## hidden singles

Post the puzzle or solving technique that's causing you trouble and someone will help

### hidden singles

I read somewhere that Sudoku was an exercise in locating naked singles and hidden singles, except in advanced puzzles. I have trouble recognizing hidden singles and wonder if I could persuade a knowledgeable person to share some wisdom in this regard. :?:
feste

Posts: 4
Joined: 03 March 2006

### Re: hidden singles

feste wrote:I have trouble recognizing hidden singles

Well, look at this puzzle for an example:
Code: Select all
`-1-  78-  --96-4  912  -857--  --6  1--4-7  -3-  926---  6-5  ---826  -7-  5-3--5  1--  --834-  258  6-79--  -64  -5-`

Look at the cell in row 4, column 2 (r4c2). Here is what could go there:

1 - No: already at r1c2
2 - No: already in Box 4 (4-7, ---, 826)
3 - No: already at r4c5
4 - No: already in Box 4 (4-7, ---, 826)
5
6 - No: already in Box 4 (4-7, ---, 826)
7 - No: already in Box 4 (4-7, ---, 826)
8 - No: already in Box 4 (4-7, ---, 826)
9 - No: already at r4c7

Only 5 remains. It is "naked" as opposed to "hidden" by convention.

Now look the cell at r3c4. Here is what could go there:
1 - No: already in Box 2 (78-,912,--6)
2 - No: already in Box 2 (78-,912,--6)
3
4
5
6 - No: already in Box 2 (78-,912,--6)
7 - No: already in Box 2 (78-,912,--6)
8 - No: already in Box 2 (78-,912,--6)
9 - No: already in Box 2 (78-,912,--6)

Well, rats! No "naked" single. But we look at the other two cells that are empty in box 2 and see that 5 cannot go there. So it must go in r3c4. It was "hidden" because 3 and 4 could also go there. The 5 goes there because it can't go anywhere else.

Mac
QBasicMac

Posts: 441
Joined: 13 July 2005

Richard

Posts: 8
Joined: 25 February 2006

### Re: hidden singles

QBasicMac wrote:
1 - No: already in Box 2 (78-,912,--6)

My reply could be confusing due to that non-standard notation.

The Box 2 I referred to looks like this:
Code: Select all
`             7 8 -             9 1 2             - - 6`

But I showed it as this: "Box 2 (78-,912,--6)" to save lines.

Similarly, Box 4 (4-7, ---, 826) meant
Code: Select all
`             4 - 7             - - -             8 2 6`

Sorry for the confusion. For an explanation I am proud of, download SSP3.zip at www.SuDoku.funURL.com. Unzip and read the tutorial.

Mac
QBasicMac

Posts: 441
Joined: 13 July 2005

### Re: hidden singles

feste wrote:I have trouble recognizing hidden singles

Naked singles are harder!

Try this:
http://homepage.oninet.pt/842mtm/index/solvingm/patterns/singlesl/hiddensi.html
foxglove

Posts: 12
Joined: 04 February 2006

Tks for the help. I have been working the easy Sudoku on Sudoku.com.au. Now those easy ones are really easy, and my speed is slowly improving. On the website that you suggested, the easy Sudoku was extremely difficult for me. Whether I succeed or not, I enjoy the challenge. One of my initial problems is when I begin a Sudoku what is the best place to start rather working from 1-9, and then looking for the naked singles, and proceeding from there. I try not to count as that is time consuming.
Tks again
Feste
feste

Posts: 4
Joined: 03 March 2006

### hidden singles

feste wrote:"..One of my initial problems is when I begin a Sudoku what is the best place to start ..."

Hi feste,
I recently put a similar request to what you raised - if you click on HERE you can read the responses I got.

I don't know what stage you are at in solving such as the placement of "candidates" in vacant cells (these are the possible numbers for a cell as distinct from the given numbers called "clues"). If you're writing these candidates directly in a newspaper it can get messy. The following links may help you with solving techniques - just click on them. You will see that you can download a program solver "Simple Sudoku" which is free. This eliminates all the painstaking time of working out the candidates as the solver does this automatically plus much more. Ask again if you need more help.
http://www.angusj.com/sudoku/hints.php
http://www.simes.clara.co.uk/programs/sudokutechniques.htm

Cec
Cec

Posts: 1039
Joined: 16 June 2005

### hidden singles, etc

Hello. Tks for your support. The puzzle that Cec included I worked in less than 15 minutes. It is similar to the easy ones at Sudoku.com.au.
I have observed that the "easy" category is open to intrepretation. The easy category in Simple Sudoku is usually quite difficult for me, as on WebSudoku.com. I appreciate MC's definitions and plan to study the locked candidates. One of the problems that I have which I cannot seem to solve is when two cells are eligible to receive the candidate, and leaves me the option of filliing in as many cells as possible thereby reducing the number of cells that are eligible to receive the candidate. I could list the possible candidates and do reduction but that is time consuming and is not what I want to do at this time. I got it figured that all I need is more practice and this will clear itself in my mind. Again I thank you for your support and patience.
Feste
feste

Posts: 4
Joined: 03 March 2006

### cell choice dilemma

Hello: I have had a continuing problem trying to locate the correct cell when I have two choices. In the below example, using the information as listed is there a way I can find 7 for square 1 or 7. Finding one will yield the other, I realize. Also 3 in square 5?
code:
2*687**9*
**8*9*746
*9*5**812
****58*37
57*****8*
82*71**6*
*8***7*5*
4*5*8*67*
*6*935428
Feste
feste

Posts: 4
Joined: 03 March 2006

### Re: hidden singles

feste wrote:"..is there a way I can find 7 for square 1 or 7. Finding one will yield the other, I realize. Also 3 in square 5?"

Hi feste,
Firsty, the little "squares" you refer to are called cells and are identified according to what row and column they occupy. The 1 and 7 you mention is presumably the "naked pair" [17] which occurs in row9 column1 (r9c1) and r9c3. The 3 and 5 you mention is presumably another "naked pair" [35] in cells r1c7 and r1c9. Click on Here for more on the recommended terminology and note how "Boxes" are defined. You'll need to know this to understand the following explanations to help you with your puzzle.
Code: Select all
`*--------------------------------------------------------------------* | 2      1345   6      | 8      7      134    | 35     9      35     | | 13     135    8      | 123    9      123    | 7      4      6      | | 37     9      347    | 5      46     346    | 8      1      2      | |----------------------+----------------------+----------------------| | 169    14     149    | 246    5      8      | 129    3      7      | | 5      7      1349   | 2346   246    23469  | 129    8      149    | | 8      2      349    | 7      1      349    | 59     6      459    | |----------------------+----------------------+----------------------| | 139    8      1239   | 1246   246    7      | 139    5      139    | | 4      13     5      | 12     8      12     | 6      7      139    | | 17     6      17     | 9      3      5      | 4      2      8      | *--------------------------------------------------------------------*`

Spotting "hidden singles" is only required to solve this puzzle which will result in the placements for the 1 and 7 as well as the 3 and 5 you refer to. Look at each row or column or Box and try spotting a candidate that occurs only once in that group. For example there is only one candidate 6 in Box 4 (centre left box), which solves cell r4c1 which thus excludes any other 6's in that row or column. In box 7 there is only one candidate 2 which solves cell r7c3 which thus excludes other 2's in that row or column. Following the above placement of 6 in Box4 which eliminates candidate 9 from r4c1 there is only one candidate 9 remaining in column1 which thus solves cell r7c1 - the remaining candidate 9's are then excluded from row7. It's just a case of keep looking until you spot these "hidden" singles to solve it - Richard's above suggested link to help identify singles seemed helpful - good luck

Cec
Cec

Posts: 1039
Joined: 16 June 2005