Sudoku Help

[tabako]

I already found the solution to this one, so it is really not necessary for anybody to go through the whole thing for my benefit.

To get past this current position, I believe I need to use what is called forcing chains. With a lot of trial and error, I found something that worked, but I have no understanding on why that particular position worked.



What strategies do you guys use to find forcing chains on the more advanced puzzles? I feel like I am just guessing blindly until I happen to pick the right boxes.

[metsandfinsfan]

going by (column,row)
if (1,5) is an 8, then (1,7) is an 3
then (2,9) (3,9) are 4 and 7
then (2,8) is 2, (2,7) is 3, and 3,7 is 5
then (2,5) is a 9, (3,5) is a 1, (6,5) is a 2, and (7,5) cannot be placed

therefore (1,5) is a 3 and work from there

I think i did this right, but i did not write it on paper so not 100% but looks right to me

[tabako]

Like I said, I understand how to do this particular one.

What I don't get is how to look at the puzzle and be able to see where potential forcing chains could appear. Right now I am basically picking a square at random and seeing if I can get the forcing chain to work.

[metsandfinsfan]

Im not sure i know the answer to that, but i think it is best to find 2 squares that have the same to choices in a column or row, and then try those for chains

[Spook]

nope, I don't think you did it right.
if (1,7) is an 8, then (2,9),(3,9) are 4 and 7...

[ScottHoward]

im pretty sure theres more than 1 solution from this point out. further i think the OP is looking for techniques like xwing and whatever, rather than just guessing? or does guessing = chain forcing?

[Stephen H]

Well, the first thing I notice is that the boxes on row 6 are all tied together; either they're a)1-9-2 or b)9-2-1; there's no other way they can be filled in. Then I simply expand these either/or scenarios until I find something that causes a contradiction, or I move on.
In this case, (using the same notation as mets did above, column, row), (6,5) is either a)2 or b)9. So (6,3) is either a)9 or b)2. Looking at square (3,3)...
a) forces (3,3) to be 2.
b) has a 9 in (3,6) and a 2 in (6,3)...no options left for (3,3). So option (a) is the only valid choice.

I hate doing these on a computer because it limits my notes for things like this. On paper I'll typically put all the choices for option a) in the upper left corner and option b) in the upper right.

[Stephen H]

I'd say that "chain forcing" *IS* guessing, and he's looking for ways to figure out which squares to guess at sooner.
I hate guessing. So instead, I take excessive notes on what implies what and so forth while I work, and if I ever reach a contradiction, I can remove that option.
I tend to do this in parallel with other solving techniques, so there you go.

[tabako]

[ QUOTE ]
Well, the first thing I notice is that the boxes on row 6 are all tied together; either they're a)1-9-2 or b)9-2-1; there's no other way they can be filled in.

[/ QUOTE ]

This was very helpful. Having the pencil marks in a different part of the box for each of the two possibilites was exactly what I was looking for in terms of being able to see these more easily.

The one I came up with was (column, row):
If you look at (3,6), regardles of whether that cell is a 1 or a 9, (1,5) must be an 8.

I disagree with anybody who says that this thought process is "guessing," as it is just as logical as any of the other ways to eliminate possibilites.