Sudoku Help

[sterlingice]

I'm not a huge sukodu fan but I'll play from time to time. Particularly, when I'm killing a few minutes somewhere, I like to play on my phone. However, I have one that has me stumped. Now, I'm not looking for a total solution- I can just cheat and get that. But I can't figure out what the next move is and, more importantly, *why* (or how you figured that out). So, a little help from the more veteran sudoku players?



3 9 1 X X X 7 2 X
2 8 X 9 X X X X 3
X 6 X 3 X X 8 9 1

4 X 8 6 3 X 1 7 9
1 X 9 X 7 X 3 4 6
6 7 3 1 4 9 5 8 2

X 3 2 X X 6 X 1 X
X 1 X X X 3 X X 8
X 4 6 X X X 2 3 7

SI

[Rizon]

http://upload.wikimedia.org/wikipedia/commons/thumb/0/02/100_Yen_lighter.JPG/793px-100_Yen_lighter.JPG

[sterlingice]

I'm not sure why you want me to burn my phone. Work might get a little mad if they can't reach me (not that I'd complain a whole lot) ;)

SI

[MCK]

One move is:

1st Row - 5th Spot = 6

Reason:

A 6 can not be in the 1st Row - 9th Spot

Not sure if that will help you.

[Butter]

I'm the same way with Sudoku, SI. I can get to a certain point, but beyond that I don't understand how you move on to solve the whole thing without just guessing.

[MCK]

Another move:

Row 9 Column 4 = 5

Reason:

No other numbers can go there.
4th column already has 1,3,6,9
9th row already has 2,4,7

That leaves 5 & 8.

After putting the 6 in row one from my previous message, row one needs an 8 either in column 4 or 6. That mean there has to be an 8 in column five, either row seven or nine.

You following me on this?

From this point you should be able to figure the rest out. Their are moves to make after you get write the 5 down. Let me know if you need the next step.

[Huckleberry]

Another move:

Row 9 Column 4 = 5

Reason:

No other numbers can go there.
4th column already has 1,3,6,9
9th row already has 2,4,7

That leaves 5 & 8.

After putting the 6 in row one from my previous message, row one needs an 8 either in column 4 or 6. That mean there has to be an 8 in column five, either row seven, eight, or nine.

You following me on this?

Correct except the column five 8 is row seven or nine.

This also puts a 1 in row nine, column six and a 2 in row eight, column five.

Right? This leads to a 5 in row three, column five and we're off.

[sterlingice]

I'm the same way with Sudoku, SI. I can get to a certain point, but beyond that I don't understand how you move on to solve the whole thing without just guessing.

I hate guessing- because there has to be a reason. After MCK pointed that move out, I get it. But I didn't see it before.

I had that space listed as 5/6/8 and the one below it 1/5/6. But in the second row, last 2 X's, I had them as 4/6 and 5/6 (and the top corner as 4/5) so one of those had to be 6. Which meant the 1/5/6 couldn't be 6 so there were no other 6's in that box.

It just had gotten to the point where I couldn't see any valid moves since I'd been staring at it too long.

SI

[sterlingice]

Another move:

Row 9 Column 4 = 5

Reason:

No other numbers can go there.
4th column already has 1,3,6,9
9th row already has 2,4,7

That leaves 5 & 8.

After putting the 6 in row one from my previous message, row one needs an 8 either in column 4 or 6. That mean there has to be an 8 in column five, either row seven or nine.

You following me on this?

From this point you should be able to figure the rest out. Their are moves to make after you get write the 5 down. Let me know if you need the next step.

I don't see how that one can be a 5 for sure. It could be either a 5 or 8 because row 1, spots 4 and 6 can each be 4/5/8 so I'm not sure how we've eliminated 8 from Row 9, spot 4.

Couldn't you do 8 in 7.5, 5 in 9.4, and 1 in 9.6 and then 1.4 is 8 and 1.6 is 4?

SI

[Huckleberry]

Yeah, I think that's incorrect.

[Huckleberry]

Can you post the original puzzle before you started it or is that the original puzzle?

[MCK]

I don't see how that one can be a 5 for sure. It could be either a 5 or 8 because row 1, spots 4 and 6 can each be 4/5/8 so I'm not sure how we've eliminated 8 from Row 9, spot 4.

Couldn't you do 8 in 7.5, 5 in 9.4, and 1 in 9.6 and then 1.4 is 8 and 1.6 is 4?

SI

My thinking is you know you need a 5/8 in R9C4 and to complete R1 you still need a 4/5/8. If you place an 8 in R9C4, that means you would have to place an 8 in R1C6 since there is an 8 in R2C2 and R3C7. If you do that then it's impossible to place an 8 in the middle box since 3,7,4 are already listed and you still need an 8 in column 5.

[sterlingice]

Can you post the original puzzle before you started it or is that the original puzzle?

Not easily, without resetting everything on my phone. But I know that those numbers so far are correct (it will make them blue if correct, red if wrong)

SI

[DaddyTorgo]

i got my mom a kickass sudoku book last year - there's all kinds of advanced theorems for solving them - she says it's fascinating but it can be a bit...much

[Maple Leafs]

My thinking is you know you need a 5/8 in R9C4 and to complete R1 you still need a 4/5/8. If you place an 8 in R9C4, that means you would have to place an 8 in R1C6 since there is an 8 in R2C2 and R3C7. If you do that then it's impossible to place an 8 in the middle box since 3,7,4 are already listed and you still need an 8 in column 5.
This is right. It must be a "5" since you'll need an "8" in the middle column.

[Rizon]

I'm not sure why you want me to burn my phone. Work might get a little mad if they can't reach me (not that I'd complain a whole lot) ;)

SI

DO IT

[Huckleberry]

This is right. It must be a "5" since you'll need an "8" in the middle column.

Yes, yes it is. My mistake.

[Huckleberry]

So now we have this


3 9 1 X 6 X 7 2 X
2 8 X 9 X X X X 3
X 6 X 3 X X 8 9 1

4 X 8 6 3 X 1 7 9
1 X 9 X 7 X 3 4 6
6 7 3 1 4 9 5 8 2

X 3 2 X X 6 X 1 X
X 1 X X X 3 X X 8
X 4 6 5 X X 2 3 7


Using similar logic to last time, if we put an 8 in R9C6, then both R1C4 and R5C4 are the only possible locations for the 8 in their 3x3 boxes. So R9C6 must be a 1.


3 9 1 X 6 X 7 2 X
2 8 X 9 X X X X 3
X 6 X 3 X X 8 9 1

4 X 8 6 3 X 1 7 9
1 X 9 X 7 X 3 4 6
6 7 3 1 4 9 5 8 2

X 3 2 X X 6 X 1 X
X 1 X X X 3 X X 8
X 4 6 5 X 1 2 3 7


Now, R8C5 must be a 2. This is because the R7C5 is 8/9, R8C5 is 2/9, and R9C5 is 8/9. Obviously an 8 must be in either R7C5 or R9C5, meaning that a 9 must be in the other one.

Also, R2C5 has become the only remaining possible location for 1 in Column 5.


3 9 1 X 6 X 7 2 X
2 8 X 9 1 X X X 3
X 6 X 3 X X 8 9 1

4 X 8 6 3 X 1 7 9
1 X 9 X 7 X 3 4 6
6 7 3 1 4 9 5 8 2

X 3 2 X X 6 X 1 X
X 1 X X 2 3 X X 8
X 4 6 5 X 1 2 3 7


This now leaves 5 as the only remaining possibility in R3C5, R5C4 as the only possible location for 2 in Column 4, and R3C6 as the only possible location for 2 in Row 3.


3 9 1 X 6 X 7 2 X
2 8 X 9 1 X X X 3
X 6 X 3 5 2 8 9 1

4 X 8 6 3 X 1 7 9
1 X 9 2 7 X 3 4 6
6 7 3 1 4 9 5 8 2

X 3 2 X X 6 X 1 X
X 1 X X 2 3 X X 8
X 4 6 5 X 1 2 3 7


Now we have 7 as the only number for R3C1, 5 as the only number for R5C2, 5 as the only number for R4C6, R4C2 as the only spot for 2 in both that 3x3 box as well as R4 and C2, and 4 in R3C3 follows similarly, 5 in R1C9, 7 in R2C6, and 8 in R5C6 as well.


3 9 1 X 6 X 7 2 5
2 8 X 9 1 7 X X 3
7 6 4 3 5 2 8 9 1

4 2 8 6 3 5 1 7 9
1 5 9 2 7 8 3 4 6
6 7 3 1 4 9 5 8 2

X 3 2 X X 6 X 1 X
X 1 X X 2 3 X X 8
X 4 6 5 X 1 2 3 7




It follows pretty easily from there. Actually, even the last two steps above are pretty straightforward. The first two steps are the ones that are tricky.