Building the Fibonacci using recursive

Help needed in displaying the fibonacci series as a row or column vector, instead of all number.

Thia is my code: I need to display all the numbers: But getting some unwanted numbers.

function y = my_recursive3(n)

% y = zeros(1,n);

y = zeros(1,n);

ca = 1;

if n == 0

y = n ;

disp(y);

elseif n == 1

y = n;

disp(y);

else

y = my_recursive3(n-1)+ my_recursive3(n-2);

%y(ca) = y;

%ca = ca + 1;

end

%disp(y);

Thanks in advance

[Piyush Gupta]

thanks

Answer 1

[Stephen23]

I doubt that a recursive function is a very efficient approach for this task, but here is one anyway:

function v = myfib(n,v)
if nargin==1
    v = myfib(n-1,[0,1]);
elseif n>1
    v = myfib(n-1,[v,v(end-1)+v(end)]);
end
end

and tested:

>> myfib(8)
ans =
    0    1    1    2    3    5    8   13
>> myfib(10)
ans =
    0    1    1    2    3    5    8   13   21   34

[Akhila M]

will this code work for myfib(1)???

[Alwin Varghese]

you can add two lines to the above code by Stephen Cobeldick to get solution for myfib(1)

function v = myfib(n,v)

if nargin==1

v = myfib(n-1,[0,1]);

elseif n>1

v = myfib(n-1,[v,v(end-1)+v(end)]);

end

if n<1

v=0;

end

[Stephen23]

@Akhila M : you could do something like Alwin Varghese suggested, but I recommend a more efficient elseif rather than defining a separate if statement:

function v = myfib(n,v)
if nargin==1
    v = myfib(n-1,[0,1]);
elseif n>1
    v = myfib(n-1,[v,v(end-1)+v(end)]);
elseif n<1
    v = 0;
end

Answer 2

[Soumya Sinha]

The code for generating the fabonacci series numbers is given as -

function [n] = abcd(x)
if (x == 1 || x==0)
    n = x; 
    return
else
    n = abcd(x-1) + abcd(x-2);
end
end

However you can use a simpler approach using dynamic programming technique -

fibonacci = [0 1];
for i = 1:n-2
    fibonacci = [fibonacci fibonacci(end)+fibonacci(end-1)];
end

This is a more efficient approach for this since recursion is exponential in complexity.

Answer 3

[Dhiraj Darji]

function f= fibor(n)
if n<=2
    f=1;
else
    f=fibor(n-1)+fibor(n-2);
end

This is working very well for small numbers but for large numbers it will take a long time

[MEERA MATHEW]

What is the solution for larger inputs ?

[Vishnu V]

The code you provided is producing the sum of the elements in the fibonacci series but not the fibonacci series

Answer 4

[Iyad Khuder]

Although this is resolved above, but I'd like to know how to fix my own solution:

function FiboSec = Fibo_Recursive( a,b,n )
k = n;
if n == 2
    FiboSec(k) = b;     
    return;
end
if n == 1
    FiboSec(k) = a;
    return;
end   
while k <= n
    FiboSec(k) = Fibo_Recursive(a,b,k-1) + Fibo_Recursive(a,b,k-2);
    k = k + 1;
end   
end

The algorithm is to start the formula from the top (for n), decompose it to F(n-1) + F(n-2), then find the formula for each of the 2 terms, and so on, untul reaching the basic terms F(2) and F(1).

I tried to debug it by running the code step-by-step. I noticed that the error occurs when it starts calculating Fibosec(3), giving the error:

"Unable to perform assignment because the indices on the left side are not
compatible with the size of the right side.

Could you please help me fixing this error?

Thanks

[John D'Errico]

Please don't learn to add an answer as a question! Your answer does not actually solve the question asked, so it is not really an answer. At best, I suppose it is an attempt at an answer though.

As far as the question of what you did wrong, Why do you have a while loop in there???????? You have written the code as a recursive one. That completely eliminates the need for a loop of any form.

1. What do you ant to happen when n == 1? It should return a. So you go that part correct.

2. What do you want it to do when n == 2? Agin, it should return b. Again, correct.

3. What should happen when n is GREATER than 2? It should use the recursive formula. NO LOOP NEEDED.

Next, why do you rename k to n?

Next, learn how to use the (if, elsef, else) form properly.

Finally, IF you want to return the ENTIRE sequence, from 1 to n, then using the recursive form is insane. Sorry, but it is. If you already have the first parts of the sequence, then you would just build them up from 1, to 2, to 3, all the way up to n. As such a fully recursive code is crazy IF that is your goal.

Below is your code, as corrected. As a test...

Fibo_Recursive(1,1,3)

ans = 2

Fibo_Recursive(1,1,4)

ans = 3

Fibo_Recursive(1,1,5)

ans = 5

Fibo_Recursive(1,1,10)

ans = 55

function FiboSec = Fibo_Recursive( a,b,n)
if n == 2
    FiboSec = b;     
    return;
elseif n == 1
    FiboSec = a;
    return;
else
    % All other cases have n > 2
    FiboSec = Fibo_Recursive(a,b,n-1) + Fibo_Recursive(a,b,n-2);
end   
end

Again, IF your desire is to generate and store the entire sequence, then start from the beginning. A for loop would be appropriate then. Note that the above code is also insanely ineqfficient, if n is at all large. And n need not be even too large for that inefficiency to become apparent.

Anyway, a simple looped code, generating the entire sequence would look like that below:

function Fibseq = loopedfib(a,b,n)
  Fibseq = zeros(1,n)
  Fibseq(1) = a;
  if n == 2;
    Fibseq(2) = b;
  for k = 3:n
    Fibseq(k) = Fibseq(k-1) + Fibseq(k-2);
  end
end

This code starts at the beginning, and works upwards. A recursive code tries to start at the end, and then looks backwards, using recursive calls. There is then no loop needed, as I said.

Do you see that the code you wrote was an amalgam of both the looped versions I wrote, and the recursive codes I wrote, but that it was incorrect to solve the problem in either form?

[Iyad Khuder]

Thanks @John D'Errico!

• Apologies. I'm new to this forum.
• I first wanted to post this as a separate question, but I was afraid it'd be repetitive, as there's already this post, which discusses the same point.
• Much apprecuated!