On Thursday 24 May 2012, Oleksandr Kazymyrov wrote:
> Dear all,
>
> In manual "ZZ ?" you can find:
>
> As an inverse to "digits()", lists of digits are accepted, provided
> that you give a base. The lists are interpreted in little-endian
> order, so that entry "i" of the list is the coefficient of
> "base^i":
>
> sage: Z([3, 7], 10)
> 73
> sage: Z([3, 7], 9)
> 66
> sage: Z([], 10)
> 0
>
> But for base more than 2^64 it doesn't work. It looks stupid, because you
> can call "digits(2^64)", but not an inverse function:
> sage: a=ZZ(randint(0,2^128-1)).digits(2^64)
> sage: a
> [1154963902035838039, 8176620537326016718]
> sage: ZZ(a,2^64)
> ERROR: An unexpected error occurred while tokenizing input
> The following traceback may be corrupted or invalid
> The error message is: ('EOF in multi-line statement', (1348, 0))
>
> ---------------------------------------------------------------------------
> OverflowError Traceback (most recent call last)
> ....
> OverflowError: long int too large to convert
>
> Of course it is possible to convert it back using Sage:
> sage: a=ZZ(randint(0,2^128-1))
> sage: a
> 201636464310824733716520014075404236490
> sage: b=a.digits(2^64)
> sage: b
> [6699442741605840586, 10930734632904602844]
> sage: sum([b[i]*(2^64)^i for i in xrange(len(b))]) == a
> True
>
> Why core library doesn't include this functionality? Are there any other
> ways to do the same as described above using another function?
It's easy: implement it and send us a patch ;) On a more serious note, it
seems you found a bug and your code above is the right fix. So it would be
great if you could open a ticket and provide a patch which falls back to the
generic code above if the base is >= 2^64.
Cheers,
Martin
--
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