Resizing pre-allocated vector of zeros using while and if terms?

Hi, i'm writing a function that starts off with two intial values x(1) and x(2), then does the secant iteration on this to give a vector (called res) with the left hand side being each value in-putted and the right hand side being f(x)

What i want to do now is as soon as NaN comes up as an answer for the left hand side, i want to stop adding any values to the vector and then finish the function (so in this case it would give a vector of 11 rows instead of 20). Also as soon as there is a repeat on the left i want to do the same, and have a vector each number only occurring once on the left hand side (so here the 1.49... number that repeats 3 times i'd only want the first time)

x=zeros(20,1);
x(1)=1;
x(2)=2;
for k=2:20;
    x(k+1)= secant iteration formula
    res=[x,f(x)];
    if  res(:,1)==0;    %if number in the left hand column=0
        x=zeros(k-1,1);  %then resize vector of zeros to have length k-1
        res=[x,f(x)]
   end
res=[x,f(x)]
end

this doesn't work so far, i have been trying to tweak it for the whole day so any help would be appreciated, thanks!

Answer 1

[valdal]

You can try something like :

n = 100;
x = zeros(n,1);

x(1) = 1;
x(2) = 2;

for i = 2:n-1
    x(i+1) = x(i) - f(x(i))* (x(i) - x(i-1)) / (f(x(i)) - f(x(i-1)));
    if (x(i+1) == x(i))
       break
    end
end

res = [x(1:i) f(x(1:i))];

[regina]

thank you so so much! this worked perfectly