How can I asign values to syms during a for loop

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Raul Reyes
Raul Reyes 2022-6-19
回答： Brahmadev 2023-9-27
Hi there,
I'm having a problem with the sysmbolics when I try to make a for loop to then plot the function for the different values of x.
The code is the following
syms w(x,y) W(x,y) g(y) yp z Ax
assume(y,'real')
assume(yp,'real')
% Ax = 300;
% Radius
R = 50;
q = 1/3;
k = 0.04;
sigmas = 1/3*(k*x+R)^2;
% c(x)
c = R^2*(1-q)/(2*sigmas);
% Tau squared
taus = 2/3*k*(k*x+R)*Ax;
w(x,y)=sqrt(c)*exp(-y^2/(2*sigmas));
g(z) = exp(-z^2/(2*taus))/sqrt(2*pi*taus);
g(y-yp);
f(x,y,yp,Ax) = g(y-yp)*w(x,yp);
% Some limits of integration
a = -1000;
b = 1000;
% Define W as the integral of f(x,yp) dy from a to b
% W(x,Ax,y) = int(f(x,y,yp,Ax), yp, a, b);
Ax = 1;
N=Ax:Ax:10*R;
%W(x,y) = zeros(length(N),1); % Pre-allocation. not sure if needed
for i = N
x = i-1;
W(x+i,y) = int(f(x,y,yp,Ax), yp, a, b);
i = i + 1;
end
So basically in the for loop the W function will be evaluated at x+deltaX and then the integral of the function on the right hand side will have x as value. Basically if we start at x=0 and deltaX=0.1, we will have W(0.1,y)=int(g(y-yp)*w(0,yp))
Any idea how could I make it work ?

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回答（1 个）

Brahmadev
Brahmadev 2023-9-27
Hi Raul,
I understand that you would like to evaluate the integral "W" for each step of "deltaX". This can be accomplished without using a for loop. You can replace the code after defining the limits "a" and "b" with the following code:
Fnew = subs(f, Ax, 1); % Symbolic substitution of Ax as 1
Ax = 1; % Defining the value of Ax for proper evaluation of Ax:Ax:50*R
% Calculating the integral after substituting the value for x as an array
% with steps deltaX
Fnew2 = subs(Fnew, x, Ax:Ax:10*R);
W = int(Fnew2, yp, a, b);
Also, it is not necessary to define "Ax" and "yp" as a symbolic variables in the first line.
Hope this helps.

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