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.

[wolf63]

I have finished it but had to guess a couple of times where there was an option of two numbers.

I am convinced that these are not computer generated puzzles but hat some evil genius is out there setting traps for us to fall in to. :-)

[flobadob]

Well done Wolf. After work I'll post an image of my finished puzzle including the workings. At times I had 4 to 5 numbers in a box as a possibility. Tough going but I'm not feeling so bad after reading through the comments.
I how O_G gets a go at it before then.
Jackdaw, I'm too much of a stickler to resort to checking if I'm right or not halfway through. If I did check I'd probably write that one off and move on afterwards.

[TonyV]

Jackdaw thanks for your link, got me started again lol

https://www.websudoku.com

[Old_Geezer]

Got about half way, you are probably further along. But already reached a point where I need to pencil in and mark one value, and make my way around noting the consequences for the rest of the table. I have to go out soon, but will look back when I can.

[Old_Geezer]

Feel free to post the solution; I'll be a fair old while.

[Togo]

15minutes to copy print solve and post. No Cheats. I love Sudoku.

https://thumbsnap.com/i/nt1QR6fc.jpg

[Togo]

One of the best tips I can give to a solver of hard puzzles is not only consider which square a certain number can go but to also eliminate the squares that it cannot go because another or other numbers have to go there. Some 9 grid boxes will have 2 or 3 squares that have to be occupied by 2 or 3 numbers meaning that other numbers have limited options. Remembering what you just discovered, as you do the process, is the devil in the detail.

[flobadob]

Hey Togo, did you actually finish it without having any workings or do you do those on another page?
My one looks like something it off the matrix with all the numbers over it.

[Old_Geezer]

Just back. Only got about half way through the tough one. Will look at it again, but it is demonic. I tend to find them a chore if forced to trace routes through looking for discrepancies. I may be some time. Impressed others have done it.

[Canary42]

IMO Togo's tip is essential for Difficult puzzles- but as Togo says, the problem is remembering as you go along.

[flobadob]

This is my effort. Not as neat as Togo but same answer

[flobadob]

https://thumbsnap.com/USfM4yoD

[Togo]

No workings at all Flobadab….that was the paper that I printed from your tinypic(hence the curve). But I confess that when I tackle the Sunday Times "Very Hard" I sometimes use just dots in squares to remind me that I have deduced something definite and to alert me to back work the heads up when I find something else in the same 9 box grid, column, or row. This is true. When I suffered my "turn" and woke up after 4 days in intensive, my Sunday Times was at my bedside, so I did the Sudukos. The doctors were astonished and even admitted that they could not do it, even though not being morphine up and nearly dead. Haha.
There are other techniques I use Canary but it is difficult to explain......sort of like mental algebra.

[Old_Geezer]

Really didn't think I'd finish it, but have just done so. Confess I too looked at the options and started tracing consequences for the table having chosen a possible value into one cell. I expected to learn something when I found a contradiction, but to my amazement, reached the end with it all compatible. Good to get it done, but still ended up more of a chore than a pleasure. Still, hopefully it keeps the dementia away for a bit longer.

[Togo]

Shot Geezer. Believe me the more you do the better you will get. If however you do not enjoy them then don't bother. They just, I suppose, suit my and many others particular way of looking at things. I do so like a conundrum.

[jno]

as TonyV says - they've got to be solved by logic. If they can't be, and you have to guess, the setter has screwed up.

[Togo]

The only time I have seen a setter screw up is when there are two or more possible solutions. Really.

[flobadob]

Have to admit, I'm completely amazed at Togo's skill, definitely should be working for NASA or something.
jno, I'd call it more of an educated guess than outright guess, because its a 50/50 so by rights one of the two options should complete the thing.
I have to admit there's a lot more satisfaction in solving them after putting so much effort in.
Togo reckons 15 minutes, I'll be not like an hour and a half :=)

[Old_Geezer]

I think looking for hidden pairs or triples is enjoyable. Looking for rectangles which indicate which corner cell holds what, ain't so bad. Looking for right angle triplets or loops starts to get a bit much. Takes so long to find them, even if you do. Ok as a challenge once in a while though.

[flobadob]

Nice one Old_Geezer, I knew you had it in you :)