## Why mention Swordfish?

Advanced methods and approaches for solving Sudoku puzzles

### Why mention Swordfish?

Just really started playing a couple of days ago, and have been reading through some of the strategy sites (mostly Simes' and Angus'), and became quite curious as to why there is a special mention of Swordfish at all, as it appears to be simply a special case of what would be a "3x3 X-Wing" (that is, if three rows/columns each have their only candidates for a given digit in the same three columns/rows, then all additional candidates for this digit in these columns/rows can be eliminated).

We can easily weaken our conditions in the above by just requiring there to be three columns/rows which capture all candidates for this digit in these rows/columns (ie, you don't necessarily get a complete 3x3 grid of candidates, but a 3x3 grid of candidates and possibly empty spaces).

Given this extension, "Swordfish" is obviously just a special case.

Just curious if the specific "Swordfish" position shows up frequently enough to warrant special mention.

Thanks.
Physcher

Posts: 3
Joined: 22 June 2005

Consider the ultimate generalisation, as someone already has. Look for N columns/rows with only two candidates for a given digit. If these fall on exactly N common rows/columns, and each of those rows/columns has at least two candidate cells, then all N rows/columns can be cleared of that digit - except in the defining cells. (I have paraphrased simes' definition here.)

X-wings, swordfishes, jellyfishes, and squirmbags are all specific cases of this general case. So should we give the general case a name and be done with it? Suppose we call the general case a gronk. To differentiate the specific cases, when N=2 we would say there is a gronk-2. Or, "I saw the most unbelievable gronk-7 in a 16x16 grid the other day."

Personally, I like the names x-wing and swordfish; they are one of the few technique names that most people agree on. When I encounter an x-wing, I "see" an x-wing: two diagonal possibilities. When I encounter an swordfish, I can conjure an image of three swordfishes spearing the unnecessary candidates and swimming away with them.

As a side-note, I would be interested to know what the origins (or etymology, if you will) of the terms x-wing, swordfish, etc. are. Who came up with them and when?
scrose

Posts: 322
Joined: 31 May 2005

scrose wrote:Consider the ultimate generalisation, as someone already has. Look for N columns/rows with only two candidates for a given digit. If these fall on exactly N common rows/columns, and each of those rows/columns has at least two candidate cells, then all N rows/columns can be cleared of that digit - except in the defining cells. (I have paraphrased simes' definition here.)

Of course, there's a simple "ultimate generalisation". My question was more-or-less why this specific "swordfish" pattern. Does it come up that frequently in actual games? (I haven't had need to use it, yet; nor have I really looked for it... but as I said in my previous post, I just started playing literally two days ago --- if it wasn't for a letter to the editor in Tues' Globe and Mail, I may not have heard of it yet --- and so I haven't done too many V. Hard puzzles.)
Physcher

Posts: 3
Joined: 22 June 2005

No, it doesn't come up very often. Or rather, it's quite infrequent. Pappocom's software (which supplies the Globe and Mail with its puzzles) does not generate puzzles that contain swordfish.

The programmers' forum has a thread with a short list of puzzles containing swordfish, if you want to examine them further.

Update: Actually the swordfish pattern may be present with a certain amount of regularity, but the swordfish technique is not required to solve all puzzles in which the pattern occurs.
scrose

Posts: 322
Joined: 31 May 2005

scrose wrote:
X-wings, swordfishes, jellyfishes, and squirmbags are all specific cases of this general case.

Just curious, what are jellyfishes and squirmbags?

Thanks,

Emily

Posts: 11
Joined: 25 June 2005

This is a generalisation of the X-wing.

Look for N columns (2 for X-wing, 3 for the Swordfish, 4 for a Jellyfish, 5 for a Squirmbag) with only two candidate cells for a given digit (4 in this example). If these fall on exactly N common rows, and each of those rows has at least 2 candidate cells, then all N rows can be cleared of that digit (except it the defining cells!).

Taken fromhttp://www.setbb.com/phpbb/viewtopic.php?t=9&highlight=jellyfish&mforum=sudoku
Last edited by simes on Sun Dec 11, 2011 9:49 am, edited 1 time in total.
simes

Posts: 324
Joined: 11 March 2005
Location: UK

Ahhhh...

So they are all terms that refer to the same logical step... the different names just refer to the total number of columns (or rows) that you are dealing with.

Much obliged.....

Em