Find value in Fibonacci sequence

Hi, i can't solve a problem. Considering the Fibonacci sequence:

function F = Fibonacci(n)

if n == 1

S(1,1) = 0;

elseif n == 2

S(1,1) = 0; S(2,1) = 1;

else

S(1,1) = 0; S(2,1) = 1;

for i = 3:n

S(i,1) = S(i-1,1) + S(i-2,1);

end

end

F = S(n,1);

end

I would like to create a function that accepts two inputs: a natural number m ∈ N and a number j that can take values 0 or 1.

If j = 0, the output is the largest number of the sequence lower than m;

If j = 1, the output is the lowest number of the sequence greater or equal to m.

I can't find a solution. Could someone help me?

Thank you

Answer 1

[Dyuman Joshi]

You don't need to explicitly define condition for n == 1 and n == 2, they can be covered by how you define the sequence.

Now, due to condition on j, while loop would be a better fitting here, as it can cover both conditions.

F=Fib(13,0)

F = 8

F=Fib(13,1)

F = 13

F=Fib(1234567,0)

F = 832040

F=Fib(1234567,1)

F = 1346269

F=Fib(19,4)

Error using solution>Fib
Error, please give a valid value for j

You can do it with a for loop as well, but that wouldn't be recommended for higher values, as the code will be slower because it would be difficult to pre-allocate. However, that will not be the case with this code. Here, the size of array S is same through out all the iterations, just the values are updated.

I've put some temporary error messages, you can change them as per your need.

function F = Fib(m,j)
S=[0 1];
if m~=floor(m)
    error('Invalid value for m');
else
    while S(end)<m
        S = [S(end) sum(S)];
    end
    if j==0
        F=S(1);
    elseif j==1
        F=S(2);
    else
        error('Error, please give a valid value for j');
    end
end
end

[Giuseppe Moio]

thank you very much