Trains, puzzles and pencil-marks

Advanced methods and approaches for solving Sudoku puzzles

Trains, puzzles and pencil-marks

I was on the train back from London the other day standing up doing a puzzle and looked down to see four people sat by me doing Sudoku puzzles.

Anyway I was perusing the message board the other day and read with interest that Wayne (Pappocom) does his puzzles without using pencil marks. I had started doing this on the easier ones but not tried it on a hard. Well I’ve now taken up the challenge and started doing all but the V. Hard ones with out using PMs. I’ll have to say I find it more fun to do them this way. It takes a bit longer but it’s not much harder. On thinking about it I’ve come to the conclusion that PMs don’t really help with unlocking the key to the puzzle, all they do is speed up the finishing off of the puzzle when you’ve cracked it. Any one else going the no PM route?

That said, I was doing a hard puzzle yesterday and failed to do it without PMs in fact even with PMs I was struggling.

Here’s the puzzle:

*4*|***|*9*
8**|4*6|**7
*67|***|28*
--------------
***|1*5|***
**2|*3*|1**
***|7*2|***
--------------
*51|***|34*
9**|3*8|**5
*7*|***|*2*

Solution follows:

I got to the stage where I carpet bombed the blanks and had these numbers in the fifth row

Cell1-457 C2-89 C3-2 C4 689 C5-3 C6-49 C7-1 C8-567 C9-69

Anyway, my logic was that it had to be a 4 in C6 because the nine had to be in C2 C4 or C9 because otherwise the 8 and the 6 would need to fill three cells.

Didn’t seem to be very extreme logic once I spotted it but I then did a V hard and unlocked it with the same method and no X-wing requirements.

Anyone else used this method or can we henceforth call it the Duncan? ;-)

Here’s the V hard if you want to give it a go.

7*4|**5|***
***|28*|***
**1|94*|8**
---------------
91*|***|**7
**2|***|3**
8**|***|*65
---------------
**8|*92|6**
***|*31|***
***|6**|7*1

When I transcribed it to my big grid I missed off the 1 in box 1 which didn’t help.
Duncan

Posts: 13
Joined: 05 May 2005

I gave up on pencil marks a while ago, and found it speeded up my solving no end - but required must greater concentration. Still can't normally do a very hard without them though :o(
Guest

Re: Trains, puzzles and pencil-marks

Duncan wrote:I got to the stage where I carpet bombed the blanks and had these numbers in the fifth row

Cell1-457 C2-89 C3-2 C4 689 C5-3 C6-49 C7-1 C8-567 C9-69

Anyway, my logic was that it had to be a 4 in C6 because the nine had to be in C2 C4 or C9 because otherwise the 8 and the 6 would need to fill three cells.

I find the following logic easier:

What cells can hold the number 5 and 7? The only cells that can be 5 or 7 are C1 and C8, which means that there is only one cell that can have 4... C6 to be precise...
Animator

Posts: 469
Joined: 08 April 2005

You can look at http://forum.enjoysudoku.com/viewtopic.php?t=143 this puzzle aswell, it uses the same technique...
Animator

Posts: 469
Joined: 08 April 2005

Just a note on PM. Solved my first "differcult" sudoku with the help of PM and CB. I was amazing how many obvious cells were available that I just could not see till I CBed. Net stop "Fiendish"
Guest

Posts: 312
Joined: 25 November 2005

Animator wrote:You can look at http://forum.enjoysudoku.com/viewtopic.php?t=143 this puzzle aswell, it uses the same technique...

Are yes, then I withdraw my name suggestion for this technique, reluctantly.
Did you try the V.hard one? I found one of these in Col 9, unless I missed somthing easier.
Duncan

Posts: 13
Joined: 05 May 2005

Yep

The very hard one has one in column 9 (2/8/9) and one in row 6 (3/4/7)
Animator

Posts: 469
Joined: 08 April 2005