need help writing a loop m file

What I'm trying to do is this problem: http://puu.sh/chhdo.png

This is what I have, but it's not working too well:

clear all
x_old = 20; 
x_new = 0; 
i = 0; 
while x_new >= 2.1
    x_new = (2*x_old)^.5 
    i = i-1 
    x_old = x_new;
end

Answer 1

[Guillaume]

Your only problem is that you start with

x_new = 0

So your

while x_new >= 2.1

is false and you never enter the loop.

Start with

x_new = 20;

and it'll all work.

[Image Analyst]

Since i is decremented after the x_new value is obtained, you might need to increment it once you've exited the loop to restore the i to what it should be.