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

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regina
regina 2015-12-1
评论： regina 2015-12-1
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!

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采纳的回答

valdal
valdal 2015-12-1
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))];

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