The New Sudoku Players' Forum

[googler]

I just got a homework assignment that's due really soon and I want to see if anyone could help me out?? I seriously have been working on this for about 4 hours and I can't come to the right answer. It's killing me. Well here it is:
Directions: Every row, column, and box of 3x3 cells must contain the numbers 1-9 exactly once.


and #2-- same directions. This one's a little harder.

[Cec]

".. I seriously have been working on this for about 4 hours and I can't come to the right answer...."

Hi googler,

To make it easier for us to help you, have you solved any of the vacant cells and, if so, post again your two grids showing how far you progressed. That way we won't be going over the "work" that you have already completed yourself.

Cec

[MCC]

As this a homework assignment you'll have to work on it yourself, but I'll give you a few clues to help you on your way.

Each group of 3x3's is a box.

Boxes are numbered:
1 2 3
4 5 6
7 8 9

#1
Look at box 7 and the number 3.
Look at box 3 and the number 5.
Look at box 2 and the number 6.
Look at column 3 and the number 9.


MCC

[Sped]

#2:

Code: Select all

 *-----------*
 |..2|6..|...|
 |73.|...|9..|
 |...|...|421|
 |---+---+---|
 |...|..4|.1.|
 |4.9|5.8|2.3|
 |.1.|7..|...|
 |---+---+---|
 |326|...|...|
 |..1|...|.46|
 |...|..9|8..|
 *-----------*


There is only one place in box 1 for a 1. Hint: It can't be in column 2 or 3 because those columns already have a 1.

Similarly, there is only one place for a 1 in box 5. There is only one place for a 1 in box 9.

Put a 2 in box 9 and a 4 in box 6. Look at putting a 6 in boxes 3 and 8.

Then you can complete row 5. It lacks a 6 and a 7. The 6 can only go one place.

Now you can complete the 6s by putting one in box 1 and box 5 and box 6.

Continue this process of finding "hidden" singles.

When I solved this, I ran out of the easy to spot hidden singles, and had to resort to "naked" singles, which, despite the name, are actually harder to see because you usually have to use pencilmarks to find them.

Here is the point at which I looked at each unsolved square and penciled in its possible candidates:

Code: Select all

 *-----------*
 |1.2|6.5|387|
 |73.|..1|965|
 |6..|.73|421|
 |---+---+---|
 |...|..4|618|
 |469|518|273|
 |.1.|7.6|594|
 |---+---+---|
 |326|..7|159|
 |..1|352|746|
 |...|169|832|
 *-----------*
 
 *--------------------------------------------------*
 | 1    49   2    | 6    49   5    | 3    8    7    |
 | 7    3    48   | 248  248  1    | 9    6    5    |
 | 6    589  58   | 89   7    3    | 4    2    1    |
 |----------------+----------------+----------------|
 | 25   57   357  | 29   239  4    | 6    1    8    |
 | 4    6    9    | 5    1    8    | 2    7    3    |
 | 28   1    38   | 7    23   6    | 5    9    4    |
 |----------------+----------------+----------------|
 | 3    2    6    | 48   48   7    | 1    5    9    |
 | 89   89   1    | 3    5    2    | 7    4    6    |
 | 5    457  457  | 1    6    9    | 8    3    2    |
 *--------------------------------------------------*


When you use pencilmarks, the naked single 5 in r9c1 (row 9, column 1) jumps right out at you.

So make r9c1 a 5, and remove the 5s from r4c1, r9c2 and r9c3. Now there is a naked single 2 in r4c1.

Continue spotting naked singles and updating the candidate grid. The puzzle solves very easily.

[QBasicMac]

When I solved this, I ran out of the easy to spot hidden singles, and had to resort to "naked" singles, which, despite the name, are actually harder to see because you usually have to use pencilmarks to find them.

Hunhh?

Pick cell r9c1. Why this one? Because there are lots of digits in the row, column and box. Good chance to isolate one candidate.

Test each candidate in your mind
1 In box and column
2 In box and row
3 In box and row
4 In column
5 ??? Don't see one
6 In column and row and box
7 In column
8 In row
9 In row

So r1c9 is clearly 5

You should never require pencilmarks for puzzles like this one that consist only of singles.

IMHO

Mac

[Pat]

#2:

Code: Select all

 *-----------*
 |..2|6..|...|
 |73.|...|9..|
 |...|...|421|
 |---+---+---|
 |...|..4|.1.|
 |4.9|5.8|2.3|
 |.1.|7..|...|
 |---+---+---|
 |326|...|...|
 |..1|...|.46|
 |...|..9|8..|
 *-----------*

--- When I solved this, I ran out of the easy-to-spot "hidden singles" --- Here is the point at which I looked at each unsolved square and penciled in its possible candidates:

Code: Select all

 *-----------*
 |1.2|6.5|387|
 |73.|..1|965|
 |6..|.73|421|
 |---+---+---|
 |...|..4|618|
 |469|518|273|
 |.1.|7.6|594|
 |---+---+---|
 |326|..7|159|
 |..1|352|746|
 |...|169|832|
 *-----------*

at this point, in c1, you can place the 9,8,2,5 (in that order)
- all as "hidden singles"

[QBasicMac]

at this point, in c1, you can place the 9,8,2,5 (in that order)
- all as "hidden singles"

True. In fact, starting with the original puzzle and restricting myself to only filling in hidden singles, I was able to solve the whole puzzle. No need even for the naked single technique.