problem with subs function

hello here is my program....it substitutes the value for the first run only; then the symbols appear again!

syms La Lb Lc
Ia=(20+6*La+.02*La^2);
Ib=(25+5.5*Lb);
Ic=(20+6.5*Lc+.005*Lc^2);
IHRa=diff(Ia,La)
IHRc=diff(Ic,Lc)

for L=0:10:90
     if L<=30
    Lb=L
else
    Lb=30;
    b=L-Lb
ihr_a=subs(IHRa,La,b)
ihr_c=subs(IHRc,Lc,b)
p=ihr_a-ihr_c

       end

end

[Walter Roberson]

When I try, I appear to get substituted values each time.

I am not sure what you mean by "the first run". Your "if L<=30" statement is true for L = 0, 10, 20, and 30, and in those cases the only action you request is Lb=L .

Perhaps your "end" for your "if" is too late? Should it be

for L=0:10:90
  if L<=30
    Lb=L
  else
    Lb=30;
  end
  b=L-Lb
  ihr_a=subs(IHRa,La,b)
  ihr_c=subs(IHRc,Lc,b)
  p=ihr_a-ihr_c
end