Code for finding Pisano Period showing wrong output

Question

I want to calculate Pisano Period for any integer 2 <= m <= 100000.

This code is currently not working for m>4, for m>4 output showing is 9.What am I doing wrong in the below code.

def fib(n):
    i=0
    f=[]
    while(i<=n):
        f.append(i)
        i+=1
    i=2
    while(i<=n):
        f[i]=f[i-1]+f[i-2]
        i+=1
    return f[n]

m=int(input())
j=2
p=[0,1,1,2]
while (p[j-1]!=1 and p[j-2]!=0):

    h=fib(j)%m
    p.append(h)
    j+=1

print(len(p))

SO is not a code-debugging service. State clearly the problem with your code. — SiHa, Jul 06 '16 at 15:22
`p[-1] in p[:-2]` not `p[::-1]!=p[0:]` Understand splices http://stackoverflow.com/questions/509211/explain-pythons-slice-notation — Dylan Madisetti, Jul 06 '16 at 15:23
@SiHa This code is not showing any output.That means the last while loop is somehow becoming infinite. — Gautam Sagar, Jul 06 '16 at 15:30

Blender · Answer 1 · 2016-07-06T15:33:23.237

1

Your stopping condition is p[::-1] != p[0:], which checks if the list is palindromic. Your stopping condition should be if the list repeats.

One way to do it would be to use the recurrence relation representing the n-th Fibonacci number, which still holds modulo m. There are only two initial values, so...

edited Jul 06 '16 at 15:33

answered Jul 06 '16 at 15:23

Blender

289,723
53
439
496

how can i check that – Gautam Sagar Jul 06 '16 at 15:28
@GautamSagar: That's for you to figure out. I already gave one hint. Another would be to check if the list literally repeats itself, which means that the first half is equal to the second half. That'll be slower, but it still works. – Blender Jul 06 '16 at 15:29

Code for finding Pisano Period showing wrong output

1 Answers1