what's the program mistake?

clear all
clc;
syms a b c 
w=zeros(3,1);
w(1,1)=3*a+b;
w(2,1)=b+2;
w(3,1)=c-a;
disp(w)
b=3;
disp(w)

[Sara]

why do you need

syms a b c

[MA]

excuse me check the question again

Answer 1

[Sean de Wolski]

w = sym(zeros(3,1))

You preallocated w as a numeric type and then tried to stuff a sym into it. If you preallocate w as a symbolic, it works as expected.

[MA]

why isn't the program replace b qauntity in w matrix in second step?

[Sean de Wolski]

You would need to subs-stitude it in

subs(w,b,3)

[MA]

subs(w,[a b c],[a 3 c]) now works,thank you so much