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Aug 26 Hard

 
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keith



Joined: 19 Sep 2005
Posts: 3355
Location: near Detroit, Michigan, USA

PostPosted: Wed Aug 26, 2009 6:49 pm    Post subject: Aug 26 Hard Reply with quote

Today's puzzle is pretty good:
Code:

+-------+-------+-------+
| . . . | . . 6 | . 1 . |
| . . 4 | . 2 . | 6 . . |
| 8 5 . | . . . | 2 . . |
+-------+-------+-------+
| . . . | . 5 9 | . . . |
| 3 . 9 | 1 . 8 | 5 . 2 |
| . . . | 6 7 . | . . . |
+-------+-------+-------+
| . . 8 | . . . | . 2 7 |
| . . 5 | . 1 . | 8 . . |
| . 4 . | 2 . . | . . . |
+-------+-------+-------+

Play this puzzle online at the Daily Sudoku site

Keith
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arkietech



Joined: 31 Jul 2008
Posts: 1834
Location: Northwest Arkansas USA

PostPosted: Wed Aug 26, 2009 9:59 pm    Post subject: Reply with quote

Glad to see someone else enjoys a good "Hard" puzzle.
This one was better than a lot of the "Very Hards".
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keith



Joined: 19 Sep 2005
Posts: 3355
Location: near Detroit, Michigan, USA

PostPosted: Wed Aug 26, 2009 10:43 pm    Post subject: Reply with quote

Here is my sticking point:
Code:
+-------------------+-------------------+-------------------+
| 279   2379  237   | 4578  39    6     | 3479  1     34589 |
| 179   1379  4     | 578   2     57    | 6     35789 3589  |
| 8     5     6     | 47    39    1     | 2     3479  349   |
+-------------------+-------------------+-------------------+
| 4     267   27    | 3     5     9     | 17    678   168   |
| 3     67    9     | 1     4     8     | 5     67    2     |
| 5     8     1     | 6     7     2     | 349   349   349   |
+-------------------+-------------------+-------------------+
| 19    139   8     | 459   6     345   | 1349  2     7     |
| 2679  23679 5     | 479   1     347   | 8     3469  3469  |
| 1679  4     37    | 2     8     357   | 139   3569  13569 |
+-------------------+-------------------+-------------------+

Quote:
Hint: Hidden pair
Big Hint: Hidden pair in R1


Keith
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Stanley P



Joined: 27 Feb 2008
Posts: 20
Location: Tennessee, USA

PostPosted: Thu Aug 27, 2009 2:17 am    Post subject: Aug 26 Hard Reply with quote

It was the dastardly hidden pair. I never expected the hidden pair. It was, in fact, more difficult for me than most of the VH puzzles.
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keith



Joined: 19 Sep 2005
Posts: 3355
Location: near Detroit, Michigan, USA

PostPosted: Thu Aug 27, 2009 2:31 am    Post subject: Reply with quote

Stan,

My arrogance says I should do these without pencil marks. After an hour, I succumbed.

Even after the hidden pair, I was stuck for a time on the pairs in C1.

I agree with you and dan, this is much more enjoyable than most of the one-step "Very Hard" puzzles.

Best wishes,

Keith
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Marty R.



Joined: 12 Feb 2006
Posts: 5770
Location: Rochester, NY, USA

PostPosted: Thu Aug 27, 2009 3:47 am    Post subject: Reply with quote

Quote:
Even after the hidden pair,

I think I'll never understand hidden pair, as opposed to (in this case) naked quad, which is so much easier to spot, at least for me. Question
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kragzy



Joined: 01 May 2007
Posts: 112
Location: Australia

PostPosted: Fri Aug 28, 2009 1:30 am    Post subject: Reply with quote

I'm with you Marty - the larger naked sets are easy to spot. They then lead to the discovery of those sneaky little hidden pairs.

More of the same in today's H puzzle (28th).

Cheers
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strmckr



Joined: 18 Aug 2009
Posts: 64

PostPosted: Fri Aug 28, 2009 9:13 am    Post subject: Reply with quote

i find hidden sets easier to find when not using pencil mark ups.

(look at the givens)

all the *'s see 58 marked with @. (leaves two cells as 58 )
Code:

 *-----------*
 |***|.*6|*1.|
 |..4|.2.|6..|
 |8@5@6|..1|2..|
 |---+---+---|
 |4..|35@9|...|
 |3.9|148|5@.2|
 |581|672|...|
 |---+---+---|
 |..8|.6.|.27|
 |..5|.1.|8@..|
 |.4.|28@.|...|
 *-----------*


where naked sets are easier to spot with pencil marks. (or asking what is missing)

Code:
 *-----------*
 |...|8.6|415|
 |@.4|725|6.8|
 |856|4.1|27.|
 |---+---+---|
 |462|359|781|
 |379|148|562|
 |581|672|...|
 |---+---+---|
 |@.8|564|.27|
 |..5|91.|8..|
 |x4*|28*|.5.|
 *-----------*


@ are missing 19 (naked pair)
* are missing 37(naked pair)
seen by x:2458
thus x=6.
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cgordon



Joined: 04 May 2007
Posts: 769
Location: ontario, canada

PostPosted: Fri Aug 28, 2009 3:49 pm    Post subject: Reply with quote

Good one!!

Agreed – harder than most very hards.

Also agreed – much easier to spot naked quads than hidden pairs. It’s the same in real life.
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strmckr



Joined: 18 Aug 2009
Posts: 64

PostPosted: Fri Aug 28, 2009 7:21 pm    Post subject: Reply with quote

how do you find its easier to spot naked sets over hidden sets?

try not using a solver to aid in the solution or with out Pm's:

hidden sets are much easier to find over naked sets when manual non-pm solving is used.
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keith



Joined: 19 Sep 2005
Posts: 3355
Location: near Detroit, Michigan, USA

PostPosted: Fri Aug 28, 2009 8:43 pm    Post subject: Reply with quote

strmckr wrote:
how do you find its easier to spot naked sets over hidden sets?

try not using a solver to aid in the solution or with out Pm's:

hidden sets are much easier to find over naked sets when manual non-pm solving is used.
I agree completely!

Be vary sparing with your use of pencil marks and, with practice, you will become quite good at finding pairs (and bigger sets), hidden or not.

Keith
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storm_norm



Joined: 18 Oct 2007
Posts: 1741

PostPosted: Sat Aug 29, 2009 12:21 am    Post subject: Reply with quote

to use or not use the sweep function Exclamation Exclamation Exclamation

that is the question.

with finding hidden pairs, I think using the sweep function might clutter the grid to the point you might not find them as easily.

finding xy-wings on the other hand, its a breeze with the sweep function. that goes for naked sets also.
its probably a little bit more intuitive to make the eliminations after recognizing a naked set. when finding hidden sets, its almost thinking about it from the opposite side. what two candidates can only go in these cells, thus eliminating all others from the cells. that is a little different than the naked sets which tell us the candidates that can't go in the "house" the set is already naked in.
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keith



Joined: 19 Sep 2005
Posts: 3355
Location: near Detroit, Michigan, USA

PostPosted: Sat Aug 29, 2009 1:10 am    Post subject: Reply with quote

Quote:
to use or not use the sweep function Exclamation Exclamation Exclamation

Norm,
My only use for computer solvers is to check for accuracy when I make postings. Otherwise, I solve all my puzzles by hand with pencil & paper.

One of the reasons is that I find the auto pencil marks just clutter up the grid, and clutter up my thinking.

Keith
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Marty R.



Joined: 12 Feb 2006
Posts: 5770
Location: Rochester, NY, USA

PostPosted: Sat Aug 29, 2009 1:15 am    Post subject: Reply with quote

Quote:
how do you find its easier to spot naked sets over hidden sets?


I don't know how, but using full pencil marks I've never seen a hidden pair but easily spot most subsets.
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storm_norm



Joined: 18 Oct 2007
Posts: 1741

PostPosted: Sat Aug 29, 2009 1:17 am    Post subject: Reply with quote

Quote:
and clutter up my thinking


I normally would clear the pencil marks to make sure I got all the singles. Rolling Eyes

its easier for me to cross hatch without the pencil marks in place.
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strmckr



Joined: 18 Aug 2009
Posts: 64

PostPosted: Sat Aug 29, 2009 3:03 am    Post subject: Reply with quote

Pencil marks are the reasons that hidden sets are "hidden".

if your used to not using them you should notice that they are in fact easier to find.

hidden sets use what is given to identify (dont need pms)
(as in the example above)

over naked sets where u write down what is missing in each of n cells and comparing for identical states to form the set

howsoever in a programmers point of view naked is easier to program compared to inverting the problem to solve hidden, as bitmaps already contain the pms of all cells from a grid.

for me i rarely use pencil marks anymore unless i'm working on some er 7+ grids.

but if i have to instead of marking down all the pencil marks just use notes on the key finds. (ie where the sets are).
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cgordon



Joined: 04 May 2007
Posts: 769
Location: ontario, canada

PostPosted: Sat Aug 29, 2009 3:51 pm    Post subject: Reply with quote

Quote:
to use or not use


I always use a soft pencil with a rubber on the end (I'm English)
2B or not 2B
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