[Python-Dev] SIGCHECK() in longobject.c (original) (raw)

Mark Dickinson dickinsm at gmail.com
Mon Oct 19 13:27:04 CEST 2009

On Sun, Oct 18, 2009 at 9:01 PM, Antoine Pitrou <solipsis at pitrou.net> wrote:

Can we remove this check, or are there people doing million-digits calculations they want to interrupt using Control-C ?

By the way, here's an example of an almost real-life use of million digit calculations.

For an elementary number theory course that I taught a while ago, there was an associated (optional) computer lab, where the students used Python to investigate various ideas, conjectures, examples, etc. One of the less open-ended questions might have been[1] something like:

"On August 23rd, 2008 a computer at UCLA found the first example of a prime with more than 10 million digits: p = 2**43112609-1. Find

(1) the exact number of digits in p, when written out in decimal (2) the last 100 digits of p (3) the first 100 digits of p."

It's wonderfully easy to get answers to these questions with Python:

import math e = 43112609 p = 2**e - 1 ndigits = int(math.ceil(math.log10(p))) last100digits = '{:0100d}'.format(p % 10**100) first100digits = str(p // 10**(ndigits-100))

Getting the first 100 digits takes a good few seconds; the other operations are faster.

But if you (not unreasonably) try to compute str(p), you'll find it's impossibly slow, and it's very handy that it's possible to interrupt that calculation, attempt to understand why it's slow, and then try different methods.

(And yes, there are better ways to get both the first and last hundred digits.)

Mark

[1] Might have been, but wasn't. Hence the almost. If I'd been teaching that course this semester I'd almost certainly have included something like this.

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