPQ(N, sigma11); % call stepPQ with N and the function sigma11
disp(sigma)
function [sigma] = stepPQ(N,eqn)
sigma =zeros((N+1)^2,(N+1)^2);
for p=0 : N
for q = 0:N
for p1 = 0:N
for q1 = 0:N
temp = eqn(p,q,p1,q1); % You have to call the equation with arguments
if isnan(temp) %check if divide by 0
temp = 0;
end
row = p*(N+1)+(q+1);
col = p1*(N+1)+(q1+1);
sigma(row,col) = temp ;
end
end
end
end
sigma = sigma + transpose(sigma);
end
Note that sigma11 isnt an equation, it's a function.
Alirio De Araujo on 7 Apr 2021
Amazing!
Thank you so much

Julius Muschaweck on 7 Apr 2021
Edited: Julius Muschaweck on 7 Apr 2021
Your "equation" seems to be a function depending on p, p1, q, q1.
I would use an anonymous function to model this behavior. An anonymous function is an object you can feed into another function and evaluate within that other function.
(You also need to define N to make this example run. With N = 3, sigma11 becomes a 4x4 matrix)
for Sigmainternal = 1:1
%sigma11 refers to 11:==> 1_: First U equation _1: fisrt term in equation.
% define N
N = 3;
% define equation
eqn = @(p, p1, q, q1) ((p*p1/(p+p1-1))*((1-(-1)^(q+q1+1))/(q+q1+1)));
sigma11 = stepPQ(N, eqn );
end
function [sigma] = stepPQ(N,eqn)
sigma =zeros((N+1)^2,(N+1)^2);
for p=0 : N
for q = 0:N
for p1 = 0:N
for q1 = 0:N
temp = eqn(p,p1,q,q1);
if isnan(temp) %check if divide by 0
temp = 0;
end
row = p*(N+1)+(q+1);
col = p1*(N+1)+(q1+1);
sigma(row,col) = temp ;
end
end
end
end
sigma = sigma + transpose(sigma);
end