Help indexing symbolic ODE in for loop

Question

Terry Poole 2021-3-31

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/789499-help-indexing-symbolic-ode-in-for-loop

评论： Terry Poole 2021-3-31

Ok so my code works,

P = [0; 22.25; 44.5];
syms w(x) v(x)
Dw = diff(w);
Dv = diff(v);
for i = 1:3
% Deflection in y direction
ode_v = diff(v,x,2) == -Izz/(E*den)*(P(i)*(L-x));
cond1_v = v(0) == 0;
cond2_v = Dv(0) == 0;
conds_v = [cond1_v cond2_v];
vSol(x) = dsolve(ode_v,conds_v);
vSol = vpa(simplify(vSol),3)
% Deflection in Z direction
ode_w = diff(w,x,2) == Iyz/(E*den)*(P(i)*(L-x));
cond1_w = w(0) == 0;
cond2_w = Dw(0) == 0;
conds_w = [cond1_w cond2_w];
wSol(x) = dsolve(ode_w,conds_w);
wSol = vpa(simplify(wSol),3)
end

And my output is:

vSol(x) =
0.0
wSol(x) =
0.0
 
vSol(x) =
1.41e+13*x^2*(x - 3.0)
wSol(x) =
3.14e+13*x^2*(x - 3.0)
 
vSol(x) =
2.82e+13*x^2*(x - 3.0)
wSol(x) =
6.29e+13*x^2*(x - 3.0)
>> 

Which is correct for all 3 load cases,

My issue is, I'm not sure how to call each case, I can't say vSol(1), because it'll assume x=1 and solve the differential equation. So how would I "index" for lack of a better word each case to generate a plot of (v,w) vs. P? I assume since its a syms function that it's not storing these values as an array, honestly it doesn't seem like it's storing them at all.

Any help on how to call vSol(x)(1), vSol(x)(2) and so on would be greatly appreciated.

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Answer 1

Walter Roberson 2021-3-31

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/789499-help-indexing-symbolic-ode-in-for-loop#answer_663949

在 MATLAB Online 中打开

wSol{i} = vpa(simplify(wSol),3)

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Terry Poole 2021-3-31

Error using sym/subsasgn (line 951)

Invalid indexing assignment.

Error in LabSix (line 84)

wSol{i} = vpa(simplify(wSol),3)

Terry Poole 2021-3-31

在 MATLAB Online 中打开

ode_w = diff(w,x,2) == Iyz/(E*den)*(P(i)*(L-x));
cond1_w = w(0) == 0;
cond2_w = Dw(0) == 0;
conds_w = [cond1_w cond2_w];
wSol(x) = dsolve(ode_w,conds_w);
wSol = vpa(simplify(wSol),3)
ws{i} = wSol

That did it! Thanks!

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