Variable x must be of data type double. It is currently of type sym. Check where the variable is assigned a value.

Hello,

I was wondering how I could change my variable 'x' to type double from syms. I've tried converting it to type double with x=double(x) and nothing has happened.

Help would be very appreciated!

U = [3,2,1;
     0,1,2;
     0,0,1]
b = [0;0;0]
x=backsub(A,b)

function x = backsub_syms(U,b)
n = length(b);
syms t
x=sym(zeros(n,1))

x(n)=sym('t')
b=(sym(b))
for i = n:-1:1
    x(i)=b(i);
    x(n)=sym('t')
    if i<n
        for j = n:-1:i+1
            
            x(i)=x(i)-U(i,j)*x(j);
            
        end
    end
    
    x(i)=x(i)/U(i,i);
end
end

Thanks for the help much appreciated!

Answer 1

[Ameer Hamza]

You need to substitute a value for symbolic variable t to get a numeric value

U = [3,2,1;
     0,1,2;
     0,0,1];
b = [0;0;0];
syms t;
x = backsub_syms(U,b);
y = subs(x, t, 1) % t = 1
double(y)

function x = backsub_syms(U,b)
n = length(b);
syms t
x=sym(zeros(n,1))
x(n)=sym('t')
b=(sym(b))
for i = n:-1:1
    x(i)=b(i);
    x(n)=sym('t')
    if i<n
        for j = n:-1:i+1
            
            x(i)=x(i)-U(i,j)*x(j);
            
        end
    end
    
    x(i)=x(i)/U(i,i);
end
end