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CAT arguments dimensions are not consistent while using eval command of symbolic jacobian

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Thayumanavan
Thayumanavan 2014-2-17
编辑： Thayumanavan 2014-4-10
syms x1 x2
%The Diffenrential Equation
f1= (x1)+(x2*x2);%x1dot Equation
f2=x1-x2; % x2dot Equation
f = [f1, f2];
% Forming the symbolic jacobian
Jac = jacobian(f)
x1=0.0;
x2=1.0;
% eval is used to convert the symbolic jacaobian to numeric jacobian
% Finding the eigen values of the Numeric jacobian
q=eval(Jac)
e=eig(q)
Jac =
[ 1, 2*x2]
[ 1, -1]
q =
1 2
1 -1
e =
1.7321
-1.7321
If i use the above code as a function to calculate the eigen values of the sol of ode45 solver as follows:
Main Program:
clear all
clc;
Tspan =[0 1];
X0=[0.0; 1]; % Initial condition
%solving the differential Equation
[t,x]=ode45(@myfunc,Tspan,X0);
%Plotting the figures
figure(1)
subplot(2,1,1)
plot(t(:,1),x(:,1))
xlabel('t');
ylabel('x1');
grid on
subplot(2,1,2)
plot(t(:,1),x(:,2))
xlabel('t');
ylabel('x2')
grid on
% Finding the eigen values
e=findeig(x)
First Function (Ode function)
function dv=myfunc(t,x,flag)
%The Diffenrential Equation
dv=[ (x(1))+(x(2)*x(2));%x1dot Equation
x(1)-x(2); ]; % x2dot Equation
Second Function to calculate eigen value of x:
function e=findeig(x)
syms x1 x2
%The Diffenrential Equation
f1= (x1)+(x2*x2);%x1dot Equation
f2=x1-x2; % x2dot Equation
f = [f1, f2];
% Forming the symbolic jacobian
Jac = jacobian(f)
x1=x(:,1);
x2=x(:,2);
% eval is used to convert the symbolic jacaobian to numeric jacobian
% Finding the eigen values of the Numeric jacobian
q=eval(Jac)
e=eig(q);
It is giving the following output with error:
x =
0 1.0000
0.0001 0.9999
0.0001 0.9999
0.0002 0.9998
0.0002 0.9998
0.0005 0.9995
0.0007 0.9993
0.0010 0.9990
0.0012 0.9988
0.0025 0.9975
0.0037 0.9963
...... (57x2 array)
Jac =
[ 1, 2*x2]
[ 1, -1]
??? Error using ==> horzcat
CAT arguments dimensions are not consistent.
Error in ==> sym.eval at 15
s = evalin('caller',vectorize(map2mat(char(x))));
Error in ==> findeig at 11
q=eval(Jac)
Error in ==> MainProgram at 23
e=findeig(x)
I am expecting the jacobian elemnts has to be replaced with x
for example:for the first row
Jac =
[ 1, 2*1]
[ 1, -1] and has to give eigen value as
1.7321
-1.7321
and so on for all rows of x.why eval is not working for array of elememts

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回答（1 个）

Christopher Creutzig
编辑：Christopher Creutzig 2014-3-31
Don't use eval on syms. Preferably, don't ever. Better use subs as in
q = double(subs(jacobian(f), {x1, x2}, {x(:,1), x(:,2)}));
(You can of course split that on multiple lines. It's just my personal preference not to.)
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Walter Roberson
Walter Roberson 2014-4-8
If you want the first row first element to be assigned to x1, then do not pass in the entire column x(:,1), only pass in the single element x(1,1)
Thayumanavan
Thayumanavan 2014-4-10
编辑：Thayumanavan 2014-4-10
for i=1:Length q = double(subs(Jac,{x1, x2},{x(i,1),x(i,2)})); e=eig(q); end
Length is the array size .
As per ur idea i used a for loop as above and got the answer.Thanks sir

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