Al GoRhythm Al GoRhythm - 1 year ago 99
Python Question

How do I print a fibonacci sequence to the nth number in Python?

I have a homework assignment that I'm stumped on. I'm trying to write a program that outputs the fibonacci sequence up the nth number. Here's what I have so far:

def fib():
n = int(input("Please Enter a number: "))

if n == 1:
elif n == 0:
return (n-1) + (n-2)

mylist = range[0:n]

I'm thinking I could use separate functions but I can't figure out how to pass the argument that calculates the fibonacci sequence. Then the next step would be to to print out the sequence of numbers up to that number.

Answer Source

Non-recursive solution

def fib(n):
    cur = 1
    old = 1
    i = 1
    while (i < n):
        cur, old, i = cur+old, cur, i+1
    return cur

for i in range(10):

Generator solution:

def fib(n):
    old = 0
    cur = 1
    i = 1
    yield cur
    while (i < n):
        cur, old, i = cur+old, cur, i+1
        yield cur

for f in fib(10):

Note that generator solution outperforms the non-recursive (and non-recursive outperforms recursive, if memoization is not applied to recursive solution)

One more time, recursive with memoization:

def memoize(func):
    memo = dict()
    def decorated(n):
        if n not in memo:
            memo[n] = func(n)
        return memo[n]

    return decorated

def fib(n):
    #added for demonstration purposes only - not really needed
    global call_count
    call_count = call_count + 1
    #end demonstration purposes

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

call_count = 0 #added for demonstration purposes only - not really needed
for i in range(100):
print(call_count) #prints 100

This time each fibbonacci number calculated exactly once, and than stored. So, this solution would outperform all the rest. However, the decorator implementation is just quick and dirty, don't let it into production. (see this amazing question on SO for details about python decorators :)

So, having fib defined, the program would be something like (sorry, just looping is boring, here's some more cool python stuff: list comprehensions)

fib_n = int(input("Fib number?"))
fibs = [fib(i) for i in range(fib_n)]
print " ".join(fibs) 

this prints all numbers in ONE line, separated by spaces. If you want each on it's own line - replace " " with "\n"