# Convert sym to double

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Mark on 16 Jun 2022
Commented: Mark on 16 Jun 2022
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
KSSV on 16 Jun 2022
Your RHS is a syms class, you cannot convert it into double unless you substitute the values of x.

Paul on 16 Jun 2022
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.
Mark on 16 Jun 2022
Thank you very much! I have solved the problem

R2022a

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