Assign variables from a solve solution

Question

Rihards ，2022-10-27

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Direct link to this question

https://ww2.mathworks.cn/matlabcentral/answers/1837078-assign-variables-from-a-solve-solution

评论： Chunru ，2022-10-27

Hello!

I am having a slight problem where I am unable to assign the results of solve/vpa solve to a variable/variable array.

It has worked perfectly on an analogue problem in the previous parts of the same script, so I am unsure what I am missing here.

I am running the following for loop to find the solutions over the range of variable D2, and in this current setup the script runs well and gives me the solutions (correct), but as soon as I add "[Kr_x, alpha_x, k_x] = " in front of the vpasolve line, I'm getting the following error:

"Unable to find variables in equations"

Without the assignment to the three variables, I get the following answer:

ans =

struct with fields:

Kr_x: 0.99981170571100763658667832907699i

alpha_x: 97.041342847663022899757805866536

k_x: 0.029864918466521086498648092576702

So it seems like, it actually can find the variables?

syms k_x Kr_x alpha_x
c0=18.1499
alpha0=0.3491
g=9.81
omega=0.54
for i=1:length(D2)
    c_x=c0*tanh(k_x*D2(i));
    Snell_x=sin(alpha0)/c0==sin(alpha_x)/c_x;
    Defraction_x=Kr_x==sqrt(cos(alpha0)/cos(alpha_x));
    Linear_x = omega^2 == g*k_x*tanh(k_x*D2(i)); 
    vpasolve([Snell_x, Defraction_x, Linear_x],[Kr_x, alpha_x, k_x])
end

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Answer 1

Chunru 2022-10-27

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1837078-assign-variables-from-a-solve-solution#answer_1085023

编辑：Chunru 2022-10-27

syms k_x Kr_x alpha_x
c0=18.1499
c0 = 18.1499
alpha0=0.3491
alpha0 = 0.3491
g=9.81
g = 9.8100
omega=0.54
omega = 0.5400
D2 = [1 2];
for i=1:length(D2)
    c_x=c0*tanh(k_x*D2(i));
    Snell_x=sin(alpha0)/c0==sin(alpha_x)/c_x;
    Defraction_x=Kr_x==sqrt(cos(alpha0)/cos(alpha_x));
    Linear_x = omega^2 == g*k_x*tanh(k_x*D2(i)); 
    % one ouput struct
    sol(i) = vpasolve([Snell_x, Defraction_x, Linear_x],[Kr_x, alpha_x, k_x]);
end
sol(1)
ans = struct with fields:
       Kr_x: 0.97020769905214767846260313371713
    alpha_x: 100.5896791353776560831304307593
        k_x: 0.17326753619233916799563671436781
sol(2)
ans = struct with fields:
       Kr_x: 0.97103085879234422686236899857365i
    alpha_x: 97.306705145538144269628933596149
        k_x: 0.12313256334502620836110299915066