[FIXED] How do I get the last number in a list that is very large?

Issue

def fib(n):
    a=0
    b=1
    for i in range(n+1):
        yield a
        a,b = b,a+b

lastnum = [num for num in fib(150000)]
lastnum[-1]

This is about the largest number (150000th number in fib) I can get from this method (Memory Error if larger). Is there ways I can improve this to get up to 2,000,000th digit? Is there a way to loop 'del' in my range loop till the last number needed in order to keep memory free by not saving EVERY number before?

tried a complicated mess of looping del and it was throwing errors or giving unexpected outputs

sorry kinda new to python

Thanks!

Solution

If you really only want the last element, then just avoid using a list in the first place. For instance, the following took some time but didn't run into any memory issues:

i = 2000000
for n in fib(i):
    result = n
print(result)

If, however, you want something more general (such as getting the last k elements or something), then you can use itertools.islice.
Here's the usage for getting the last element as requested, but you can tweak this to get other "slices" as necessary:

from itertools import islice
i = 2000000
print(list(islice(fib(i), i, i + 1))[0])

Docs: https://docs.python.org/3/library/itertools.html#itertools.islice

Answered By - ShlomiF