How to get nth result from simple function in python?

Question

I apologize for the simplicity of this question, I have searched multiple times but it may be so simple it hasn't been asked before.

I wrote a Fibonacci function that prints every Fibonacci number under 3000.

def fibonacci():
    a, b = 0, 1
    while b < 3000:
        a, b = b, a + b
        print a
    return a

How can I make it so it returns the first n Fibonacci numbers?

Also, how can I make it just print the nth value? For example print [6], which would return 8. I tried to make it a string:

a = str(fibonacci())
print a[6]

but that didn't work, and I'm not sure why. Thanks for helping.

Print prints the intermediate results to standard output, but otherwise they are lost. You'll have to store them somewhere. — flup, Jul 21 '13 at 23:52
possible duplicate of [Python Fibonacci Generator](http://stackoverflow.com/questions/3953749/python-fibonacci-generator) — flup, Jul 21 '13 at 23:54
You can use `yield` in a method to turn the results into a iterable generator — cdbitesky, Jul 21 '13 at 23:55
That makes sense, but I am such a novice I'm not sure how I'd go about doing that. How do you store all the intermediate results? — ChillerObscuro, Jul 21 '13 at 23:55

score 0 · Answer 1 · answered Jul 22 '13 at 00:04

First, your function needs to know what n to use. That's simple enough - just tell it n as an argument:

def fibonacci(n):

If you want to return the first n fibonacci numbers, you'll need to keep a list of them:

    numbers = [0, 1] # Start off with the first 2 numbers

Then, compute the fibonacci numbers pretty much the way you did before, except stopping at the nth one, and adding together the last two list elements instead of a and b:

    # xrange(2, n) is a sequence going from 2 to n-1
    for i in xrange(2, n):
        # Add together the last two fibonacci numbers and append the result
        # to the list
        numbers.append(numbers[-2] + numbers[-1])

Finally, return the list:

    return numbers

The end result:

def fibonacci(n):
    numbers = [0, 1]
    for i in xrange(2, n):
        numbers.append(numbers[-2] + numbers[-1])
    return numbers

This will return a list of the first n fibonnaci numbers (for n >= 2). You'll need to special-case n == 0 or n == 1 if you want to handle those values.

score 0 · Answer 2 · answered Jul 22 '13 at 00:12

You could turn your function into a generator and use it in a list comprehension. The last number in the list created is the value of nth fibonacci number (fibonacci(n)):

def fibonacci(n):
    a, b = 0, 1
    count = 0
    while count < n:
        a, b = b, a + b
        count += 1
        yield a

fibs = [fib for fib in fibonacci(6)]
print fibs
print 'fibonacci(6):', fibs[-1]

Output:

[1, 1, 2, 3, 5, 8]
fibonacci(6): 8

score 0 · Answer 3 · answered Jul 22 '13 at 00:14

Create a generator:

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

Create a wrapper to call it n times

def fibn(n):
    g= fibonacci()
    return [next(g) for _ in xrange(n)][-1]

>>> fibn(5)
5
>>> fibn(15)
610

Fredrik · Accepted Answer · 2013-07-22T01:21:58.033

There are several ways to do this; here's a semi-clever one:

First, change print to yield, so the function returns the numbers instead of printing them:

def ifibonacci():
    a, b = 0, 1
    while b < 3000:
        a, b = b, a + b
        yield a

Then use itertools.islice to slice out the numbers you want:

import itertools

print list(itertools.islice(ifibonacci(), 10))
# prints [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]

print list(itertools.islice(ifibonacci(), 6, 6+1))
# prints [13] which isn't quite right; see below

Note that your function doesn't output the initial 0, so the indexes are off by one. To fix that, move the yield up one line:

def ifibonacci():
    a, b = 0, 1
    while b < 3000:
        yield a
        a, b = b, a + b

print list(itertools.islice(ifibonacci(), 6, 6+1))
# prints [8]

(also, this still won't print numbers larger than 3000. fixing that is left as an exercise)

How to get nth result from simple function in python?

4 Answers4