## swordfish ??????

Advanced methods and approaches for solving Sudoku puzzles

### swordfish ??????

hi people, can someone please help me with this prob? doing the simple suduko program puzzles and have got to swordfish 004.ss ! been doing ok with the other swordfish puzzles so far (001 to 003 plus others ) but find that 004 has a swordfish in the 7's candidates but cannot understand why/how it is supposed to work! the 'hint' says use a swordfish to erase a total of eight candidates (7's) in diff rows/cols but i just cannot fathom why! as it is the puzzle has a total of 25 candidate sevens!, -- before applying the swordfish!---and i also just cannot see whether the swordfish is in the cols or in the rows--- pleeeeease could someone look at this puzzle and explain to me (in idiots language plse) the reasoning behind the erasure of the eight 7's and just how this particular swordfish works
to enable the erasure of the specific eight candidate sevens?? many thanks in advance and cheers goldie
goldie5218

Posts: 37
Joined: 27 May 2005

Last edited by zebedeezbd on Mon Sep 19, 2005 4:02 pm, edited 1 time in total.
zebedeezbd

Posts: 60
Joined: 14 September 2005

Code: Select all
` *-----------* |3.1|.9.|...| |..6|8..|...| |.8.|.5.|3.4| |---+---+---| |1..|..5|8..| |8..|.2.|..6| |..4|9..|..7| |---+---+---| |5.3|.8.|.1.| |...|..2|7..| |...|.4.|5.9| *-----------* {3}    {247}  {1}    {2467} {9}    {467}  {26}   {8}    {5}    {2479} {5}    {6}    {8}    {37}   {347}  {29}   {279}  {1}    {279}  {8}    {27}   {1267} {5}    {167}  {3}    {2679} {4}    {1}    {2679} {27}   {467}  {67}   {5}    {8}    {249}  {3}    {8}    {379}  {5}    {1347} {2}    {1347} {149}  {49}   {6}    {26}   {236}  {4}    {9}    {136}  {8}    {12}   {5}    {7}    {5}    {467}  {3}    {67}   {8}    {9}    {46}   {1}    {2}    {46}   {146}  {9}    {5}    {136}  {2}    {7}    {346}  {8}    {267}  {1267} {8}    {1367} {4}    {1367} {5}    {36}   {9}    `

There are 3 rows that each have 3 cells
containing the candidate 7. These 9 cells
are confined to 3 columns. This pattern
of 9 cells is the swordfish.

In the final solution, each of those 3 rows must have 1
cell containing a 7.

Each of those 3 cells must be in a different
column of the pattern (because the final solution
can't have multiple 7's in a column).

Therefore we can eliminate 7 as a candidate from each
cell in the 3 columns, except for the cells in
the pattern. (We can't eliminate 7 from any of the
cells in the pattern, because we aren't sure yet
which of those cells will have a 7 finally.)
Last edited by Nick67 on Mon Sep 19, 2005 5:56 pm, edited 1 time in total.
Nick67

Posts: 113
Joined: 24 August 2007

Actually, this description might be more helpful.
Let's take advantage of Simple Sudoku's
visual aids.

On your screen, I believe the pattern cells
are highlighted in blue, right?

Well, in the final solution, each of the blue
rows must contain a 7. So there are 3 blue
cells that will ultimately contain a 7.

Since the pattern has only 3 columns,
each of these 3 special cells must be
in a different column.

So we know that in the final solution,
exactly 1 of the blue cells in each column is
going to contain a 7. So we can eliminate
7 as a candidate from all the non-blue
cells in the column.
Nick67

Posts: 113
Joined: 24 August 2007

### swordfish ??????

hi Nick67, and thanks for your reply and patience in trying to get me to see the prob. i guess the most frustrating part of trying to help newbies is their inability to see what seems soooooo obvious to the experienced solvers! ok let me try and explain, my current concept of the swordfish is that you find them by looking for cols/rows that contain ONLY three of the particular candidate? now in 004 you say there are three rows that hold three sevens in each? hey Nick67, you gotta know how i looked for those three rows before i wrote in for help? well now you can see there are only two rows with three candidates and one row with only two candidates! yes and i can hear all the old solvers saying "so what? you must know that that is a swordfish anyway" ok so thats what was throwing me ! a swordfish can in fact have up to three candidates per row/col??? not so ?? and if that statement is in fact correct then i am away!!! and thanks again for your help Nick67 - once i have identified the darn thing i have no probs in what to erase and how to proceed from there- my prob was in the identification and the confusing solver notes which simply added to my mis understanding - so in summary i can take it as carved in stone that a swordfish can be found by looking for "up to three " candidates per col/row (????) cheers goldie (hopefully a swordfish finder now)
goldie5218

Posts: 37
Joined: 27 May 2005

Hi Goldie,

I believe you've got it exactly right.

Look for 3 columns, such that each column has
up to 3 cells containing the candidate,
and all those cells are confined
to 3 common rows. (You can eliminate the candidate
from each cell outside the pattern in those 3 rows.)

You can also swap "row" with "column" in the above:

Look for 3 rows, such that each row has
up to 3 cells containing the candidate,
and all those cells are confined
to 3 common columns. (You can eliminate the candidate
from each cell outside the pattern in those 3 columns.)

(Thanks to MadOverlord for his thoughts on the swordfish definition in this post.)
Nick67

Posts: 113
Joined: 24 August 2007

### when is it not a swordfish??

hi guys, a little help please? i know the general rules re x-wings ie must be only two candidates per row/col and both must fall on the same row/col then erase all other candidates but for the four "corner cells" - then for A swordfish a somewhat similar situation but this time find three rows/cols each holding up to three candidates and again these three to share common rows/cols, then erase all those candidates on the row/col other than the identifying cells - now my prob is this! how and what tips me off to show me that what i am looking at is NOT a swordfish? and can there be more than one per puzzle? my specific example is from the simple suduko puzzle reference swordfish 013.ss i find a swordfish in the '3s' in cols C2R3,C2R5 and C7R3,C7R7 and C8R5,C8R7 I also find
a swordfish in the '7s' in cols C2R3, C2R5 and in C4R3,C4R7 and in
C6R5, C6R7 - there is also an x-wing in the "7s" C3R2,C3R6 and C5R2, C5R6 ! so when can i ignore and when do i act on what i find re sworfishes ? or do i treat them all the same ie if found action as per normal? but in this case only the swordfish in the "3s" allows the puzzle to be solved? so how do i know which of the swordfishes is the correct one ? any comments much appreciated - cheers - goldie
goldie5218

Posts: 37
Joined: 27 May 2005

### Re: when is it not a swordfish??

goldie5218 wrote:how and what tips me off to show me that what i am looking at is NOT a swordfish?

If it doesn't comply with all criteria it's not a swordfish.
http://angusj.com/sudoku/hints.php#swordfish

goldie5218 wrote:and can there be more than one per puzzle?

Definitely. However, since one swordfish in a puzzle is rare, it's much rarer to find more than one.

Here's a puzzle requiring 2 swordfish ...
Code: Select all
` *-----------* |7..|42.|.6.| |.43|5..|2..| |.1.|.3.|59.| |---+---+---| |...|..4|321| |...|.9.|...| |164|3..|...| |---+---+---| |.71|.6.|.3.| |..9|..3|61.| |.2.|.41|..9| *-----------*`

goldie5218 wrote:so how do i know which of the swordfishes is the correct one ?

If it's a proper swordfish it can't be 'incorrect'.
angusj

Posts: 306
Joined: 12 June 2005

### swordfish????

hi angusj, thanks for your reply and help with my prob. i notice that you make no comment re the puzzle simple suduko 013.ss having two swordfishes but only one is the correct one? or did i get this wrong and am seeing what i think is a swordfish incorrectly in this particular puzzle? in my humble opinion both are "correct" swordfishes but only one leads to a solution of the puzzle? was that intended do you think? your comment re "if its a proper swordfish it cant be incorrect" confuses me, as i believe both meet the swordfish criteria but only one leads to a solution of the puzzle ? - cheers - goldie - and thanks for your patience with us newbies!
goldie5218

Posts: 37
Joined: 27 May 2005

Angusj,

I just joined the forums, and I've found two references to your web site. But when I click the links, I get "Page not found" errors. Is your site up?

Thanks!

RobR
RobR

Posts: 6
Joined: 24 September 2005

### swordfish????

hi angusj, thanks for your reply , i solved the puzzle you posted by finding a swordfish in the 7's in columns --- in cols 4,6,7 and in the 8,s in rows 2,6,8. very elegant ! nice problem ! not too severe for the experts but very good fun for us newbies - thanks for all your help! you guys really make the forum worthwhile for us beginners! cheers - goldie and thanks again for your help !
goldie5218

Posts: 37
Joined: 27 May 2005

### Re: Is your website up?

RobR wrote:I just joined the forums, and I've found two references to your web site. But when I click the links, I get "Page not found" errors. Is your site up?

No, it's been down for about 24hrs. I'm trying to contact my web hoster to see what's up. (However, since his site's down too I'm sure he'll be working hard to get it sorted.)
angusj

Posts: 306
Joined: 12 June 2005