Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: Richard Heathfield <rjh@cpax.org.uk>
Newsgroups: comp.theory
Subject: Re: Every sufficiently competent C programmer knows
Date: Tue, 11 Mar 2025 19:06:10 +0000
Organization: Fix this later
Lines: 67
Message-ID: <vqq1j2$23vhr$2@dont-email.me>
References: <vqntaq$1jut5$1@dont-email.me> <vqp388$1tvqa$1@dont-email.me>
 <vqpdv9$202b2$2@dont-email.me> <vqperb$20c9k$2@dont-email.me>
 <E6mcnWv3nMa66036nZ2dnZfqnPWdnZ2d@brightview.co.uk>
 <vqpv2u$23vhr$1@dont-email.me>
 <a3304ec1dc4a774e1570a5bcaee4732327635974.camel@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Injection-Date: Tue, 11 Mar 2025 20:06:10 +0100 (CET)
Injection-Info: dont-email.me; posting-host="e140e83e0fdcc599ec997207e186ae4a";
	logging-data="2227771"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1+Fn84YG2LLLutYfRUCk5rlg78eqGWmfPcKxz068K+wqw=="
User-Agent: Mozilla Thunderbird
Cancel-Lock: sha1:Zztajt/XfQLSwPJ39CFStr6buHM=
In-Reply-To: <a3304ec1dc4a774e1570a5bcaee4732327635974.camel@gmail.com>
Content-Language: en-GB
Bytes: 3353

On 11/03/2025 18:33, wij wrote:
> On Tue, 2025-03-11 at 18:23 +0000, Richard Heathfield wrote:
>> On 11/03/2025 17:42, Mike Terry wrote:
>>> Finally, if you really want to see the actual HHH code, its in
>>> the halt7.c file (along with DDD) that PO provides links to from
>>> time to time.  However it's not very illuminating due to
>>> bugs/design errors/misunderstandings which only serve to
>>> obfuscate PO's errors in thinking.
>>
>> [I've now seen the code. Oh deary deary me.]
>>
>> Thank you for a spirited attempt to be cogent - or at least as
>> cogent as it is possible to be in the circumstances!
>>
>> I think PO's first step must be to demonstrate that HHH()
>> correctly diagnoses some easy functions, such as these:
>>
>> int rha(unsigned int i)
>> {
>>     while(--i > 0)while(--i > 0);
>>     return 0;
>> }
>>
>> int rhb(unsigned int i)
>> {
>>     if(i > 0)
>>     {
>>       rhb(i/10);
>>     }
>>     return putchar(i + '0');
>> }
>>
>> int rhc(unsigned int i)
>> {
>>     typedef int(*pf)(unsigned int);
>>     pf arr[3] = {rha, rhb, rhc};
>>     return arr[i % 3];
>> }
>>
>> and other such obvious tests.
>>
>> HHH(), the procedure that decides whether a program halts, is
>> required to work for all programs and all inputs. Does it work on
>> those cited above? I'm guessing it doesn't.
>>
> 
> No TM can simulate itself.

It doesn't have to. For a start, it can take source code as input 
and analyse it in much the same way that a compiler does.

> Proving HP in this way is dead end.

Proving it any way is a dead end, because the answer to the 
Halting Problem is already known, and has been known since at 
least 1936.

But if the OP is /going/ to write a decision function to attack 
the Halting Problem, he either writes a general purpose decision 
function or risks tackling the wrong problem.

-- 
Richard Heathfield
Email: rjh at cpax dot org dot uk
"Usenet is a strange place" - dmr 29 July 1999
Sig line 4 vacant - apply within