Talk:1271: Highlighting

2013-09-30T07:53:13Z

131.215.169.224:

Personally, I dislike it when the highlighted area includes either the beginning indent/tab or the ending indent/tab, so according to my standards, I'm satisfied with the highlighting in paragraphs 1-3, but not with 4-6. <span style="color:green;">Saibot84</span> 04:33, 30 September 2013 (UTC)
:There is nothing marked in paragraph 6, is there? --[[User:Chtz|Chtz]] ([[User talk:Chtz|talk]]) 07:07, 30 September 2013 (UTC)

Why does he spell highlight like "hilight"? [[Special:Contributions/62.209.198.2|62.209.198.2]] 06:47, 30 September 2013 (UTC)
:Good question. Wiktionary allows {{Wiktionary|hilite}} as ''informal'', but says that {{Wiktionary|hilight}} is a "common misspelling". --[[User:Chtz|Chtz]] ([[User talk:Chtz|talk]]) 07:07, 30 September 2013 (UTC)

I'm so used to doing this that I know a few tricks and tiny strips of areas to click on in order to achieve symmetry in some tricky situations. [[Special:Contributions/131.215.169.224|131.215.169.224]] 07:53, 30 September 2013 (UTC)

1266: Halting Problem

2013-09-22T02:55:53Z

131.215.169.224: /* Explanation */ Wording changes.

{{comic
| number = 1266
| date = September 18, 2013
| title = Halting Problem
| image = halting_problem.png
| titletext = I found a counterexample to the claim that all things must someday die, but I don't know how to show it to anyone.
}}

==Explanation==
In 1936 {{w|Alan Turing}} proved that it's not possible to decide whether an arbitrary program will eventually halt, or run forever. This was later called the {{w|Halting problem}} by {{w|Martin Davis}}. The official definition of the problem is to write a program (actually, a {{w|Turing Machine}}) that accepts as parameters a program and its parameters. That program needs to decide, in finite time, whether that program will ever halt running these parameters.

The halting problem is a cornerstone problem in computer science. It is used mainly as a way to prove a given task is impossible, by showing that solving that task will allow one to solve the halting problem.

[[Randall]], however, is providing a simpler solution. He implements his own code for the question ''"Does it halt?"'' which always returns "true", and directs us to think about the bigger picture.

From a '''physical''' perspective, according to our current understanding of physics, this is right. Given enough time, any program will halt. This is due to factors external to the actual program. Sooner or later, electricity will give out, or the memory containing the program will get corrupted by cosmic rays, or corrosion will eat away the silicon in the CPU, or the {{w|second law of thermodynamics}} will lead to the {{w|Heat death of the universe}}. Nothing lasts forever, and this includes a running program.

From a '''mathematical''' point of view, this is not true: a Turing machine will never have a hardware failure because it's not a physical machine. It's a theoretical construct, and it's '''defined''' mathematically, independent of any physical hardware. Similarly, 1/3 + 1/3 + 1/3 = 1 no matter what any physical hardware you are computing it on claims.

From a '''practical''' point of view, there are of course times that a programmer would want to return "false", since some programs can be mathematically shown to run forever (or at least to run longer than the application is willing to wait).

The title text further relates to this issue by claiming to have found a case where something need not die, but Randall does not know how to actually show it to anyone, because just the fact everyone will die sooner than this thing it doesn't prove it will not die. The wording of the title text might also be a reference to {{w|Fermat's Last Theorem}}.

It should be noted that Randall's solution, barring its unsoundness, solves more than the halting problem in the form it is usually stated. The halting problem requires two parameters (a program and its parameters), while Randall's function only accepts one (the program). The question of whether a program halts for every input can be shown to be even harder to solve than the halting problem, meaning that even if a Turing machine had an additional instruction allowing it to check whether a program halts with given parameters, it still could not always confirm that a given program that halts for all parameters does so.

==Transcript==
:[Today's strip is in the form of a short computer program.]
'''Define''' DoesItHalt(program):
{
'''return''' true;
}
:The big picture solution to the halting problem.

==Trivia==
*A similar useless computer program can be found here: [[221: Random Number]].

{{comic discussion}}
[[Category:Computers‏‎]]
[[Category:Programming]]

explain xkcd - User contributions [en]

Talk:1271: Highlighting

1266: Halting Problem