Fish
Swordfish
A swordfish is an X-Wing with a third row. Three rows each hold a digit in only two or three cells, and every one of those cells sits inside the same three columns. Between them the three rows must place the digit three times, and the only columns they can use are those three, so the digit leaves the rest of each column.
- Level
- Advanced
The pattern
What it does.
The X-Wing proves a digit is spent in two columns from two rows. Swordfish is the same argument with three rows and three columns, and it removes the digit from three columns at once.
The rows don't each need three possible cells. A row with two homes and two rows with three is a swordfish, as long as no cell falls outside the three columns. What has to hold is the total: three rows, three columns, and no row reaching beyond them.
It works the other way round too. Three columns whose homes for the digit all sit inside the same three rows clear the digit from the rest of those rows. Both directions are the same shape read from a different side.
Example
A worked example.
Follow one digit through the rows
Pick a digit and, for each row, count how many cells can still take it. You're looking for rows with two or three homes, and you can ignore rows with four or more.
Line up three of them
Compare three of those rows. Gather the columns their homes sit in. If the three rows together use exactly three columns, you have the pattern.
Check nothing spills over
A single home in a fourth column breaks it. The whole argument depends on the three rows having no way to place the digit outside the three columns, so one stray candidate invalidates the swordfish.
Clear the three columns
Remove the digit from every cell in those three columns that isn't part of the pattern. The rows still have to place it, and only those cells can hold it.
Spotting it
How to spot it.
- Work one digit at a time across the whole grid. Nothing about a swordfish is visible from a single cell or a single unit.
- Start from rows that are nearly solved. A row with two homes for a digit is the raw material, just like for an X-Wing.
- If a digit keeps surviving your X-Wing scan, look for a third row. A pattern that almost worked with two rows is often a swordfish with three.
- Try the columns as well. A swordfish read from three columns is invisible from the rows and just as common.
Where it stops working
Like every fish it removes candidates rather than placing digits, so its value depends on what the removals uncover. It also demands a complete candidate list: a fourth possible cell in any of the three rows means the pattern isn't there, and applying the eliminations anyway can delete the digit the puzzle needed.
Keep going
Related topics.
New to sudoku? The rules take two minutes to read, and the first-puzzle walkthrough covers the basics that come before any of these.
Beyond the lessons.
The game's lessons stop at X-Wing, so there's no Swordfish lesson. The eight it does teach will get you through almost any puzzle; this page is here for when you run into the pattern anyway.