user1698174 - 1 year ago 144

Python Question

The Collatz conjecture

what i am trying to do:

Write a function called collatz_sequence that takes a starting integer and returns the sequence of integers, including the starting point, for that number. Return the sequence in the form of a list. Create your function so that if the user inputs any integer less than 1, it returns the empty list [].

background on collatz conjecture:

Take any natural number n. If n is even, divide it by 2 to get n / 2, if n is odd multiply it by 3 and add 1 to obtain 3n + 1. Repeat the process indefinitely. The conjecture is that no matter what number you start with, you will always eventually reach 1.

What I have so far:

`def collatz_sequence(x):`

seq = [x]

if x < 1:

return []

while x > 1:

if x % 2 == 0:

x= x/2

else:

x= 3*x+1

return seq

When I run this with a number less than 1 i get the empty set which is right. But when i run it with a number above 1 I only get that number i.e. collatz_sequence(6) returns [6]. I need this to return the whole sequence of numbers so 6 should return 6,3,10,5,16,8,4,2,1 in a list.

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Answer Source

You forgot to append the `x`

values to the `seq`

list:

```
def collatz_sequence(x):
seq = [x]
if x < 1:
return []
while x > 1:
if x % 2 == 0:
x = x / 2
else:
x = 3 * x + 1
seq.append(x) # Added line
return seq
```

Verification:

```
~/tmp$ python collatz.py
[6, 3, 10, 5, 16, 8, 4, 2, 1]
```

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