dailysudoku.com

[Nenthorn]

I really can't do this one. I'm here:

900 745 003
730 962 001
402 138 070
009 027 000
003 000 700
070 693 000
090 376 102
300 200 057
127 050 006

The hint gives 9 at row5, column 9, but I can't see how the 9 next to it in column 8 can be ruled out. Will some kind genius help me, please?

[Steve R]

Here is one suggestion.

In the first column, 2 and 6 can be placed only in box 4. That is, in box 4, 2 and 6 must occupy column1.

With this in mind, test row 5 for cells which admit 2s and 6s.

Steve

[David Bryant]

Hi, Nenthorn!

The short answer is that there's a "hidden pair" {2, 6} in r5c1 & r5c8.

-- Neither "2" nor "6" can lie in r5c4-6 because of the "2" and "6" already entered in the middle center 3x3 box.

-- In the middle left 3x3 box both "2" and "6" are constrained to lie in column 1. Note that the "6" in column 1 must lie in the middle left 3x3 box, so there can't be a "6" at r5c2.

-- Neither "2" nor "6" can appear at r5c9, so the only two cells in row 5 that can hold the pair {2, 6} are r5c1 & r5c8. dcb

[Nenthorn]

Thanks, guys. The bit I was missing was, as David says so eloquently, "In the middle left 3x3 box both 2 and 6 are constrained to lie in column 1." I was so busy looking for pairs I'd neglected the more basic principles. Maybe I'm going for "candidate profiling" too early. Do you think that's a possibilty?

Best wishes . . .

[Angel]

When I did it I noticed the hidden quad {1,4,5,8} in row 5, and completely missed the hidden pair! Obviously this gives the same result of 9 at r5c9.

[jabejochke]

Nenthorn wrote:
Maybe I'm going for "candidate profiling" too early. Do you think that's a possibilty?


Nenthorn

Thanks for triggering some thoughts in me. I continue to learn much from the discussions on this site.

I classify myself as a ‘plodder’ – have been able to solve all “dailys” since I joined, however, never less than 15 minutes and from 45 to 80 minutes for “hard & very hard”.

I usually go for "candidate profiling" when there are no other alternatives that I see (there are no implications from existing solutions or pairs) and there are no remaining groups (rows, columns or blocks) that have 4 or more solutions to apply the pairing strategy against. For me this means about 25% of the time I have to go for profiling.

At that point I tend to commit to the Profile starting with the group having the most filled-in status. Usually one of three scenarios results:


1. Break-thru occurs while working the first group
2. Get thru first group (row, column or block), start to recycle thru the other two groups and break-thru occurs along the way
3. Or no break-thru after recycling, and I look first for hidden subsets and only then for xy-wing, swordfish, etc. (which means searching for notes on what they are). For me finding hidden subsets I missed or once-in-a-while finding an xy-wing will provide the break-thru.

In all cases where I’ve gone for profiling and solved, I felt as if “it would have been solved faster if………..).

Right now, after committing to profiling but before processing, I would like to scan about the board to select an optimum strategy to take in completing the profile. Do I:

1. Profile by row, column or block (which one, why)?
2. Search for specific configurations on the board and profile at specific rcb’s first?
3. Look at frequencies on the board, to inform the profiling strategy?

I would guess there is no one answer, but any thoughts would be most welcome.

Regards,

Jack

[juggler]

I am a relative newcomer to sudoku. I've only been doing them about two months. I am able to do most of them, even the very hard ones. This March 30 one has me stumped though. I'm behind the rest of you in that I don't understand how you see a naked quad in row 5, or even why you say 2 and six are only in column 1 and 8. Isn't there a 6 in column 2? There seem to be five 8's in row five. So how can there be a naked quad?
Tell me the steps I've missed that would make it so. Thanks.

[keith]

Let's start with the full set of possibilities for the situation posted at the beginning of this thread:

[Guest]

Thanks Keith. I can't believe I missed that the 6 in box 4 had to be in c1. What a stupid mistake! I must have been doing puzzles too long or too late at night. Linda