how to solve error "Error using sym/min"? in my code

2 次查看（过去 30 天）
shir levi
shir levi 2019-5-29
回答： Walter Roberson 2019-5-29
my code :
format long
sprintf('%16.f',2332456943534324);
l=30;
L=300;
m=10^6;
g=9.81;
M=556594.32;
k1=2.0201*10^12;
c1=9.769*10^10;
c2=0;
s=2i;
sym k2;
a0=(k1*(m*g*L+k2)-(m*g*L*(m+M)+k2*M)*g*L)/(m*(M*L*l)^2);
a1=(c1*(m*g*L+k2)+c2*(k1-M*g*L))/(m*(M*L*l)^2);
a2=(c1*c2+m*M*g*L*(L^2-l^2)+k1*m*l^2+k2*m*(L-l)^2+k2*M*L^2)/(m*(M*L*l)^2);
a3=(c1*(l^2)*m+c2*m*(L-l)^2+c2*(L^2)*M)/(m*(M*L*l)^2);
G1=-(s^2+c2*s/m*(l^2)+(m*g*L+k2)/(m*(l^2)))/((M*L)*((s^4)+(s^3)*a3+(s^2)*a2+(s)*a1+a0));
mag=abs(G1);
[ym,ind]=min(mag);
---------------
the error:
Error using sym/min (line 123)
Input arguments must be convertible to floating-point numbers.
-------------------------------------
i have no idea how to fix so i can find the index of the minimum

请先登录，再回答此问题。

回答（1 个）

Walter Roberson
Walter Roberson 2019-5-29
You need
syms k2;
instead of
sym k2;
But your main problem is that mag is an expression with a symbolic variable, and you cannot use min() on an expression with an unresolved symbolic variable.
To find the k2 that minimizes mag, you can observe that abs() is never negative, so if there is a value of k2 that makes mag 0 then that is a minima. So examine mag and observe that it has
abs(k2/900000000 - 73/100)
divided by something.
>> solve(k2/900000000 - 73/100)
ans =
657000000
and this will be a minima, yielding 0.
You can also find the maximum.
solve(diff(mag,k2),k2)
and the maxima will be at the value that is not 657000000 . It turns out that you get the same maxima at k2 = -inf

类别

在 Help Center 和 File Exchange 中查找有关 Conversion Between Symbolic and Numeric 的更多信息

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by