code on finding the roots using simple bisection method

what's wrong in the following code:

function s=bisectionpart(s);

xl=5;

xu=100;

xr=xl;

while(1)

    xrold=xr;

xr=(xl+xu)/2;

error=abs((xr-xrold)/xr)*100;

if error<0.1

      break,

end;

test=fs(xl)*fs(xr);

if test<0;

      xu=xr;

else,
    xl=xr;

end
end

s=xr;

plot(t,s);

end

function f=fs(s);

s0=10;

vm=0.5;

ks=2;

t=0:50;

f=s0-(vm*t)+ks*log(s0/s)-s;

end

[Geoff Hayes]

B - try formatting your above code that it is readable by others, or attach the code to your question using the paperclip button.

A couple of comments: why is s an input parameter to your bisectionpart function. How is it used, and what would it be when you call this function? Also, don't use error as the name of a variable as this is also the name of a built-in MATLAB function.

When I try to run the bisectionpart function, I get an error at line 17

 Error using  * 
 Inner matrix dimensions must agree.

 Error in bisectionpart (line 17)
 test=fs(xl)*fs(xr);

This is because the result returned from fs is an array of 51 elements. Is this expected? What is the purpose of fs i.e. what does it represent or model?