# Sudoku - sharing skills



## 2020hindsight (8 July 2008)

Whenever I fly I have a go at these damned Sudoku things - must be hundreds of "Qantas magazines" flying around up there with half completed Sudoku puzzles I started. 

I figure we could give each other some lessons.  I'll start with an example I posted on the "Games" thread.

Proposal is to have a go at a medium hard sudoku..  



> THere's an easy one posted a couple of months ago - already posted on games thread). ....  here :-
> https://www.aussiestockforums.com/forums/showthread.php?p=143586&highlight=website#post143586




http://www.websudoku.com/

you can repeat any puzzle you wish ( they all have numbers), 

so  here's "Medium" difficulty Puzzle 2,557,675,180   (or level 2) - proposal is to publish the answer. - including explanation of the steps. 

I believe I've filled in all the obvious squares. (eg added a 9 , then a 6 , then a 6 , then a 1, then a 2) etc 

How do you experts out there proceed from here? 

ok - I have now set the options to permit two options and/or text to be written in a square - (I'm fairly sure it will be necessary)

One way I have (very occasionally) found helpful is to write down two optional places for the 1 for instance - eg in the second row I have written 1a and 1b - i.e. it must be in one of these positions.

Incidentally I've also written 6a and 6b (one or other of these is a 6)- 
also 6c and 6d ( ditto) etc etc 

to be honest I tend to get stuck on these harder ones - so if anyone's an expert out there , I''d appreciate your advice 

PS 
Ahhh I see a light ...
... 4b is a 4 for a start - (from looking at the 5th row) -  to be continued


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## 2020hindsight (8 July 2008)

then there's a 5 in the last column of the 4th row
reason - in this "cluster of 9" , there is missing a 1, a 5 and an 8
because either 1c and 1d must be a 1, then the other unknown square of this cluster must be a 5 or an 8 - by inspection it must be a 5 ( there's already an 8 in that row)

After this the 1 and the 8 of this cluster are also fairly easy .

then the right hand column can be filled in , 
then the 5th row can be completed , etc etc 
then fill the "centre left" cluster (3 9 6 etc)


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## 2020hindsight (8 July 2008)

Then the left column can be filled, 
Then change a few to eg "3,5", meaning this must be a 3 or a 5 etc

Now the first row second column is interesting because it must be a 2, 3 or 5. but the other two emply squares in this cluster are each 3 or 5 ( hence one must be 3 the other5) which only leaves 2.

eventually 3, 5, and 7 are missing from second last column - hence since two of these are 3 and 5 , we've found the 7 

then the 8 in the topright cluster falls out because it cant be 3 or 5 , yet it must be 3, 5 or 8

then the 5 th column 3rd row can only be a 3 
then all the ones with "3 or 5" fall out pretty much 
only the three central clusters to go - the highlighted "6b" must be a 3
this triggers 6d = 6 , then 6a = 6
then they all fall out


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## 2020hindsight (8 July 2008)

and - stats show that 100% of people would be faster, lol

PS best if you can avoid assigning more than two options to a square IMO


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## 2020hindsight (14 December 2008)

I'll have another go to get this thread to fly. (not that it ever took off in the first place lol). Might be a change from listening to bludy Xmas carols whilst shoping in supermarkets. 

I've picked this puzzle based on today's date (2008 / 12/ 14 in ISO format)

Go to this website  http://www.websudoku.com/?select=1&level=1
Click "Select a puzzle"
at "Number" type in 2008 012 014

Incidentally, this is how I copy pictures and keep them at a reasonably small size (memory wise) :-
Press "Alt+Print Screen" = screendump
Then go to Irfanview (much easier to crop in Irfanview than Paint if you ask me)
Press "Cntl+V" will paste the screendump
with rectangle selected in toolbox, drag rectangle around the bit you want to select,
go to Edit>Crop Selection
"Save as"  "sudoku01" 
done .  
PS  this advice just in case you want to advance this problem and post your contribution - plus practice for posting pictures.  Incidentally screen dumps automatically control the size of the picture - "wysiwyg" etc. 

PS I have no idea how appropriate this puzzle is - but it's called "easy" so here's hoping lol.  

PS I can't tell you how many Airline magazines I've thrown back in the back of the chair in front, pissed off that i couldn't solve the damned Sudoku puzzle therein.


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## 2020hindsight (14 December 2008)

I'd set the options as follws :- 
 most important is to "allow multiple numbers to be typed into a cell" 
(alias "pencil marking / pencilling in options")


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## 2020hindsight (14 December 2008)

I personally use two options for "multiple numbers" 
(refer post #3) 
i.e. "4a... 4b" in two cells means that the 4 is in one or other of fthese cells
"2, 5" in one cell means that this cell has either a 2 or a 5.
I never XXX  I try not to use more than 2 pencilled in options. 

Hey I'm not pretending to be an expert - but *I'd like to learn from any real experts out there !* 

Any assiatance appreciated. 

Having said that, I'll get it started off.  (hopefully to give it a fraction of  momentum) ...

I trust everyone can agree that Cell a31 = 1 (i.e. third row, 1st column).


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## 2020hindsight (14 December 2008)

Using this "single row" or "single column" analysis, you can fill in the following , 1y, 2y etc :-

I've also added the first of the pencilled in options, namely 9a and 9b in the centre.  (sudoku03.jpg)

Better go do some mowing - feel free to contribute.


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## 2020hindsight (14 December 2008)

OK, when I said that was all you could get from looking at "rows and columns", there are at least 3 more cells (possibly more) that can be found that way ... 

Your mission, should you choose to accept it, is to find them ?


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## 2020hindsight (14 December 2008)

geepers - wait for you blokes, we'd be here all day .. 

there are 3 off "3y" 's added. 

Next rule I call "if it's one or other, then it can't be another"
and hence 
"if it's one or other in a Row, Column or 3x3 Square, then there can't be another in that R,C, or S"

Hence I've added the 9y as well. (based on the 9a and 9b = one or the other of these must be a 9)
sudoku05 :-


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## 2020hindsight (14 December 2008)

Next rule is "process of elimination"
you can add a 5y in a column ( last one standing)
and also in a column (because I've upgraded "9a and 9b" to "1,9"  and "1,9" - i.e. one must be 1 and the other must be 9.

Note that it is an ugrade because of the fact that they are in the same row (or column or square) - and hence the inference that one must be a 1, and the other a 9 still holds.  
NB. But if they were NOT in the same R,C or S, then you couldn't assume that one being a 1 would make the other a 9 for example.

Note "9a and its companion 9b" are a pair - one or the other is a 9.
likewise "9c and 9d".  
Quite often it follows that 9a and 9c also must follow this rule , i.e. one is a 9 , the other isn't - why? because they are on the same column etc.


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## 2020hindsight (14 December 2008)

Cells a41 etc must be 5a, 5b, 5c, 5d.  
Because 5a and 5c are in same column, (and 5c and 5d are in same square) one or other of 5a and 5c must be a 5.

Therefore in the first row, cell a11 must be a 5 or 7 - it cannot be a 5, must be a 7.


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## 2020hindsight (14 December 2008)

Then cell a1,9 becomes a 5, which triggers 
5d not a 5, 
5c= 5y, (yes) 
5a not a 5
5d = 5y (yes)

Apologies I've renamed 5e and 5f which is what I should have done all along.


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## 2020hindsight (14 December 2008)

Self explanatory ...
I'm hoping I've broken its back ...


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## 2020hindsight (14 December 2008)

from previous post, cell a8,7 must be an 8
cell a7,9 must be a 4 ...
Column 5 means a7,5 must be an 8 ...
... und so weiter ...


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## It's Snake Pliskin (14 December 2008)

20/20 do you realise I am the only other poster on this thread?

But seriously good work with what you have posted.


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## 2020hindsight (14 December 2008)

It's Snake Pliskin said:


> 20/20 do you realise I am the only other poster on this thread?
> 
> But seriously good work with what you have posted.



thanks snake
It might be read over Xmas break maybe ... 

If not ... At least I'm clarifying it in my own mind 

PS Someone else might have a crack at a medium or a difficult one,  who nose.


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## 2020hindsight (14 December 2008)

OK we've done a medium , and an easy - now for a hard one.  (might take a week).. we'll see. 

Hard #101 270 313

The central 3x3 square falls out ( the missing numbers 7, then the 2, and 1 are easily allocated)..


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## 2020hindsight (14 December 2008)

From which a row and part of a column can be completed :-


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## 2020hindsight (14 December 2008)

The 5y and the 8y follow from "row and column analysis"
Thw 3y can be found as follows :-

in the 3x3 Square, the missing numbers are 2, 3,  and 9.  The only only that fits at cell a7,3 is the "3y".


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## 2020hindsight (14 December 2008)

etc ..
Consider the 4th row 
missing numbers are 4,5, and 6
therefore cell a4,2 must be a 5, because there are already a 4 and a 6 in that column. 

Likewise, (but probably apropos o nothing) in the third column, the missing numbers are 4,6,7, and 9.
There are already a 4 and a 9 in the 5th row, therefore cell a5,3 is a "6,7" = either 6 or 7.


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## 2020hindsight (14 December 2008)

... - you get to a point where cell a9,3 must be a 9 by checking concurrently rows, columns, and the 3x3 square.


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## 2020hindsight (14 December 2008)

... 
you get to where 2c must be a 4 because of simple column analysis.
This in turn triggers a few more in lower right hand square :-


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## lesm (14 December 2008)

Hi 2020,

You wouldn't happen to do cryptic crosswords as well?

Cheers


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## 2020hindsight (14 December 2008)

... etc to the end...


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## 2020hindsight (14 December 2008)

lesm said:


> Hi 2020,
> 
> You wouldn't happen to do cryptic crosswords as well?
> 
> Cheers



lol no m8,
but I'm sure they're fun - and I'm sure I will one day 

well done btw,  doesn't help the concentration when you're vacuuming for the Xmas tree area, feeding the dog etc, trying to remember what you did in what sequence - so towards the end I cheated and ran a check before posting each step  

STill I rarely do these things, (except on planes) -  and quite chuffed I did a hard one - same principles, just a bit more "free-rein" and "lateral thinking" yes? . 

If there are some tricks I've missed (or inefficiencies) please correct me  - thanks.


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## 2020hindsight (14 December 2008)

to be honest I prefer the logic puzzles, viz:-

PS I've chosen this one because of the humourous cartoon


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## lesm (14 December 2008)

2020hindsight said:


> lol no m8,
> but I'm sure they're fun - and I'm sure I will one day
> 
> STill I rarely do these things, (except on planes) -  and quite chuffed I did a hard one - same principles, just a bit more "free-rein" and "lateral thinking" yes? .
> ...




hehe...the fun part is when you realise that you have made an error and you need to backtrack and work your way forward again.

Used to do these with a couple of colleagues, as well as cryptics, in our spare time or when we were bored. You should try an 'evil one' some time 

The main thing is to be logical and methodical, after a while you will start recognising particular patterns and that will assist in selecting the possible numbers to include or exclude.

It doesn't hurt to take on the harder puzzles at times, as you will learn from these. The easier ones will then start to appear to be trivial exercises in comparison.

Cheers.


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## 2020hindsight (14 December 2008)

lol - I looked into the maths behind them on wiki or somewhere.  I was curious as to how they'd be sure there was 
a) a unique solution, and 
b) just adequate information, or finally 
c) superfluous info (as there obviously is with the easy ones).

Decided not to spend too much time checking the equations - and trust that the people who post these things know 
d) what they're doing,    and 
e) whether they're easy, medium, hard etc , lol


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## lesm (14 December 2008)

2020hindsight said:


> to be honest I prefer the logic puzzles, viz:-
> 
> PS I've chosen this one because of the humourous cartoon




The logic puzzles are fun, but it has been a long time since I played around with the ones similar to your post.


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## 2020hindsight (14 December 2008)

...
now to do something productive lol 
cheers les


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## 2020hindsight (15 December 2008)

Largesse from another thread said:
			
		

> you can teach a 5year old to do a rubik's cube in under 3 mins.....
> hardly a measure of logic, reasoning or intellect. just simple pattern recognition.



Largesse, 
lol - see how long it takes you to teach a 5 year old to do this rubik's cube 
PS that sentence of yours needs a disambigulator


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## 2020hindsight (27 December 2008)

That website (websudoku.com) has various toughnesses, "easy" to "evil".
Here's an evil one. 
At this point in time I think I'm stumped - but then again, sometimes you go away from it and return, and the way forward appears from nowhere. - and you kick yourself that you didn't see it before. 

I've taken to using 
"1r, 1r"  = one of these two, in this row, is a 1. 
"1c, 1c" = one of these two, in this column, is a 1.
"1cr" = at the interection of such a row and such a column
2,3 = this cell is either a 2 or a 3
2r,3r = look along this row for another 2r and a 3r.  However, not necessarily limited to a 2 or a 3.

For this Evil puzzle, I've decided to permit three options, eg 
1,2,5 = this cell is either a 1,2 or 5
I've even added 9c, 9c, 9c  = (see solumn 5) = where one of these three in this column is a 9 - gotta feeling it's a mistake to do that. 

PS btw, If these are too easy - there's a link to another website where you can download a free trial - (including using the alphabet etc) "Web Sudoku Deluxe".

http://www.websudoku.com/deluxe.php?fe


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## 2020hindsight (27 December 2008)

PS "1r,9c" could be "1r, 9cr" maybe - but the text keeps getting smaller - and there's a limit of 5 characters in the "pencilled in options" of each cell I believe  

("1r9cr" maybe - i.e. omit the comma - still if that's the biggest problem here, there aren't any problems lol - I wish)


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## 2020hindsight (27 December 2008)

some other "different" options ...


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## lesm (27 December 2008)

2020,

The solution below.


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## 2020hindsight (27 December 2008)

lesm, 46 minutes, excellent !  
bettn me thas for sure.


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## 2020hindsight (27 December 2008)

hey les, now for the bad news 
the average (median) time for "evil" is 11m 58s 
the most common time (mode whatever) is about 8m 25s 
(yeah right lol)
based on "four million puzzles during one week - when people did not use the "how am I going" button"" - or where using that button did not lead to a correction whatever.

http://www.websudoku.com/faqs.php#statistics


> How are the statistics calculated?
> 
> The statistics for solution times are based on four million puzzles solved during one week on websudoku.com.
> 
> ...




PS How did I get 8m 25s lol ? - I copied your results , trying to time it to coincide with the peak of that statistical distribution 

Like I said, I copied your numbers - first time was 1m 30s lol.   So the statistical average for the week would include my 1m30s


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## 2020hindsight (27 December 2008)

46m 31s then becomes the 98th percentile (or 2nd if you pefer)
(gotta be nonsense)


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## lesm (27 December 2008)

2020,

Have been trying to work out how they get down to some of these times.

Maybe should try a plug-in solver and type the solution in , but that would be cheating

The average (median) 'easy level' is around 5 minutes, mine is around 7 minutes 16 seconds, without error or using the 'how am I going" button.

At the 'evil level', I can get down to sub 40 minutes without error. Once you get the first 6-8 numbers at the 'evil level', it becomes quite straight forward and you pick up speed finishing it off. Hence, losing time in early part.

Maybe just need to learn to move the mouse cursor and type numbers faster.

Cheers.


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## 2020hindsight (27 December 2008)

lol - embarrassment prevents me from saying how long I took on that evil problem .  cheers


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## Dowdy (27 December 2008)

I not bad at sudoku. I was able to do a medium in 10min from the site you gave without any help


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## 2020hindsight (29 December 2008)

well done dowdy lol
noone is sharing any skills I notice - but no probs 

Here's one of those logic puzzles.  
Firstly the clues- and the initial naughts and crosses implied therein :-


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## 2020hindsight (29 December 2008)

next the addition of another naught (dead easy - process of elimination)  ; 

And after that the second naught (based on the large truncated rectangle thingo)

(or if you prefer, truck at zebra = Monday, 
and truck at zebra = 10.15am
therefore Monday = 10.15am)


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## 2020hindsight (29 December 2008)

then some crosses in the same way ,  (the two blue ones at least)
(the green crosses are dead easy of course)

you can add the blue crosses based on inequalities or whatever
if Monday equals 10.15am
and Monday doesn't equal Osbourne
then Osbourne doesn't equal 10.15am etc

and so on till the whole thing falls out. (maybe re-reading the clues when you get bogged down)


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## lesm (29 December 2008)

2002,

If you have the time.

A simple example, find the value in the cell containing the '?' and show the steps used.

Cheers.


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## 2020hindsight (29 December 2008)

les 
ahh pretty easy that one 
central 2
then 9th row
then 4th column. 
E&OE of course.

When I think about it , you could go straight to 4th column I guess.


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## lesm (29 December 2008)

2020,

Hehe...it was, but I wanted to see how you approached it and show you an alternative approach. Apologies if you are already aware of it.

Not sure of what you are actually aware of, but if you see particular sequences in rows or columns, then you can find the value in a particular cell.

Really helps with the harder puzzles. There are pair and triple combinations that can assist in solving the puzzles faster.

If you have three cells (in a row or column) which contain the same pairs, but one cell has an additional value, then the value of that cell (with the additional value) can be determined.

See below.


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## 2020hindsight (29 December 2008)

lesm said:


> 2020,
> 
> 1. Not sure of what you are actually aware of, ...
> 
> 2. If you have three cells (in a row or column) which contain the same pairs, but one cell has an additional value, then the value of that cell (with the additional value) can be determined.



1. yep - first thing a new teacher has to establish lol. 
And as you say - a simple example in this case - but the principles, when extended, will assist for the evil ones (I'm hoping lol)

2. btw, I recently became aware of that (as you say - when you get to the harder ones - and hence I started usuing 3 numbers for instance), but I have only rarely used it to be honest .  Thanks - will keep it in mind 

Furthermore - if you have two cells with (2,or 8),  - in one row, or one column , or one 3x3 square - then you know one is 2, and the other is 8.  So that only leaves a missing 1 in that column.  - almost exactly as you have, but without the need to use 3 options.

PS (but I hear you , I think - sometimes you need to go to three I'm sure)


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## 2020hindsight (29 December 2008)

Incidentally, for these easy ones, I'd do as many rows and columns as possible - then , when there are only a few missing in a RCS (row column or 3x3 square), then I'd say eg - "well missing in this row are a 4 and a 7,  and by inspection, they must go here and here etc"


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## lesm (29 December 2008)

Correct. The 4 & 7 fall into place and the 8 & 1 in the row below fall into place by elimination.

Another example of the use of pairs, used a hard puzzle this time.

If two cells can only contain 1 of 2 values then any cell in the same row or column cannot contain those values.

Refer to the example below (for illustrative purposes only):

Rows 1 & 2, Column 4 contain the values 1 & 9 and can only contain those values. Therefore, any other cell containing a 1 or 9 in that column can have that value safely removed, as in rows 5 & 6 in the example, where the 1's can be removed.

Where this occurs within a 3x3 square no other cell within that square can contain these values either.


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## 2020hindsight (29 December 2008)

thanks les
I'll (hopefully) try out your method on an 'evil' in the next week or so 
 cheers 2020


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## lesm (29 December 2008)

2020...No probs.

I vary the approach depending on how each puzzle is presented, especially since there are different levels of complexity within a given level.

Either working on rows and colums or the 3x3 squares or a combination of both.

Another approach is to start by working through the numbers from 1 through 9 and find all the obvious ones in each of the 3x3 squares.

Part of it is reducing a higher level down to the next lower level.

Let us know how you go and if using pairs assists.

You run the risk of peeving the Quantas passengers that come behind you, when you start completing them inflight and they don't have a puzzle to finish off. 

Cheers.


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## 2020hindsight (30 December 2008)

lesm said:


> You run the risk of peeving the Quantas passengers that come behind you, when you start completing them inflight and they don't have a puzzle to finish off.



specially if you f*** it up.


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## RUSHIAT (30 December 2008)

Hi, hindsight. I usually do the "evil"  sudokus, but I have a different technique to yours. I've attached an image of my first pass of  Hard #101 270 313. As you can see I've "pencilled" in all possibilities in each square. The 7,2,1 in the center are self explanatory,(shot 1) which means cell 4,2 must be a 5. Cells 5,1 5,2 5,3 together have to have 3,6 and 7 in them (order unclear at present), which means cells 5,7 5,8 and 5,9 have to have 1,5 and 8 in them (order unclear at present)(shot 2). Cells 6,8 and 6,9 have to share 4 and 7, therefore cell 6,3 must be 2; cell 4,7 must be 6; cell 4,1 must be 4. Cells 1,6  2,6 and 3,6 share numbers 6,7 and 9 (order unclear at present), so cells 7,7 and 7,9 have to share 5 and 8 (order is obvious) and 2,5 has to be 3. Cells 1,5 and 8,5 share 1 and 4, so 9,5 has to be 7(shot 3). In the bottom left square(set of 9 cells) 7,3  8,3 and 9,3 are the only cells with 5 and 8 in them, therefore cells 1,3 2,3 and 3,3 will not have 5 or 8 in them; that leaves 8,3 with 5; 7,1 and 7,3 share 2 and 3, so 9,2 =9 and 9,3 =8, and discard 2 & 3 from the rest of row 7, which leaves 8,4 =3 and 9,9 =3 and 9,8 =2.(shot 4) From there it's a walk in the park.


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## 2020hindsight (30 December 2008)

RUSHIAT said:


> Hi, hindsight. I usually do the "evil"  sudokus, but I have a different technique to yours. I've attached an image of my first pass of  Hard #101 270 313. As you can see I've "pencilled" in all possibilities in each square. The 7,2,1 in the center are self explanatory,(shot 1) which means cell 4,2 must be a 5. Cells 5,1 5,2 5,3 together have to have 3,6 and 7 in them (order unclear at present), which means .... etc
> 
> From there it's a walk in the park.




thanks rush 
one thing's for sure - you're more efficient with space lol.  
I take about 6 or 7 posts (#18-24) to give details of my "method" - you take one lol. (same puzzle)
cheers , thanks

PS you're very similar to les I believe. 

Incidentally, you'd literally need a pencil surely (and an eraser) - so many "workings" during the drafts.


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## 2020hindsight (30 December 2008)

I'll throw this one in ..
Sorry haven' t triple checked it .. 
I was trying to do it this afternoon before being distracted by family stuff ...
But at that point in time I believed I was stumped ... 

btw, the "  *  "  means "7 or 9"
for discussion 
cheers 2020


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## lesm (30 December 2008)

Here is the puzzle a number of moves further in. See if you can work out how to get there and we can discuss in a follow up post.


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## lesm (31 December 2008)

The same puzzle after another set of moves.


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## lesm (31 December 2008)

The completed puzzle


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## 2020hindsight (31 December 2008)

I'm still scratching my head at this stage lol.
(as I said before 1346* = 134679, because you can only fit 5 numbers in a cell)
(probably some stupid oversight - then again the explanation could be the holy grail  )


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## 2020hindsight (31 December 2008)

hey les,
Maybe I'll just have to accept the fact that I can do medium and hard, but at this point in my life at least, I can do no evil


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## lesm (31 December 2008)

2020,

Just takes time to get used to the evil puzzles, but use the lower levels to gain experience. and try different approaches out.

I have included a link to a web page on Sudoku hints that you may find useful below:

http://angusj.com/sudoku/hints.php

It explains pairs, triples and quads (hidden and naked).

I have included a copy of the same 'evil' puzzle and taken a different approach to solving it.

You will note that in R8 C5&6 that these cells contain '4,7' & '4,7,8' respectively. You can remove the '8' from R8C6, as you have actually have a pair in those two cells. The '4' can be removed from R8C2, etc.

Try it and see how you go from there.

Have fun.


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## lesm (31 December 2008)

Hi Rushiat,

Good to see you join in. Used to use the full notation approach when starting to learn Sudoku, but have reduce using it to assist in identify candidates in rows, columns or squares.

A number of the Sudoku games allow you to pencil in the full set of numbers from 1 to 9. Whereas, WebSudoku permits only '5' numbers to be pencilled in. May as well leave the cell blank and when more numbers are discoverd consider pencilling the missing numbers into the cell, as required.

When trying to beat the clock on Sudoku you need to minimise notating  potential candidate numbers unless it is necessary, as this really slows you down.

Cheers.


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## 2020hindsight (31 December 2008)

thanks les
I'll study it when I have some spare time
have a great NY 
Suspect I'll be working harder next year, trying to master some new software at work etc - still might fit in the odd sudoku  

btw, If you're ever on a Qantas plane, and you hear someone in economy class scream out "Yahooooo !! Beudyy!! - GOT YOU OUT, YOU BAS-TARD!! "  ...  - it'll be me. 

cheers 2020


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## lesm (31 December 2008)

2020hindsight said:


> thanks les
> btw, If you're ever on a Qantas plane, and you hear someone in economy class scream out "Yahooooo !! Beudyy!! - GOT YOU OUT, YOU BAS-TARD!! "  ...  - it'll be me.
> cheers 2020




Be careful they don't think it's a terrorist, especially in today's environment

You can start the puzzles in varies ways and still arrive at the same final same result. Some approaches are faster than others.

Sometimes you appear to get stymied on a puzzle and then all of a sudden see the light, wonder how you overlooked it, and make progress again.

All the best for the New Year.

Any questions let me know.

Cheers.


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## 2020hindsight (1 January 2009)

lesm said:


> Be careful they don't think it's a terrorist, especially in today's environment



yep - like the time I saw my mate Jack a few seats away - and I called out Hi. 

btw, if you can see a way forward from that predicament I ended up in (back there a few posts, #61), I'd appreciate it.  cheers. no hurry - some time this year maybe lol.


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## lesm (1 January 2009)

2020hindsight said:


> yep - like the time I saw my mate Jack a few seats away - and I called out Hi.
> 
> btw, if you can see a way forward from that predicament I ended up in (back there a few posts, #61), I'd appreciate it.  cheers. no hurry - some time this year maybe lol.




You will note that in R8 C5&6 that these cells contain '4,7' & '4,7,8' respectively. You can remove the '8' from R8C6, as you have actually have a pair in those two cells.

Cheers.


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## 2020hindsight (21 July 2009)

great site ... simple sudoku :-
http://simple-sudoku.en.softonic.com/

Even when the computer gives you a massive clue, it still isn't obvious, lol.
I mean , a decision can be made about the highlighted cell ( obtained by hitting the yellow question mark = hint.)
Lower left it says " Exclude based on multiple colours".  
Note that every square with a 9 in it is coloured green ( or some other colour) 

so 
a) what number can be excluded? , and
b) why?

Now try doing that without the computer to give you all the candidates, and also look for the colour patterns ( multiple colour patterns in this case) 

(i.e. Hence I think I'm convinced that you can only get some out with the help of a computer. )


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## 2020hindsight (21 July 2009)

I mean, even if you've coloured in your INFlight magazine with all the possible "9" squares, you get this :- 

and you have to work out that there's a decision that can be made about the highlighted square.   (you're brighter than I am if you can see that one )

PS Simple Sudoku give 5 grades :-
Easy Standard Hard Expert and Extreme.   This is from an extreme one.  
PS In Flight magazines fortunately don;t go to the extreme. 
(PS sorry if my earlier post - first draft was confusing)


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## 2020hindsight (21 July 2009)

(finally) This is what you're looking at without a computer

There is only one 9 that has been solved 
yet you can work out that there is no 9 in the yellow square !(sheesh)
Guess if I was on a deser tisland for a coupla years I might work it out. :-


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## 2020hindsight (21 July 2009)

OK if you read the clues attached to the simple sudoku program, you find this explanation ...

Blue and Lime cells, one set is "true" the other not.
Pink and Brown , ditto.
(in each case there are only 2 options in a row or column or "square")

Then - because Pink and Blue are on same column, then
"Pink and Blue cannot both be true, 
therefore their opposites ( i.e. Brown and Lime) cannot both be untrue,
ie. one of Brown or Lime is true. 
Therefore the highlighted cell (common to both brown and lime) is untrue.
Conclusion : You can delete 9 as a candidate for that cell. 


Sheesh - and that's a breakthrough at this level lol.  
Like I say, you need a computer, or a real lot of time.


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## biggles (21 July 2009)

Hi , I have become a Sudoku fan but have got to a level where alas I am stuck. Talked to a friend who completes all the evils and he told me you get to a level where you have to take a guess to continue on. He places a dot in the corner of where his first guess is and works forward and back from that point.
Too hard for me to cope with this, nearly gone cross eyed with staring and rubbing with my mechanical pencil


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## 2020hindsight (21 July 2009)

biggles said:


> Hi , I have become a Sudoku fan but have got to a level where alas I am stuck. Talked to a friend who completes all the evils and he told me you get to a level where you have to take a guess to continue on. He places a dot in the corner of where his first guess is and works forward and back from that point.
> Too hard for me to cope with this, nearly gone cross eyed with staring and rubbing with my mechanical pencil



sh#t!! - why didn't I think of that ?? !! lol
but you're right - life's arguably too short for this nonsense. 
(then again - it's as good as watching "Dancing with the stars" 

PS Actually - and in truth, I'm a bit of a purist.  If I type "4" on a cell that has the option of say 4, 9, or 2 - and if it is accepted , (meaning it was a lucky guess), - and then I realise my logic was flawed, I will "undo ( = Cntl+Z) and persist in trying  to "crack the code". 

But as I conclude back there - in some cases I reckon your friend is right - either use a computer, or use "trial and error".  - cheers m8


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## bigdog (22 July 2009)

Sudoku fans

Please try the free training sudoku lessons based on the 1 to 9 technique.

I purchased the full package and found very good.

http://www.sudokuprofessor.com/thanks

Lesson #1:
http://www.sudokuprofessor.com/1stVideoLesson.php

Lesson #2:
http://www.sudokuprofessor.com/2ndVideoLesson494.php

Lesson #3:
http://www.sudokuprofessor.com/3rdVideoLesson648.php

Lesson #4:
http://www.sudokuprofessor.com/4thVideoLesson836.php

Lesson #5:
http://www.sudokuprofessor.com/5thVideoLesson428.php

231


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## 2020hindsight (22 July 2009)

Question, bigdog ( and remember I've already admitted I couldn't progress that puzzle on post #71 without a computer, where I can use the multiple colour stuff - and where, with the click of F10 key, you can ask for  eg "all cells with 9 as a candidate" - as on the previous post, #70)

but getting back to that question ....
could you (or the good professor - looks like a humourous dude  ) advance that puzzle from it's current position?
(Let me tell you - it's a stinker! - or maybe you can show me that it isn't) 
cheers 2020

PS I'll understand if you don't take this challenge - it will take a while. (imo anyway).


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## bigdog (22 July 2009)

2020hindsight said:


> Question, bigdog ( and remember I've already admitted I couldn't progress that puzzle on post #71 without a computer, where I can use the multiple colour stuff - and where, with the click of F10 key, you can ask for  eg "all cells with 9 as a candidate" - as on the previous post, #70)
> 
> but getting back to that question ....
> could you (or the good professor - looks like a humourous dude  ) advance that puzzle from it's current position?
> ...




2020, I was able to complete #70 using the 1 to 9 technique.
-- It took quite a bit of time!
-- you only record two possible numbers only
-- you do not record three or more numbers
-- start from finding possible 1's, then 2's on to 9's

The yellow square for #71 = 6

Can you access the videos?


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## 2020hindsight (22 July 2009)

bigdog said:


> 2020, I was able to complete #70 using the 1 to 9 technique.
> -- It took quite a bit of time!
> -- you only record two possible numbers only
> -- you do not record three or more numbers
> ...



bigdog 
 well done if you did it unaided. (exceedingly well done imo)
cheers - (videos later)


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## bigdog (22 July 2009)

2020,

*71#*

Please review the video's for the technique and then try the following to find the only places two for numbers.

*First complete 1's*
- Top five rows 1's have been completed
- Row six column four and five is the only place for 1's
- Row seven column four and five is the only place for 1's 
- Bottom two rows 1's have been completed

*The 2's have been completed for all rows*

*Complete 3's where possible*
- 3's for rows four to ninth have all been completed
- There are no 3's in the first three rows
- column 2 rows 1 and 3 
- column 8 rows 2 and 3 
- column 4 (3's could be in rows 1, 2 or 3)

*Complete 5's where possible*
- row 1 column 1 and 2 
- row 6 column 5 and 6 
- column 6 rows 7 and 8
- column 7 rows 8 and 9

*Complete 6's where possible*
- row 3 column 5 and 6 
- row 6 column 5 and 6 

*Complete 7's where possible*
- row 3 column 4 and 6 
- row 4 column 4 and 6 

*Complete 8's where possible*
- row 1 column 9
- row 3 column 8

- row 7 columns 4 , 5 or 6 (all three possible)
- therefore row 9 columns 8 and 9 must be 8 

*Complete the missing numbers where there are seven numbers in a box*
- row 5 column 2 = 4 9 (are the only possibles)
- row 6 column 1 = 4 9 (are the only possibles)

- row 4 column 7 = 4 9 (are the only possibles)
- row 5 column 8 = 4 9 (are the only possibles)

- row 7 column 2 = 5 9 (are the only possibles)
- row 9 column 1 = 5 9 (are the only possibles)


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