Convert syms struct to numerical values

Question

JC.UT 2021-9-21

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1457444-convert-syms-struct-to-numerical-values

回答： sai charan sampara 2024-4-25

I need to add in initial condition (x,y)=(1,1). The solution, S will yield struct with fields. I'm trying to convert them to numerical values in order to plot directional fields. How do I use subs or double to convert them , I'm having a difficult time using documentation to solve this one.

syms x(t) y(t)
ode1=diff(x)==2*x+y;
ode2=diff(y)==x+y;
cond=;
odes=[ode1 ode2 cond];
S=dsolve(odes)
S = 
  struct with fields:
    y: [1×1 sym]
    x: [1×1 sym]

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

KSSV 2021-9-21

Read about subs and double.

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

sai charan sampara 2024-4-25

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1457444-convert-syms-struct-to-numerical-values#answer_1447541

在 MATLAB Online 中打开

Hello,

To solve the ode with initial condition as (x,y)=(1,1). You can do the following:

syms x(t) y(t)
ode1=diff(x)==2*x+y;
ode2=diff(y)==x+y;
cond1=x(0)==1;
cond2=y(0)==1;
odes=[ode1 ode2 cond1 cond2];
S=dsolve(odes)
S = struct with fields:
    y: exp((t*(5^(1/2) + 3))/2)*(5^(1/2)/10 + 1/2) + (5^(1/2)*exp(-(t*(5^(1/2) - 3))/2)*(5^(1/2) - 1))/10
    x: exp((t*(5^(1/2) + 3))/2)*(5^(1/2)/2 + 1/2)*(5^(1/2)/10 + 1/2) - (5^(1/2)*exp(-(t*(5^(1/2) - 3))/2)*(5^(1/2)/2 - 1/2)*(5^(1/2) - 1))/10
double(subs(S.x,t,0))
ans = 1
double(subs(S.y,t,0))
ans = 1

As shown above you can use "subs" to substitute the value of symbolic variable "t" with the required value in the solutions for both "x" and "y". Then "double" can be used to simplify the value from symbolic expression to "double" data type. In the same way "subs" and "double" can be used to get the value of "x" and "y" for any time value. In the above case as required the initial values of "x" and "y" are 1.

To get the directional fields you can use the following code:

[x, y] = meshgrid(0:0.5:2, 0:0.5:2);

dx_dt=@(x,y) 2*x+y;

dy_dt=@(x,y) x+y;

dx=dx_dt(x,y);

dy=dy_dt(x,y);

quiver(x, y, dx, dy);

xlabel('x');

ylabel('y');