Sudoku solver

[Ian Kelly]

On Thu, Mar 26, 2015 at 8:23 AM, Marko Rauhamaa <marko at pacujo.net> wrote:
> Ian Kelly <ian.g.kelly at gmail.com>:
>
>> I don't think that I have used trial and error, in my head or
>> otherwise, in any sudoku I have ever solved.
>
> Of course you have. "This here can't be a 2 because if it were a 2, that
> there would have to be a 5, which is impossible. Thus, the only
> remaining alternative is 3, so I mark that down."
>
> That's trial and error, aka, reductio ad absurdum.
>
> (Additionally, sudoku solvers are known to pencil all kinds of markings
> on the sudoku sheets to help the deduction work.)

Okay, I've probably used single-lookahead trial and error in my
reasoning at some point. But the example you give is equivalent to the
deductive process "That can't be a 5, so I remove it as a candidate.
The only place left for a 5 is here, so I remove the 2 as a candidate
and fill in the 5."