how to easily change the function paramters in contour plot

Hi fellow, I am using contour to plot something as shown below

if true
  % plot([0,100],[100,0])
hold on
[x,y] = meshgrid(0:0.1:100);   
f = - (1000*exp((9*x)/2000 - (9*y)/5000))/27 - (1000*exp(- (27*x)/5000 - (9*y)/2000))/27 - (1000*exp((27*y)/5000 - (9*x)/10000))/27          
contour(x,y,f,50);     

end

My question is how can I easy change the parameters in the function f. f is a function defined on another function f1, i.e, pass some parameters to f1, and then get the expression for f. At the moment, I get the expression for f in this way

if true
 load DD
p=[1/3,1/3,1/3];
r=0.009;
edw=100;
plot([0,100],[100,0])
hold on
dd=[DD(1,1:3)-1;DD(1,4:6)-1];
syms x y
f= f1( p,r,dd,edw,[x;y])
end

But I want see the how the plot change with different parameters, for example, r. I have to change r and run the above code in a separate script. When I get the expression for f, then copy paste the code in the first part code in another script. This is very inconvenient. Could you suggest some smarter way of doing this?

PS. I tried this way but it I get the error message "Warning: Contour not rendered for constant ZData " which I don't understand.

if true
  % plot([0,100],[100,0])
hold on
load DD
p=[1/3,1/3,1/3];
r=0.009;
edw=100;
plot([0,100],[100,0])
hold on
dd=[DD(1,1:3)-1;DD(1,4:6)-1];
f =@(x,y)  -eucara( p,r,dd,edw,[x;y])        
ezcontour(f,[0,100],50);     
end

Cheers, Xueqi