Sudoku Question.

[flobadob]

I've recently picked up a sudoku book that's been lying about the house for years. I've always found that I've been able to complete most sudoku puzzles without having to guess in any way, just using logic.
In this book however there is a section called Very Hard which I'm finding difficult to solve using my normal technique.
My query is whether or not it is deemed acceptable, after having filled in the boxes with what I think are all the possible solutions for each box, which can sometimes be up to 4 possibilities. Is it then okay to lightly fill in one of the boxes with only 2 possible numbers and work out others from that, and if it's wrong then to change for the other number and do the same?
Or should the sudoku puzzle always be solvable without resorting to this?
I hope this is making sense and I can get a few opinions of sudoku etiquette.

[chelle7272]

Well I admit that is what I do with the very difficult ones, as I find I can't keep track of it all mentally these days. Interested to hear other opinions myself :)

[ToraToraTora]

try some of these moves:
https://www.kristanix.com/sudokuepic/sudoku-solving-techniques.php

[Old_Geezer]

Do whatever it takes. Most stuff in newspapers are trivial, some make you think. But if desired some can be made so awkward IMO checking what ifs and tracing consequences through the table, is the only logical way forward.

Pencil (and eraser) use is very useful on the non-trivial puzzles.

[wolf63]

I do my Sudokus on the computer and I usually do the very hard ones.

I find that if you go through every box to see if their is only one option you find it easier to solve.

I find that,sometimes, I am completely unable to finish one but I am not sure if the puzzle of more difficult or my brain is having a bad day. It has many bad days.

[TonyV]

That was my tactic to right down options next to square and then work out options, but can't remember the last time I did one lol

[Calibax]

I'm sure trial and error is a perfectly acceptable technique.

[Canary42]

My understanding is that they should be solvable without resorting to such trial and error tactics.

But I very occasionally have to go down that route.

[TonyV]

Never like trial and error, it's only logic.

[flobadob]

I've found with these so called Very Hard ones that after I've filled out all the possibilities then I'm stuck with having to make a choice on one of the boxes with two options and working things out from there.
So, for example, if I've got a box in which only 5 or 6 can go, then I can put in a 5 lightly and start working everything else out based on that.
Saying that, the logic would be that if that doesn't work out then by erasing those and putting a 6 instead of the 5, then it should work out that time so long as all other assessments are correct.

[TonyV]

Flob would try something in different direction to see what rules out the five or the six. That the technic I used to use, there will always be an obvious one somewhere.
I have tried your technique and got in a horrible mess

[Jackdaw33]

To anyone who is into Sudoku this site may be of interest. Saves buying books or papers.

https://www.websudoku.com/?level=2

[flobadob]

TonyV, by the stage I mentioned I've basically already filled all possibilities and can't rule out either the 5 or 6 conclusively.
Therefore it's a 50/50 situation. If I try the 5 and it doesn't work, then it should definitely solve using the 6.

[Old_Geezer]

Intrigued. Maybe you should post where you have reached and folk can suggest how they'd proceed.

[chelle7272]

Yes I agree flobadob, the very hard ones can't always be solved even checking each individual square. You'd have to mentally track from one option until either solved or proved woring, so I pencil it in as you suggest.

[Prudie]

I do masses of sudokus and some are definitely trial and error. Pencil is the way to go or use an online grid. Sometimes if you fill every blank square with all the possibilities you will find one that stands out as must be a unique number.

[flobadob]

Old Geezer I'll see if this works. This is a puzzle I worked on but could not finish without eventually having to try a 50/50. It would be interesting to see if others have techniques that could finish it without having to do this. Hope the image loads.
http://i67.tinypic.com/2mn3cyc.jpg

[10ClarionSt]

My wife is brilliant as Sudoku. She looks for the most difficult ones. But I think it's trial and error, as you describe, flobadob. I must admit, I use a pencil with a rubber at the other end, and if I'm struggling, I write the numbers in the corner, a bit smaller. If they're wrong, I just rub them out! Simples!!!

[Old_Geezer]

Yay ! Except that as that's unstarted one may not come to the same state that you have reached. I'll write it out and give it a go.

[flobadob]

Nice one Old_Geezer, I'll be interested to see how you get on. I've still got the puzzle with my workings in the boxes from my point of having to go 50/50.

[Jackdaw33]

The advantage of the link I posted above, although perhaps not in the true spirit, is that if you enter a wrong number it will tell you immediately.