Using the bisection method

I'm not sure if I used the bisection method correctly. Do I have to have two separate .m files? Is the code below okay?

T_o = 300;
T = 1000;
u_o = 1360;
q = 1.7e-19;
n_i = 6.21e9;
p_desi = 6.5e6;
N = [0 2.5e10];

n = @(N) 0.5*(N + sqrt(N.^2 + 4*n_i^2));
u = u_o*(T/T_o)^-2.42;
p = @(N) (1./(q*n(N)*u)) - p_desi;

hold on
plot(N,p(N))

x_l = 9.8e9;
x_u = 9e9;

for i = 1:1:100
  x_r = (x_u+x_l)/2;
  if ((p(x_l).*p(x_r)) < 0)
      x_u = x_r;
  else
      x_l = x_r;
  end
  plot(x_r,p(x_r),'k-')
end
hold off

[Matt J]

Can't you tell if it's working by testing it?

I hope this is homework, BTW. Otherwise, you are unnecessarily re-inventing the wheel. FZERO is already available for finding roots, and may do better than bisection.

[Aaron]

I did run it but the correct graph was a decreasing curved line. Mine is just a straight, decreasing line.

Its not really homework, just practice stuff for the homework to come. I am aware of the FZERO option, but just wanted to try this out.