How to put a plot of iteration count vs relative error?

Hello guys, im not very good with matlab and i just have to ask a quick question. I need to put a plot of the relative error of the bisection method vs the iteration count. 'You should also include a plot of the relative error as a function of the iteration number. ' is what my teacher said. Here is the code I have at the moment.

if true
 f = @(x) cos(x)-x; 
x1 = 0.7; 
x2 = 0.8; 
TOL = 1e-4; 
counter = 0; 
while abs(x2-x1) > TOL 

   xmid = (x2+x1)/2; 
    if (f(x2)*f(xmid)) <0 
        x2 = xmid;
    else
        x1 = xmid; 
   relerr = abs(x1 - x2) / xmid
   counter = counter + 1; 
   end
figure
hold on
plot(counter, relerr)
axis ([0 11 0 0.0002])

Now whenever i try to create the plot i get something that is completely empty. And if i put the create plot command inside the while loop, i get 11 empty plots. i use an fprintf command to print out results for error, iterations and the root of the fn and they all work fine, but the only thing i cant get to work is the plot. Anyone got any ideas?

Answer 1

[Walter Roberson]

Change the

   relerr = abs(x1 - x2) / xmid
   counter = counter + 1;

to

   counter = counter + 1; 
   relerr(counter) = abs(x1 - x2) / xmid;

Change the

plot(counter, relerr)

to

plot(relerr)

By the way: consider that when you write

if something
   do first thing
else
   do second thing
   make an assignment
end

then the assignment is only done if the "something" was false. The "else" part extends right to the "end" (or next "elseif").

[Nikhil Mehta]

thank you very much!