Question

I’m a beginner at Python and I’m getting a strange out of bounds error.

The idea is that I need a cache initialized using:

arr = [0]*1000000

then accessed in the same function by calling

def func (i) :
    k=1
    a = i
    arr = [0]*1000000
    while (i>1):    
        if arr[i] != 0:
            k = k + arr[i] - 1
            break
        if i%2 == 0:
            i = i/2
        else:
            i = 3*i + 1
        k += 1
    arr[a] = k
    return k

if the value i is over 1500 it gives me an out of bounds error. The cache, however, is supposed to be initialized to a million ints. Am I missing something? Thanks

Answer 1

Updated now that the code is posted:

I don’t seem to get any problems up to 1500. I do get an IndexError for func(1819), for which the i evolution begins

1819
5458
2729

and winds up at

851290
425645
1276936
[...]
IndexError: list index out of range

but that’s not a bug, that’s simply a fact that it goes higher than you made room for. You could use a dictionary instead of a list to avoid this problem.

—

To be clear, here’s the sort of thing I had in mind:

def func_with_dict(i) :
    k=1
    a = i
    arr = {}
    while (i>1):    
        print i
        if i in arr:
            k = k + arr[i] - 1
            break
        if i%2 == 0:
            i = i/2
        else:
            i = 3*i + 1
        k += 1
        arr[a] = k
    return k

which produces

1819
5458
2729
8188
[...]
851290
425645
1276936
638468
319234
[...]
20
10
5
16
8
4
2

and a final answer of 162. I don’t think I’d use arr this way myself, though, but what I would do depends upon what you’re trying to do.