问题:

I'm getting an error on line 3 "TypeError: 'int' object is not iterable," and its been bothering me. Any advice/fixes appreciated.

Example test: collatz_counts(4) → 3 # 4 -> 2 -> 1 (3 steps)

Code I have:

def collatz_counts(x):
    num = 0    
    for i in (x):
        if i == 1:
            num += 1
            return num
        elif i % 2 == 0:
            num(i) / 2
            num += 1
            num.append(i)
        else:
            num = (i*2) + 3
            num += 1
            num.append(i)     
    return num

回答1:

Use a while loop. Just modify x in place until you get to 1 and keep track of the number of steps each time you run a cycle.

def collatz_counts(x):
    steps = 0
    while x != 1:
        if x % 2:
            x = x * 3 + 1
        else:
            x = x // 2
        steps += 1
    return steps

回答2:

This can be solved recursively:

def collatz_length(n):
    if n == 1:
        return 1
    return 1 + collatz_length(3*n+1 if n%2 else n//2)

Which lends itself to be memoized if you are going to be calling for a range of numbers, e.g. in Py3

import functools as ft

@ft.lru_cache(maxsize=None)
def collatz_length(n):
    if n == 1:
        return 1
    return 1 + collatz_length(3*n+1 if n%2 else n//2)

Which will run through the first million collatz sequences in about 2.31s vs about 28.6s for the iterative solution.