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From: vallor <vallor@cultnix.org>
Newsgroups: comp.os.linux.advocacy
Subject: Re: C Non-Programming Non-Challenge (was: C Programming Challenge)
Date: 16 Jun 2024 08:19:11 GMT
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> On Jun 15, 2024 at 3:20:19 PM EDT, "Farley Flud" <ff@linux.rocks> wrote:
> 
>> On Sat, 15 Jun 2024 14:29:04 +0000, Farley Flud wrote:
>> 
>> 
>>> Write a C program to compute the subfactorial of an integer N.
>>> 
>>> 
>> Nobody got it (as predicted).
> 
> Because no one bothered.

I actually took the opportunity to install the GMP documentation
on my system, look up what a "subfactorial" is, and do
an implementation that works for !N where N is 0 through 50.

Furled Fart lost the opportunity to show his prowess with
the GMP library by not demonstrating use of gmp_printf().

> This is all trivial stuff.  These math "problems" have already been
> solved multiple times.  That you think this is a "programming challenge"
> just shows how utterly clueless you are.

It's more of a "let's learn libgmp" challenge, which is a library
that he already has familiarity with.  I don't think he learned anything.
And it's not that difficult a library to use.

> Hint: people were doing this 60 years ago on mainframes using Fortran.
> That you have only now discovered this proves AGAIN how utterly clueless
> you are.
>   
>   
> What's next? Compute Pi to a million decimal places? That also has
> already been done many times. Sounds like another great "programming
> challenge" for your feeble brain.

$ time ./try_mpz 
0:1
1:0
2:1
3:2
4:9
5:44
[...]
49:223774392215649610092605161415154686480227084177314431854406736
50:11188719610782480504630258070757734324011354208865721592720336801

real	0m0.002s
user	0m0.001s
sys	0m0.001s

I also was interested in the recursive version.  Here's the
run with N=50 with long long int's, which gives the wrong answer
due to overflow:

$ time ./try2
7531320031745615777

real	0m35.541s
user	0m35.530s
sys	0m0.001s

So: don't bother with the recursive version, except as
a pedantic exercize.

*Do* use GMP if you ever need to work with big numbers...
maybe numpy in Python can do the same thing, I wouldn't
know about that.

$ ll /usr/share/doc/gmp-doc/gmp.pdf.gz 
-rw-r--r-- 1 root root 812615 Nov 16  2020 /usr/share/doc/gmp-doc/
gmp.pdf.gz

$ grep FLAGS Makefile
CFLAGS=-Wall -O3 -g -Wpedantic

-- 
-v System76 Thelio Mega v1.1 x86_64 NVIDIA RTX 3090 Ti
   OS: Linux 6.9.4 Release: Mint 21.3 Mem: 258G
   "Unrecoverable Error #666: Armegeddon in 30..29..28..27.."