How can I update initial value of an array in a for loop?

a(1)=0.1, a(2)= 0.11; 
for k=2:10^10 
a(k+1)= a(k)+a(k-1)+..... 
if a(k+1)> 5.0
Up = k ;
end 
end. 
It takes a lot of memory as iteration number is very high. 
How can I update the initial value like a(3)=a(2); a(1)=a(2) 
and find new a(3) till my condition is satisfied.

[Greg]

Do a little research on preallocation.

[Greg]

What is the purpose of

count = count + 1;

[suman Dhamala]

I removed count. I need to save the value of k when the condition is satisfied.

[suman Dhamala]

I have a(1) and a(2) from which I will calculate a(3). After that change a(2) to a(1) and a(3) to a(2) until my condition a(3) > my threshold is satisfied. I just need to know what value of k or iteration my condition is satisfied

[Greg]

I'm assuming the initial conditions (first two values), or the threshold change? Otherwise, this is a trivial problem and k never changes.

[Greg]

My comment on preallocation comes from the ambiguity of a(k+1)= a(k)+a(k-1)+..... This made me think you needed the entire vector.

Answer 1

[Greg]

Use 3 separate variables, with a slight fix to the value assignment you suggested in the question. Also, throw a break inside the condition - or better, use a while loop (with the counter).

one = 0.1;
two = 0.11;
for k=2:10^10 
  three = one + two;
  if three > 5.0
    break
  end
  two = three;
  one = two;
end