error in build rows of matrix

i have this code for Computing 'dim' as number of rows of 'M' : But this code gives an error:

Error using zeros Leading inputs must be numeric.

Error in Untitled3 (line 108) M=zeros(dim,9);

the code:

 clc
  clear all
  close all

S0 =0.02;
b = 3;
Q = 29
n = 0.017
g = 9.81;
z = 0
ystart = 2
dy=0.1;

A1 = (b+z*ystart)*ystart;
P1 = b + (2*ystart*sqrt(1+z^2));
R1 = A1/P1;
T1= b+ 2*z*ystart;
D1=A1/T1;
F1 = (T1 * Q^2)/(g*(A1)^3); 
Sf1 = (Q^2)*(n^2)/((R1^(4/3))*((A1)^2));
Gy1 = (1-F1)/(S0 - Sf1);

syms yc
A=(b+z*yc)*yc;
T=b+2*z*yc;
P=b+2*yc*sqrt(1+z^2);
R=A/P;
D=A/T;

eqn=(Q ^2)/g==(A^3)/(T);
y=vpasolve (eqn);
if z>0
for ii=1:6
    if real(y(ii,1))>=0 , imag(y(ii,1))==0;

          yc=y(ii,1);
      end
  end
  else
    for ii=1:3
      if real(y(ii,1))>=0 , imag(y(ii,1))==0;

          yc=y(ii,1);
      end
    end  
  end
  yc;

fr = Q / (A1*sqrt(g*D1));
if fr > 1
    disp('supcr')
elseif fr < 1
    disp ('subcr')
elseif fr == 1
    disp ('cr')
end

syms yn
An=(b+z*yn)*yn;
Tn=b+2*z*yn;
Pn=b+2*yn*sqrt(1+z^2);
Rn=An/Pn;
Dn=An/Tn;

Eq=Q==(An/n)*S0^(0.5)*(Rn)^(2/3);
yn=vpasolve(Eq,yn,1)

      if yn > yc
          if ystart > yn
              disp ('M1')

          y2=yn;
          elseif ystart > yc

              disp ('M2')
            y2=yn;
          elseif ystart < yc
              disp ('M3')
             y2=yc;
          end
      elseif yc > yn
          if ystart > yc
              disp ('S1')
               y2=yc;

          elseif ystart > yn
              disp ('S2')
            y2=yn;
          elseif ystart < yn
              disp ('S3')
             y2=yn;
          end

      end

dim=floor((ystart-y2)/dy+1)

M=zeros(dim,9);

[Rik]

As was said in the question that was deleted, you need to say what that line should do before we can give a meaningful answer. I could suggest M=zeros(max([1 dim]),9);, which would remove the error, but I can't say if that makes any sense at all for your code.

Have a read here and here. It will greatly improve your chances of getting an answer.