Please help me with XY-chain link

[pdxsudo]

Hello all. I'm new to Sudoku and I'm trying to learn the XY-chain link. I am having a difficult time to understand this particular technique. For example, I'm trying to solve this puzzle using Sudocue. The "Hint" suggested that XY-chain link for the digit 4. But the start and end point belong to the same cell! That means the digit 4 must belong to that cell.

Does it have any restrictions on what 2 digit cells I can use to link? In the example below, I'm linking the cell R6C5 twice. I'm quite confused...

This screen print shows the Puzzle I'm working on.

Thanks in advance for all the helpful tips!

[Ruud]

You have discovered a bug in the XY-Chain function. Although the eliminations are correct, it should not have been reported as an XY-Chain.

This is what happens:

either r7c5=3 or r7c5=4

if r7c5=3, then r6c5=5 => r3c5=6 => r3c3=9 => r6c3=1 => r6c9=5

the program should have stopped there, because there are two digits 5 in row 6, but instead the program continues:

r6c9=5 => r6c5=3 => r7c5=4

Thanks for this nice example. I finally figured out what is going wrong. This is obviously not a good example to learn about XY-chains. There should be 2 yellow candidates.

cheers,
Ruud

[pdxsudo]

Thanks Rudd. I thought I was going crazy! Your explanation makes perfect sense.

[Ruud]

I changed the code to avoid retracing a chain.

The program now finds this XY-Chain instead:



Looks like a valid XY-Chain to me

Ruud

[pdxsudo]

I'd been printing out the puzzle to play on paper for a month now. I have to admit, the "retracing a chain" technique is actually very useful in placing a number! Afterall, the ultimate goal of eliminating possible numbers is to place a number!