i confuse using syms variable

dear all

i run the following code :

clc
clear all
N=6; % number of disks
L=0.08;   % the length of each segment
x = sym('x', [1 3*N], 'real');
for i=1:N
    alf(i)=x(1,3*i-2);
    bet(i)=x(1,3*i-1);
    gam(i)=x(1,3*i);
    tet(i)=sqrt((alf(i))^2+(bet(i))^2);
    calf(i)=cos(alf(i));    salf(i)=sin(alf(i));
    cbet(i)=cos(bet(i));    sbet(i)=sin(bet(i));
    cgam(i)=cos(gam(i));    sgam(i)=sin(gam(i));
    ctet(i)=cos(tet(i));    stet(i)=sin(tet(i));    vtet(i)=1-cos(tet(i));
end
plcl(:,1)=zeros(3,1); % rows for xyz position, cols for disk, 3rd dim for time
for i=1:N
  plcl(:,i+1)= L*[(bet(i)*vtet(i))/(tet(i)^2);-(alf(i)*vtet(i))/(tet(i)^2 );(stet(i))/tet(i)];  
end

and i got this error :

The following error occurred converting from sym to double:
DOUBLE cannot convert the input expression into a double array.
If the input expression contains a symbolic variable, use VPA.
Error in ENERJYplus1 (line 36)
  plcl(:,i+1)=
  L*[(bet(i)*vtet(i))/(tet(i)^2);-(alf(i)*vtet(i))/(tet(i)^2
  );(stet(i))/tet(i)];

i really appreciated if someone help me that y i got this error and what s the solotion ?

and i should mention that i need to use diff for R at my next step

and i think i could use this code in past ver of matlab

Answer 1

[Steven Lord]

The symbolic expression on the right side of the equals sign:

L*[(bet(i)*vtet(i))/(tet(i)^2);-(alf(i)*vtet(i))/(tet(i)^2);(stet(i))/tet(i)];

still contains a symbolic variable. When you try assigning into the double array plcl (which is double because it was preallocated using zeros) MATLAB can't evaluate the symbolic expression to get a number. It's as though I asked you the numeric value of sin(z) without telling you what z is. Is it -1, 1, or something inbetween?

Your two main choices:

1. Substitute values into the symbolic expression using subs so the result is a number, containing no symbolic variables.
2. Preallocate plcl to be a symbolic array using sym(zeros(3, 1)).