# How to write the Fibonacci Sequence?

### Question

I had originally coded the program wrongly. Instead of returning the Fibonacci numbers between a range (ie. startNumber 1, endNumber 20 should = only those numbers between 1 & 20), I have written for the program to display all Fibonacci numbers between a range (ie. startNumber 1, endNumber 20 displays = First 20 Fibonacci numbers). I thought I had a sure-fire code. I also do not see why this is happening.

``````startNumber = int(raw_input("Enter the start number here "))
endNumber = int(raw_input("Enter the end number here "))

def fib(n):
if n < 2:
return n
return fib(n-2) + fib(n-1)

print map(fib, range(startNumber, endNumber))
``````

Someone pointed out in my Part II (which was closed for being a duplicate - https://stackoverflow.com/questions/504193/how-to-write-the-fibonacci-sequence-in-python-part-ii) that I need to pass the startNumber and endNumber through a generator using a while loop. Can someone please point me in the direction on how to do this? Any help is welcome.

I'm a learning programmer and I've run into a bit of a jumble. I am asked to write a program that will compute and display Fibonacci's Sequence by a user inputted start number and end number (ie. startNumber = 20 endNumber = 100 and it will display only the numbers between that range). The trick is to use it inclusively (which I do not know how to do in Python? - I'm assuming this means to use an inclusive range?).

What I have so far is no actual coding but rather:

• Write Fib sequence formula to infinite
• Display startNumber to endNumber only from Fib sequence.

I have no idea where to start and I am asking for ideas or insight into how to write this. I also have tried to write the Fib sequence forumla but I get lost on that as well.

1
133
1/20/2018 1:42:48 AM

There is lots of information about the Fibonacci Sequence on wikipedia and on wolfram. A lot more than you may need. Anyway it is a good thing to learn how to use these resources to find (quickly if possible) what you need.

## Write Fib sequence formula to infinite

In math, it's given in a recursive form: In programming, infinite doesn't exist. You can use a recursive form translating the math form directly in your language, for example in Python it becomes:

``````def F(n):
if n == 0: return 0
elif n == 1: return 1
else: return F(n-1)+F(n-2)
``````

Try it in your favourite language and see that this form requires a lot of time as n gets bigger. In fact, this is O(2n) in time.

Go on on the sites I linked to you and will see this (on wolfram): This one is pretty easy to implement and very, very fast to compute, in Python:

``````from math import sqrt
def F(n):
return ((1+sqrt(5))**n-(1-sqrt(5))**n)/(2**n*sqrt(5))
``````

An other way to do it is following the definition (from wikipedia):

The first number of the sequence is 0, the second number is 1, and each subsequent number is equal to the sum of the previous two numbers of the sequence itself, yielding the sequence 0, 1, 1, 2, 3, 5, 8, etc.

If your language supports iterators you may do something like:

``````def F():
a,b = 0,1
while True:
yield a
a, b = b, a + b
``````

## Display startNumber to endNumber only from Fib sequence.

Once you know how to generate Fibonacci Numbers you just have to cycle trough the numbers and check if they verify the given conditions.

Suppose now you wrote a f(n) that returns the n-th term of the Fibonacci Sequence (like the one with sqrt(5) )

In most languages you can do something like:

``````def SubFib(startNumber, endNumber):
n = 0
cur = f(n)
while cur <= endNumber:
if startNumber <= cur:
print cur
n += 1
cur = f(n)
``````

In python I'd use the iterator form and go for:

``````def SubFib(startNumber, endNumber):
for cur in F():
if cur > endNumber: return
if cur >= startNumber:
yield cur

for i in SubFib(10, 200):
print i
``````

My hint is to learn to read what you need. Project Euler (google for it) will train you to do so :P Good luck and have fun!

244
9/8/2017 12:47:10 PM

# Efficient Pythonic generator of the Fibonacci sequence

I found this question while trying to get the shortest Pythonic generation of this sequence (later realizing I had seen a similar one in a Python Enhancement Proposal), and I haven't noticed anyone else coming up with my specific solution (although the top answer gets close, but still less elegant), so here it is, with comments describing the first iteration, because I think that may help readers understand:

``````def fib():
a, b = 0, 1
while True:            # First iteration:
a, b = b, a + b    # a will now be 1, and b will also be 1, (0 + 1)
``````

and usage:

``````for index, fibonacci_number in zip(range(10), fib()):
print('{i:3}: {f:3}'.format(i=index, f=fibonacci_number))
``````

prints:

``````  0:   0
1:   1
2:   1
3:   2
4:   3
5:   5
6:   8
7:  13
8:  21
9:  34
10:  55
``````

(For attribution purposes, I recently noticed a similar implementation in the Python documentation on modules, even using the variables `a` and `b`, which I now recall having seen before writing this answer. But I think this answer demonstrates better usage of the language.)

# Recursively defined implementation

The Online Encyclopedia of Integer Sequences defines the Fibonacci Sequence recursively as

F(n) = F(n-1) + F(n-2) with F(0) = 0 and F(1) = 1

Succinctly defining this recursively in Python can be done as follows:

``````def rec_fib(n):
'''inefficient recursive function as defined, returns Fibonacci number'''
if n > 1:
return rec_fib(n-1) + rec_fib(n-2)
return n
``````

But this exact representation of the mathematical definition is incredibly inefficient for numbers much greater than 30, because each number being calculated must also calculate for every number below it. You can demonstrate how slow it is by using the following:

``````for i in range(40):
print(i, rec_fib(i))
``````

# Memoized recursion for efficiency

It can be memoized to improve speed (this example takes advantage of the fact that a default keyword argument is the same object every time the function is called, but normally you wouldn't use a mutable default argument for exactly this reason):

``````def mem_fib(n, _cache={}):
'''efficiently memoized recursive function, returns a Fibonacci number'''
if n in _cache:
return _cache[n]
elif n > 1:
return _cache.setdefault(n, mem_fib(n-1) + mem_fib(n-2))
return n
``````

You'll find the memoized version is much faster, and will quickly exceed your maximum recursion depth before you can even think to get up for coffee. You can see how much faster it is visually by doing this:

``````for i in range(40):
print(i, mem_fib(i))
``````

(It may seem like we can just do the below, but it actually doesn't let us take advantage of the cache, because it calls itself before setdefault is called.)

``````def mem_fib(n, _cache={}):
'''don't do this'''
if n > 1:
return _cache.setdefault(n, mem_fib(n-1) + mem_fib(n-2))
return n
``````

## Recursively defined generator:

As I have been learning Haskell, I came across this implementation in Haskell:

``````fib@(0:tfib) = 0:1: zipWith (+) fib tfib
``````

The closest I think I can get to this in Python at the moment is:

``````from itertools import tee

def fib():
yield 0
yield 1
# tee required, else with two fib()'s algorithm becomes quadratic
f, tf = tee(fib())
next(tf)
for a, b in zip(f, tf):
yield a + b
``````

This demonstrates it:

``````[f for _, f in zip(range(999), fib())]
``````

It can only go up to the recursion limit, though. Usually, 1000, whereas the Haskell version can go up to the 100s of millions, although it uses all 8 GB of my laptop's memory to do so:

``````> length \$ take 100000000 fib
100000000
``````