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Nov 29, 2005 - Hard

 
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How long did it take you?
< 30 min
40%
 40%  [ 2 ]
< 1 hr
40%
 40%  [ 2 ]
< 2 hr
20%
 20%  [ 1 ]
< 6 hr
0%
 0%  [ 0 ]
> 12 hr
0%
 0%  [ 0 ]
Total Votes : 5

Author Message
smith55js



Joined: 29 Nov 2005
Posts: 9
Location: Logan, UT

PostPosted: Tue Nov 29, 2005 2:55 am    Post subject: Nov 29, 2005 - Hard Reply with quote

For anyone who needs help, here' s the solution and how I got it....

1.5i5
2.8e4
3.8b8
4.9d4
5.5f8 - R1C4, R1C6,R4C4,R4C6 force {7,9} to be in r7c5,r9c5, eliminating 1,2,5 from possiblities
6.4i1 - a 4 must appear in r2c4 or r2c6 to satisfy box 2, eliminating it from r2c8 and r2c9
7.5e3 - group 2 - only possiblity
8.5b2 - column 2 must have a 5, only possiblity
9.2c2
10.1i9 - row 9 - only possiblity
11.9i2 -
12.1a2
13.6a1
14.9b3
15.6d8
16.9g8
17.3a8
18.3h7
19.8h2
20.4f2
21.3d2
22.8f3
23.3g3
24.6i3
25.4d5
26.2a5
27.4h4
28.5a4
29.6c4
30.7b5
31.4b6
32.3c6
33.2g4
34.1h5
35.6g6
36.7i6
37.1g1
38.2h3
39.2e1
40.1d3
41.3e5
42.6f5
43.1e6
44.2f6
45.2i8
46.7h8
47.1f7
48.2d7
49.6b7
50.7c7
51.9e7
52.7e9
53.4a7
54.5c9
55.9a9

638 729 154
152 364 789
794 158 326

516 987 243
279 436 815
843 512 697

467 291 538
381 645 972
925 873 461

Jake


Last edited by smith55js on Wed Nov 30, 2005 10:24 pm; edited 5 times in total
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puzzle
Guest





PostPosted: Tue Nov 29, 2005 4:50 am    Post subject: solution Reply with quote

Please help. I'm a newbie. I don't understand your 55 step "how I got it".
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geoff h



Joined: 07 Aug 2005
Posts: 58
Location: Sydney

PostPosted: Tue Nov 29, 2005 6:43 am    Post subject: Reply with quote

Hi there Puzzle,

Smith55 has used the notation of naming each column in the puzzle as a letter from a to i. For example, Column 1 is represented by the letter "a" and Column 5 is represented by the letter "e". After that, each Row is simply represented by the number of the Row.

Therefore, in his first step, he refers to "5i5". This means he placed the number 5 in Column9, Row 5. In his second step, he palced the number 8 in Column 5, Row 4. And so on.

Personally, I think it's a lot easier to simply refer to them by their designated places in the Rows and Columns. For example, I would refer to the cell "i5" as cell number r5c9.

Cheers.
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Guest






PostPosted: Tue Nov 29, 2005 7:25 pm    Post subject: Reply with quote

Can someone explain the logic used to come up with this

5.5f8

Thanks
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David Bryant



Joined: 29 Jul 2005
Posts: 559
Location: Denver, Colorado

PostPosted: Tue Nov 29, 2005 8:28 pm    Post subject: Why is it 5f8? Reply with quote

One might almost argue that this move should have been preceded by 5e3, which is actually recorded as the 7th move in the solution posted above. No matter -- here's the explanation.

Notice the values {7, 9} lying in columns 4 & 6. Because of the way these are positioned, the values {7, 9} must fit in r7c5 and r9c5 in the bottom center 3x3 box ... there's no other way to fit either of them in that box. So the "5" in the bottom center 3x3 box has to fit in r8c6 -- there's nowhere else left for it.

This formation (the {7, 9} in r7c5 and r9c5) is known as a "hidden pair." dcb
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Guest






PostPosted: Tue Nov 29, 2005 8:38 pm    Post subject: Reply with quote

I'm obviously new to SuDoku. Thanks for the clear explanation. I will have to remember this "hidden pair" going forward.
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guest
Guest





PostPosted: Wed Nov 30, 2005 12:03 pm    Post subject: Reply with quote

I did not come up with a unique solution to this puzzle, and was glad to find a posted explanation.

I do not understand step 10, "1i9"

I have 1, 7, 9 as candidates at this point, and I have 1 as possible in h7. I did finally place 1 in i9, but I do not see why it is so at this point.
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guest
Guest





PostPosted: Wed Nov 30, 2005 12:15 pm    Post subject: Reply with quote

guest wrote:
I did not come up with a unique solution to this puzzle, and was glad to find a posted explanation.

I do not understand step 10, "1i9"

I have 1, 7, 9 as candidates at this point, and I have 1 as possible in h7. I did finally place 1 in i9, but I do not see why it is so at this point.


Solved the puzzle, but, I still do not see why you can assign 1 to i9 at step 10. Could somebody explain this?
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mjb
Guest





PostPosted: Wed Nov 30, 2005 4:25 pm    Post subject: Reply with quote

I had problems with this and needed the hints above...

You can discount the 7 from I9 because in the bottom right 3x3 box a 7 must go in the top or bottom row and for the bottom middle 3x3 box a 7 must go in the top or bottom row.

I think I also deduced the same for the 9 leaving only a 1 to go in I9, but cant quite see it yet
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mjb
Guest





PostPosted: Wed Nov 30, 2005 4:28 pm    Post subject: Reply with quote

where says bottom right 3x3 - read bottom left 3x3

Embarassed Embarassed Embarassed
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David Bryant



Joined: 29 Jul 2005
Posts: 559
Location: Denver, Colorado

PostPosted: Wed Nov 30, 2005 5:20 pm    Post subject: Why a "1" at r9c9? Reply with quote

Guest wrote:
Solved the puzzle, but, I still do not see why you can assign 1 to i9 at step 10. Could somebody explain this?

-- There has to be a "1" in row 9 somewhere.

-- It can't go in r9c1 or r9c3 because of the "1" at r8c3.

-- It can't go in r9c5, either, because of the "hidden pair" {7, 9} that occupies r7c5 & r9c5. Note the {7, 9} pair in column 4, and also in column 6 -- it's clear that the two digits "7" and "9" _must_ occupy the two cells r7c5 & r9c5, because there's no other way to fit these two digits into the bottom center 3x3 box.

-- Therefore "1" must appear at r9c9 because it can't fit anywhere else in row 9. dcb
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Guest






PostPosted: Wed Nov 30, 2005 6:30 pm    Post subject: Reply with quote

Yeah, that would be it. Thanks.
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chuckfurok
Guest





PostPosted: Wed Nov 30, 2005 9:31 pm    Post subject: Reply with quote

I got step 10 but do not understand step 11.....
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smith55js



Joined: 29 Nov 2005
Posts: 9
Location: Logan, UT

PostPosted: Wed Nov 30, 2005 10:28 pm    Post subject: Reply with quote

Quote:
I got step 10 but do not understand step 11.....


step 6 removed a 4 from possibilities for r2c9
step 9 removed a 2 from possibilities for r2c9
step 10 removed a 1 from possiblities for r2c9

leaving 9 as the only possible solution to r2c9.
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chuckfurok
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PostPosted: Wed Nov 30, 2005 10:44 pm    Post subject: Reply with quote

smith55js wrote:
Quote:
I got step 10 but do not understand step 11.....


step 6 removed a 4 from possibilities for r2c9
step 9 removed a 2 from possibilities for r2c9
step 10 removed a 1 from possiblities for r2c9

leaving 9 as the only possible solution to r2c9.


now i understand and fill a bit silly. Thanks!
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puzzle
Guest





PostPosted: Thu Dec 01, 2005 4:06 am    Post subject: solutions Reply with quote

I'm learning so much from you all. Many thanks for taking the time to explain to those of us just starting!
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alanr555



Joined: 01 Aug 2005
Posts: 194
Location: Bideford Devon EX39

PostPosted: Thu Dec 01, 2005 10:36 pm    Post subject: Reply with quote

Code:

Are we to have a poll on the time taken for evey puzzle?
If so, perhaps this should be a feature of the site rather than
being left to the initiative of a forum user.

However, I fail to understand what one may glean from the
results of the poll. It all comes down to the "sampling frame".
With the methodology adopted there is a lot of self-selection.
In particular those who do not finish will not enter as there is
no "radio button" for [unfinished] - and even if there were, one
would have to consider what should happen if someone who has
it unfinished subsequently completes the puzzle.

+++
For the record, I found this one fairly difficult - and had to move
from Mandatory Pairs to significant analysis of Candidate Profiles
in order to reach the solution.

I find Candidate Profiles difficult in that they require meticulous
attention to detail and one error can negate everything. As such, I
needed two goes at this one. The first lasted about an hour, got
bogged down in profiles and resulted in an error due to negligence
on my part. Start again! The second time (with no reference to my
work sheet from the first time), the puzzle came out in under an
hour. There are some puzzles where the Mandatory Pairs approach
turns out to counter-productive and this may well have been one of
them - although I did use it before before and after the work done
on creating and scanning the Candidate Profiles.

Alan Rayner BS23 2QT

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dotdot



Joined: 07 Dec 2005
Posts: 29
Location: oberseen

PostPosted: Wed Dec 07, 2005 10:13 am    Post subject: Why 5e3 before 5f8? Reply with quote

David Bryant wrote:
One might almost argue that this move should have been preceded by 5e3, which is actually recorded as the 7th move in the solution posted above. No matter -- here's the explanation.


I am curious as to why you say this, because you seem to imply that initially the 5e3 move is more obvious than the 5f8 move.

You are in good company: Draw's hint is 5e3.
The Sudoku-san assistant also chooses 5e3, based on a hidden triplet in that column.

Is there a way of getting 5e3 first without invoking this triplet?
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David Bryant



Joined: 29 Jul 2005
Posts: 559
Location: Denver, Colorado

PostPosted: Wed Dec 07, 2005 3:44 pm    Post subject: Why 5e3 before 5f8? Reply with quote

dotdot wrote:
Is there a way of getting 5e3 first without invoking this triplet?

I think so. It's a consequence of the hidden pair {7, 9} in r7c5 & r9c5. In fact, I can't even notice the triplet {1, 2, 3} that remains in column 5 until I've entered the "5" at r3c5.

The only reason I said "5. 5e3" might be a more natural move than "5. 5f8" is psychological -- it's a matter of where one focuses one's attention. Logically, both moves are pretty simple once one notices the hidden pair {7, 9} at the bottom of column 5.

I saw the {7, 9} pair in two places -- in r1c4 & r4c4, and also in r1c6 & r4c6. Since r8c5 was already occupied, I recognized that the pair {7, 9} was going to fill the cells r7c5 & r9c5. But since my attention was already focused on the top center 3x3 box and I was thinking about column 5, I naturally noticed the "5" at r3c5 before I thought about the one at r8c6, and that's all. dcb
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dotdot



Joined: 07 Dec 2005
Posts: 29
Location: oberseen

PostPosted: Wed Dec 07, 2005 4:26 pm    Post subject: Re: Why 5e3 before 5f8? Reply with quote

David Bryant wrote:
In fact, I can't even notice the triplet {1, 2, 3} that remains in column 5 until I've entered the "5" at r3c5.


So both moves - 5e3 and 5f8 - followed the intermediate step of recognising the shared pair {7,9}.
I had half expected that you used a similarly powerful step leading preferentially to 5e3.

Thanks
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