The New Sudoku Players' Forum

[jsmarba]

I have a puzzle that has 2 swordfish with the same candidate at the same time. All my previous moves have been verified by "Simple Sudoku"
which shows one as correct and solves the puzzle. The other swordfish is false and breaks the puzzle. This is not the first time I have had a "false" swordfish. Can anyone explain or help?

[Jeff]

Please post the grid at the point where the 2 swordfish were encountered.

[jsmarba]

The grid involved all of that candidate left on the board at that time.

The false SF
R1-C235 R3-C358 R4-C238

The true SF
R2-C17 R5-C137 R9-C13

[Shazbot]

The first one isn't a swordfish. It may have 3 columns involved, but 4 rows. A swordfish involves 3 columns and 3 rows only.

[jsmarba]

Thanks for the clarification, much appreciated

[sweetbix]

The first one isn't a swordfish. It may have 3 columns involved, but 4 rows.

It doesn't look like 4 rows. Was the post changed?

Edit : OK I see. Columns.

[Animator]

Not 4 rows but 4 columns.

R1-C235 R3-C358 R4-C238 : Columns: 2, 3, 5, 8.

[jsmarba]

Yes 3 rows. 4 colunms. thanks everyone.

[Shazbot]

yes, I've done it again. I'm off to look in the yellow pages for Dyslexics Anon.

[wkrfresno]

Double swordfish are very common. I suspect people find one and go on without looking for more.

[CathyW]

Have been using Simple Sudoku to practise some techniques (selecting 'Extreme' difficulty) and found a double swordfish (1s) in this puzzle:

Code: Select all

 
Original grid
 *-----------*
 |9..|...|.3.|
 |.5.|4..|..9|
 |.3.|.5.|8.4|
 |---+---+---|
 |5..|.83|.2.|
 |...|...|...|
 |.8.|79.|..3|
 |---+---+---|
 |7.8|.2.|.1.|
 |3..|..1|.9.|
 |.9.|...|..6|
 *-----------*

1st swordfish found at this point:

 *-----------*
 |9..|...|53.|
 |85.|43.|..9|
 |.3.|.5.|8.4|
 |---+---+---|
 |5.9|.83|.2.|
 |..3|...|9..|
 |.8.|79.|..3|
 |---+---+---|
 |7.8|.2.|.15|
 |3..|..1|.9.|
 |.9.|...|..6|
 *-----------*

{9}      {1246}   {12467} {1268}  {167}   {2678}  {5}      {3}     {12}   
{8}      {5}     {*1267}  {4}     {3}     {267}   {*12}    {67}    {9}     
{126}    {3}      {1267}  {1269}  {5}     {2679}  {8}      {67}    {4}     
{5}      {1467}   {9}     {16}    {8}     {3}     {1467}   {2}     {17}   
{1246}   {12467}  {3}     {1256}  {146}   {2456}   {9}     {458}   {178}   
{*1246}  {8}     {*1246}  {7}     {9}     {2456}  {*146}   {45}    {3}     
{7}      {46}     {8}     {369}   {2}     {469}   {34}     {1}     {5}     
{3}      {246}    {2456}  {568}   {467}   {1}     {247}    {9}     {278}   
{*124}   {9}     {*1245}  {358}   {47}    {4578}  {2347}   {478}   {6}     

And after the eliminations from that you get a second swordfish as follows:

{9}     {+1246}  {2467}  {1268}  {+167}  {2678}   {5}     {3}     {+12}   
{8}     {5}      {1267}  {4}     {3}     {267}    {12}    {67}     {9}     
{26}    {3}      {267}   {1269}  {5}     {2679}   {8}     {67}     {4}     
{5}     {+1467}  {9}     {16}    {8}     {3}      {467}   {2}     {+17}   
{246}   {+12467} {3}     {1256}  {+146}  {2456}   {9}     {458}   {+178}   
{1246}  {8}      {1246}  {7}     {9}     {2456}   {146}   {45}     {3}     
{7}     {46}     {8}     {369}   {2}     {469}    {34}    {1}      {5}     
{3}     {246}    {2456}  {568}   {467}   {1}      {247}   {9}      {278}   
{124}   {9}      {1245}  {358}   {47}    {4578}   {2347}  {478}    {6}     


Grid after applying double swordfish
 *-----------*
 |9..|...|53.|
 |85.|43.|..9|
 |.3.|15.|8.4|
 |---+---+---|
 |5.9|683|.2.|
 |..3|...|9..|
 |.8.|79.|..3|
 |---+---+---|
 |7.8|.2.|.15|
 |3..|..1|.9.|
 |.9.|...|..6|
 *-----------*
   


And the final solution after another swordfish! Made small so as not to be 'spoiler' but it didn't seem to work when in code as well! (What's the syntax when you want to use formatting e.g. bold or font size or colour within code?)

*-----------*
|914|867|532|
|857|432|169|
|632|159|874|
|---+---+---|
|579|683|421|
|463|215|987|
|281|794|653|
|---+---+---|
|748|926|315|
|326|541|798|
|195|378|246|
*-----------*

Cathy

[Shazbot]

Unfortunately I don't think you CAN use other formatting within code - it shows your text exactly as-is and disregards any BBCode you've added within the Code /Code tags.

[tso]

Double swordfish are very common. I suspect people find one and go on without looking for more.

Puzzles *requiring* finding two swordfish are not common -- assuming that you always apply simpler tactics first (x-wings, locked candidates, naked and hidden subsets, singles -- your selection may vary)

At the same time, the meaning of "common" depends on where you get your puzzle. If you use Simple Sudoku to generate "extreme" puzzles, most of them will have a Swordfish -- again, depending on you solving methods. One could apply much more complex tactics to circumvent anything simpler. If on the other hand, you use Pappocom Sudoku, you will never, ever need a Swordfish. "Random" Sudoku's composition depend an the construction methods, which vary, but generally, random puzzles are pretty easy and fish-free. But you can solve even the simplest puzzles by filling in the candidate lists and looking for swordfish right off the bat.

Have been using Simple Sudoku to practise some techniques (selecting 'Extreme' difficulty) and found a double swordfish (1s) in this puzzle:

And after the eliminations from that you get a second swordfish as follows:

Code: Select all

{9}     {+1246}  {2467}  {1268}  {+167}  {2678}   {5}     {3}     {+12}   
{8}     {5}      {1267}  {4}     {3}     {267}    {12}    {67}     {9}     
{26}    {3}      {267}   {1269}  {5}     {2679}   {8}     {67}     {4}     
{5}     {+1467}  {9}     {16}    {8}     {3}      {467}   {2}     {+17}   
{246}   {+12467} {3}     {1256}  {+146}  {2456}   {9}     {458}   {+178}   
{1246}  {8}      {1246}  {7}     {9}     {2456}   {146}   {45}     {3}     
{7}     {46}     {8}     {369}   {2}     {469}    {34}    {1}      {5}     
{3}     {246}    {2456}  {568}   {467}   {1}      {247}   {9}      {278}   
{124}   {9}      {1245}  {358}   {47}    {4578}   {2347}  {478}    {6} 

... except that there only one place for a 1 in row 3 at this point. Though it's valid to make your exclusions from this second swordfish, nothing but singles is required after the first one.

[ronk]

1st swordfish found at this point:
................
And after the eliminations from that you get a second swordfish as follows:

I find it amazing that the 2 swordfish exist simultaneously! The first(*) in rows 2, 6, and 9 ... the second(+) in columns 2, 5, and 9.

Code: Select all

 9     +1246   12467  | 1268  +167    2678   | 5      3     +12 
 8      5     *1267   | 4      3      267    |*12     67     9   
 126    3      1267   | 1269   5      2679   | 8      67     4   
----------------------+----------------------+-------------------
 5     +1467   9      | 16     8      3      | 1467   2     +17 
 1246  +12467  3      | 1256  +146    2456   | 9      458   +178
*1246   8     *1246   | 7      9      2456   |*146    45     3   
----------------------+----------------------+-------------------
 7      46     8      | 369    2      469    | 34     1      5   
 3      246    2456   | 568    467    1      | 247    9      278
*124    9     *1245   | 358    47     4578   | 2347   48     6   

[CathyW]

... except that there only one place for a 1 in row 3 at this point. Though it's valid to make your exclusions from this second swordfish, nothing but singles is required after the first one.

You are right, of course! Often when looking for more complex patterns one overlooks the singles and pairs that should be obvious.