Convert sym to double

Question

Mark 2022-6-16

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1741535-convert-sym-to-double

评论： Mark 2022-6-16

I try to find out the DW (The last line of code I showed below). I used sym x y at the beginning because I need to differentiate W with respect to second order x. After the differentiation, I substitute x and y with real value, X and Y. But when I tried to run the code, there is an error indicated "Unable to perform assignment because value of type 'sym' is not convertible to 'double'." Therefore, I am curious that if I can transfer sym to double or if there is alternative method to find out DW. Thank you very much!!

clc
clear
format long
v=0.3;
E=(209e+3)*10^6;
G=E/(2*(1+v));
q=-0.1*10^6;
h=15*10^(-3);
D=(E*h^3)/(12*(1-v^2));
I=(h^3)/12;
a=600*(10^-3);b=2400*(10^-3);
syms x y
c=13
for f = 1:c
    k=[3:2:1+2*c];
    mn{f}= 1:2:k(f)
end
for f=1:c
    len=length(mn{f});
    rfa_m=zeros(1,len);
    for i=1:len
        m=mn{f}(i)
        rfa_m(i)= (m.*pi*b)./(2*a)
    end
    Am=zeros(1,len);
    for i=1:len
        m=mn{f}(i)
        Am(i)=-2*(rfa_m(i)*tanh(rfa_m(i))+2)./((pi^5)*(m^5)*cosh(rfa_m(i)))
    end
    Bm=zeros(1,len);
    for i=1:len
        m=mn{f}(i)
        Bm(i)=2./((pi^5)*(m^5)*cosh(rfa_m(i)))
    end
    W=zeros(1,len);
    for i=1:len
        m=mn{f}(i)
        W(i)=(q/(24*D))*(x^4-2*a*x^3+(a^3)*x)+((q*a^4)/D).*sum((Am(i).*cosh(m(i).*pi*y./a)+Bm(i).*(m(i).*pi*y/a).*sinh(m(i).*pi*y./a)).*sin(m(i).*pi*x./a))
    end
    for i=1:len
        m=mn{f}(i)
        DW(i)=diff(W(i),x,2)
    end
    X=600*(10^-3);
    Y=0;
    DW=subs(DW,{x,y},{X,Y})
end

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KSSV 2022-6-16

Your RHS is a syms class, you cannot convert it into double unless you substitute the values of x.

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Answer 1

Paul 2022-6-16

2
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https://ww2.mathworks.cn/matlabcentral/answers/1741535-convert-sym-to-double#answer_986845

在 MATLAB Online 中打开

Hi @Mark,

The first problem with the code is in this area

    W=zeros(1,len);
    for i=1:len
        m=mn{f}(i)
        W(i)=(q/(24*D))*(x^4-2*a*x^3+(a^3)*x)+((q*a^4)/D).*sum((Am(i).*cosh(m(i).*pi*y./a)+Bm(i).*(m(i).*pi*y/a).*sinh(m(i).*pi*y./a)).*sin(m(i).*pi*x./a))
    end

W is a double array, but the rhs of the assignment to W(i) is a sym because of the use of x and y. Based on the rest fo the code, I suspect it really should be

    W=sym(zeros(1,len)); % create a sym vector
    for i=1:len
        m=mn{f}(i)
        W(i)=(q/(24*D))*(x^4-2*a*x^3+(a^3)*x)+((q*a^4)/D).*sum((Am(i).*cosh(m(i).*pi*y./a)+Bm(i).*(m(i).*pi*y/a).*sinh(m(i).*pi*y./a)).*sin(m(i).*pi*x./a))
    end

and then W sill be a sym, which can then be diff()ed in the next for loop. But even after that change, another error message popped up, which you'll need to sort out.