"Error using symengine Unable to convert expression into double array" when using solve function
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Hi,
I'm trying to set up a symbolic solver to help solve an equation I can't solve analytically. I need the output of the code in a usable format (double) so that I can use it in another equation. However, when I attempt to run the code, it throws the following error:
Error using symengine
Unable to convert expression into double array.
Error in sym/double (line 698)
Xstr = mupadmex('symobj::double', S.s, 0);
Error in script (line 8)
f_x(counter) = double(solve(eqn, x));
Does anyone have ideas for how to remedy this?
Code for reference:
%% Constants
g = 1.2; y = 0.1; counterlim = 50
while counter < counterlim
syms x
eqn = y == c * sqrt((x ^ (2 / g) - x ^ ((g + 1) / g)));
f_x(counter) = double(solve(eqn, x));
c = c + 0.01
counter = counter + 1;
end
采纳的回答
Walter Roberson
2020-10-9
%% Constants
C_v = 0.5;
A_o = 1;
P_upstream = 1;
MW = 0.004;
Z = 0.95;
R = 8.314;
T1 = 100;
gamma = 1.660;
mdot_desired = 0.1;
counterlim = 50;
f_Pratio = zeros(1,counterlim);
for counter = 1 : counterlim
syms Pratio
eqn = mdot_desired == C_v * A_o * P_upstream * sqrt((2 * MW / ...
(Z * R * T1)) * (gamma / (gamma - 1)) * (Pratio ^ (2 / gamma) - ...
Pratio ^ ((gamma + 1) / gamma)));
sol = vpasolve(eqn, Pratio);
f_Pratio(counter) = double(sol);
A_o = A_o + 0.01;
counter = counter + 1;
end
Note: the answers are all complex valued. In each case, the imaginary component is roughly 2*counter .
There are two solutions for each equation; the solutions are complex conjugates of each other.
1 个评论
Walter Roberson
2020-10-13
For the revised question, and selecting down to real values:
g = 1.2; y = 0.1; counterlim = 50;
c0 = 1; %CHECK THIS
delta_c = 0.01;
syms c x real
eqn = y == c * sqrt((x ^ (2 / g) - x ^ ((g + 1) / g)));
sol = solve(eqn, x, 'returnconditions', true);
cvals = c0 + (0:counterlim-1) * delta_c;
eqns = subs(sol.conditions, c, cvals);
fx_cell = arrayfun(@(eqn) double(subs(sol.x, sol.parameters, solve(eqn))).', eqns, 'uniform', 0);
fx = vertcat(fx_cell{:});
plot(cvals, fx)
There are two real solutions for each c value, at least in the range being worked over.
Note that your revised question did not initialize c, so I had to speculate that your c was taking the place of your A0 of your original question, which you had initialized to 1. If that is not correct, then adjust the c0 variable.
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