How do I save the answers from a for loop that solved an equation symbolically?

Question

NewGuy 2024-8-9，20:04

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2144279-how-do-i-save-the-answers-from-a-for-loop-that-solved-an-equation-symbolically

评论： NewGuy 2024-8-19，16:01

I am trying to save the answers from a for loop in which I solved an equation symbolically.

T_c = 33.145;      %crit temp of hydrogen (K)
P_c = 1.3e6;         %crit pressure (Pa)
R = 8.314472;      %universal gas constant (J/(mol*K)
a = (0.427487*(R^2)*(T_c^2.5))/P_c;
b = (0.08662*R*T_c)/P_c;
x = 0.000051473700293409386773440257598528; %V_m
for P = 2e6:2e6:70e6
    syms T_g
    eqn = ((8.314472*T_g)/(x-b)) - (a/(sqrt(T_g)*x*(x+b))) == P;
    solx = solve(eqn,T_g,"Real",true);
    solx = vpa(solx);
end

With this code, I only have the final answer.

I tried using the code below to save the answers from the for loop, but I could not save them and got the answer in the screenshot.

k=0;
for P = 2e6:2e6:70e6
    k=k+1;
    syms T_g
    eqn = ((8.314472*T_g)/(x-b)) - (a/(sqrt(T_g)*x*(x+b))) == P;
    solx = solve(eqn,T_g,"Real",true);
    solx = vpa(solx);
    T_g(:,k) = solx;
end

Can somebody show how to save the answers from a for loop?

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Answer 1

Hassaan 2024-8-9，20:13

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https://ww2.mathworks.cn/matlabcentral/answers/2144279-how-do-i-save-the-answers-from-a-for-loop-that-solved-an-equation-symbolically#answer_1497274

在 MATLAB Online 中打开

T_c = 33.145;      % critical temperature of hydrogen (K)
P_c = 1.3e6;       % critical pressure (Pa)
R = 8.314472;      % universal gas constant (J/(mol*K))
a = (0.427487*(R^2)*(T_c^2.5))/P_c;
b = (0.08662*R*T_c)/P_c;
x = 0.000051473700293409386773440257598528; % V_m
% Preallocate a cell array to store the solutions
T_g_solutions = cell(1, 35);
k = 0;
for P = 2e6:2e6:70e6
    k = k + 1;
    syms T_g
    eqn = ((8.314472*T_g)/(x-b)) - (a/(sqrt(T_g)*x*(x+b))) == P;
    solx = solve(eqn, T_g, "Real", true);
    solx = vpa(solx);
    
    % Store the solution in the cell array
    T_g_solutions{k} = solx;
end
% Now T_g_solutions contains the symbolic solutions for each pressure
T_g_solutions
T_g_solutions = 1x35 cell array
  Columns 1 through 7

    {[34.91961423273...]}    {[40.85084631403...]}    {[47.10283616829...]}    {[53.61292670149...]}    {[60.33079643031...]}    {[67.21676758485...]}    {[74.23975776148...]}

  Columns 8 through 14

    {[81.37540390794...]}    {[88.60451998793...]}    {[95.91189116355...]}    {[103.2853532664...]}    {[110.7150969967...]}    {[118.1931430909...]}    {[125.7129454872...]}

  Columns 15 through 21

    {[133.2690897439...]}    {[140.8570623343...]}    {[148.4730728503...]}    {[156.1139159119...]}    {[163.7768630682...]}    {[171.4595775147...]}    {[179.1600462953...]}

  Columns 22 through 28

    {[186.8765260074...]}    {[194.6074990105...]}    {[202.3516378662...]}    {[210.1077762750...]}    {[217.8748851753...]}    {[225.6520529714...]}    {[233.4384690868...]}

  Columns 29 through 35

    {[241.2334102084...]}    {[249.0362287247...]}    {[256.8463429606...]}    {[264.6632288904...]}    {[272.4864130767...]}    {[280.3154666267...]}    {[288.1499999999...]}

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Torsten 2024-8-11，15:39

编辑：Torsten 2024-8-11，15:46

在 MATLAB Online 中打开

Since the "solve" command always returns exactly one value for "solx", you can use

T_c = 33.145; % critical temperature of hydrogen (K)

P_c = 1.3e6; % critical pressure (Pa)

R = 8.314472; % universal gas constant (J/(mol*K))

a = (0.427487*(R^2)*(T_c^2.5))/P_c;

b = (0.08662*R*T_c)/P_c;

x = 0.000051473700293409386773440257598528; % V_m

% Preallocate a cell array to store the solutions

T_g_solutions = zeros(1, 35);

k = 0;

for P = 2e6:2e6:70e6

k = k + 1;

syms T_g

eqn = ((8.314472*T_g)/(x-b)) - (a/(sqrt(T_g)*x*(x+b))) == P;

solx = solve(eqn, T_g, "Real", true);

solx = vpa(solx);

% Store the solution in the cell array

T_g_solutions(k) = solx(1);

end

% Now T_g_solutions contains the symbolic solutions for each pressure

plot(2e6:2e6:70e6,T_g_solutions)

grid on

But the code might not give the "correct" value of T if your equation has more than one real solution (I just chose the first one arbitrarily).

NewGuy 2024-8-19，16:01

Sorry for the late reply. Thank you very much for your answer! It's working now.

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How do I save the answers from a for loop that solved an equation symbolically?

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