Last Updated: 12/3/2010
Through December 31 2010, Bad Weasel Games is donating $1 to The Water Project for each copy of Snow Day Sudoku
sold! The Water Project is a US-based non-profit committed to
providing clean, safe drinking water to those who suffer needlessly
without it. Nearly one billion people worldwide lack access to
safe drinking water. Dirty water causes illnesses such as diarrhea and
typhoid. Diarrhea alone kills 1.8 million children each year. Unsafe
water causes a cycle of sickness and poverty and communities struggle
to teach their children, grow food, go to school and earn a
living. This Christmas season Bad Weasel Games has pledged to
give something back. Help us reach our goal of financing several
clean water systems this year. Check out The Water Project at thewaterproject.org
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This guide can be found at:
This guide is a living doument and it is being edited this week. It should be completed today or tomorrow.
iPad Instruction Guide
Some of the images in this gude are from Big Bad Sudoku Book
as Snow Day Sudoku
is part of the Sudoku Book family, but the concepts are the same.
is a number
placement game of logic. Sudoku requires no math. A simple
definition of the word Sudoku is "one of each". The object is to
fill in all the blank cells with the correct numbers following these
three simple rules:
- Every row must include one of each of the numbers 1 through 9 in any order.
- Every column must include one of each of the number 1 through 9 in any order.
- Every 3x3 block must include one of each of the numbers 1 through 9.
A sudoku game begins with several cells already filled in. We
call these starting numbers "givens". Traditional thinking is
that the more givens you start with the easier the puzzle will
be. However this isn't always the case. In Snow Day Sudoku
the difficulty of the puzzle is determined by the types of solving techniques required to finish the puzzle.
Here's an example of a simple row in Sudoku:
To solve this row simply think about which of the numbers between 1 and
9 are missing from the row. The answer is obviously 6. Usually to
begin with there will be more than one number missing, but there are
always enough clues to help you solve the puzzle.
As you fill in the board with correct answers, the remaining empty
cells become easier and easier to solve. Later in this guide we
will go over several basic and advanced techniques for solving
puzzles. A valid sudoku puzzle only has one correct solution.
Snow Day Sudoku
is a universal application that can be installed on your iPhone, iPod Touch
, and iPad
This version of the documentation is for the iPad. For
instructions on using the iPhone and iPod Touch version go to:
If you're already familiar with how to play Sudoku, here is how to interact with the Snow Day Sudoku.
If you need a more detailed information you should also read over the Glossary
and check out the Solving Sudoku
Snow Day Sudoku
on the iPad can be operated in either horizontal or vertical orientation.
One key difference between Snow Day Sudoku and other sudoku games
is that it includes an advanced way of highlighting cells to help solve
harder puzzles. These highlights can be confusing to first time
players of our game because they are exclusive to Snow Day Sudoku and
other Sudoku Book games. The thing to remember about highlights
is that they don't
effect the game in any way other than to help 'highlight' areas of the
gameboard. You don't have to use highlights, however, once you
figure out how powerful they are you won't go back.
Highlighted cells are a light blue color. We'll go over all the things that you can do with highlights
later in this guide. For now just know that you can clear all the highlights by tapping outside of the game board.
To enter a number in a cell, select it by tapping once on the
cell. The selected cell is a brighter blue color with an icy
Any time a cell is selected the ornament box appears and you can tap a number
to enter a value in that cell.
To remove the ornament box from the screen tap any where that is off the
gameboard. This will deselect the Selected Cell
and remove the ornament box from the screen. In some cases this will reveal the status area and icons
Rubbing a Cell
On the iPad version of the game you can erase a
cell by using the black X ornament, or you can rub the cell in a
circular motion to "Rub Out" or
erase your answer.
Basic game play is tapping empty cells and entering in the missing
numbers. When the whole board is filled in, if all the numbers
are in the correct spots the game will be Solved!
- One of the individual 91 squares that can contain a number.
- A horizontal unit of 9
cells. A completed row has exactly one instance of each of the
numbers 1 through 9. For a row to be correct each number has to
be in the correct position for that puzzle. In a solved sudoku
each row has a unique arrangement of the numbers.
- A vertical unit of 9
cells. A completed column has exactly one instance of each of the
numbers 1 through 9. For a column to be correct each number has
to be in the correct position for that puzzle. In a solved sudoku
each column has a unique arrangement of the numbers.
- 91 Cells arranged in a grid of 9 rows by 9 columns.
- A 3 x 3 cell subsection
of the Game Board. On the game board the blocks are emphasized by
the thicker grid lines. Again, a completed block has exactly one
instance of each of the numbers 1 through 9. For a block to be
correct each number has to be in the correct position for that
puzzle. In a solved sudoku each block has a unique arrangement of
- A row, column or
block. When learning solving techniques it is helpful to
understand the concept of a Unit. Another way to explain the
rules of Sudoku is to combine the three rules into one and say that
“each Unit must include exactly one of each of the numbers 1 through
9”. The point is that any solving technique that applies to rows
also applies to columns and blocks, and vice-versa. Some solving
guides call the Unit a Group because it's a group of 9 cells.
- A puzzle's starting
numbers. Cells containing givens can not be changed. In Big
Bad Sudoku Book givens are shown in black.
The Ornament Box
- This is the input device used to enter answers into selected cells.
- Snow Day Sudoku
has 4 types of hints: Found A Mistake, Found A Single, Found A Hidden
Single. Each hint type will be explained in more detail later in
this guide. Getting a hint negatively effects your score. Read
more about hints
- Also knows as pencil marks. Marks are small numbers placed in a
cell to help keep track of what values that cell could or could not be
(candidates). Typically players will set whatever marks could
be the answer for that cell, and then narrow them down based on several
solving techniques. When there is only one mark left it should be the
answer. Read more about marks
My Marks vs True Marks
- Snow Day Sudoku keeps track of two independent sets of marks. My
Marks are marks that you enter in and can edit. Your marks could be
accurate or not, because you can set them however you want. They are
shown in blue like the ones in the picture above. True Marks are a
computer calculated set of marks that are always correct. Read
more about marks
- Snow Day Sudoku
uses the standard Sudoku Book scoring system that awards points for each answer you
enter. Point values are weighed by the difficulty of the answer
at the time you entered it, and by the number of remaining
possibilities for that cell. Score is not based on time.
Read more about scoring
Starting New Puzzles
start a new puzzle at any time press the plus icon on the lower left of
the screen. Note that when you select a new puzzle the current
puzzle, whether in progress or competed, will be erased. If you
want to save the current puzzle in progress and start a new game use profiles
When you tap the new puzzle icon you'll be given the choice of 5
difficulty ratings. Tap a card to see puzzles to choose from in
that category or tap off the cards to dismiss the new puzzle picker.
The difficulty ratings in Snow Day Sudoku are based on the solving
techniques you will needed to use to answer the puzzle. To learn
more about the different solving techniques and what they mean check
out the Solving Sudoku
Easy puzzles have 33-37 givens and should be singles only.
Medium puzzles have 31-34 givens and could have up to 6 hidden singles.
Hard puzzles have 24-34 givens, 6-17 hidden singles, and will have occassional naked pairs and hidden pairs.
puzzles have at least 5 hidden singles, at least 2 naked pairs or
hidden pairs, and up to 2 high level problems such as pointing pairs or
puzzles are very hard to solve and have at least 2 high level problems
and can have any number of the lower level problems. They do not
require guessing but will require the more complex solving techniques.
After picking a difficulty card you'll be shown several game cards to choose from:
Tap a game card to load that game and start playing it. Tap MORE
to see six more games in that level. Tap the backgound or the difficulty card to cancel and remove the new game picker.
Random vs Pattern
puzzles - All
puzzles are generated randomly. However, in random puzzles the givens are placed
randomly around the game board, while in pattern puzzles the givens
follow a symetrical pattern. We provide both types to offer some
variation in puzzles and to make the game more fun. Sometimes in random
puzzles the givens might be heavy on one side or, like in the example
above, you might find a block that is amlost completely full while
another one is completely empty. This is part of the puzzle being
random and can still be fun to play. With out visual game picker you
can alway tap MORE until you find a puzzle that looks interesting.
Highlights are an exclusive feature of the Sudoku Book family of products that can be
used to help focus your view to a subset of the game board. Highlights
visualize what your brain is already doing when you attempt to solve a
sudoku puzzle. Most of the time you are analyizing the board and
looking at rows, columns and blocks for missing numbers.
Highlights are also an easy way to momentarily store a deduction that
Highlights can be cleared by tapping outside the game board. An
easy place to tap on the small iPhone screen to clear Highlights is the
Snow Day Sudoku title at the top.
several automatic highlighting features that can be accessed through
touch gestures. We think that once you use highlights you'll never go back.
Highlight Units (Long Tap Empty Cell)
Tapping on an empty cell selects it so that you can enter in a value.
Tapping and holding on an empty cell, or the long tap, selects it and
also highlights all 3 units associated with that cell: the row, column
and block that it's in.
This helps focus your attention to the adjoining cells that matter for
that cell. In the example above we can look at all of the highligted
cells and look at what numbers are missing. The answer is 7.
Highlight All Numbers (Long Tap A Number)
Tapping and holding on a number, whether a given or one of your
answers, will highlight all copies of that number in the game board.
This makes it easy to see what units are missing that number.
Highlight All Units for All Numbers (Long Tap A Number Twice)
Tap and hold the number a second time and it will highlight every unit
for every copy of that number. This shows every cell that the
number CAN'T be in. This feature is almost too good and some
people consider it cheating. But there are times when it comes in handy.
In the example above it shows where the 3 remaining 8's go. Using
this feature does negatively effect your score. However if you do
it manually it does not effect your score. Here's how to do it
Highlight A Row or Column (Swipe In To A Row or Column)
Swipe (or drag your finger) in to a row or column from outside the game
board and it will highlight that row or column. These swipes are
additive and won't de-highlight anything that was already highlighted.
Combine these swipes with the Highlight All Numbers explained above and
you can easily get to the same conclusion as the Highlight All Units
for All Numbers, except that you did the work. There is no score
penalty when you do the work.
Custom Highlight Cells (Draw on the Game Board)
The final highlighting feature is simply drawing on the game
board. This type of highlighting is very useful in analying the
complex chains that can be found in Insane puzzles.
people call them Pencil Marks but we just call them Marks. Marks
are small numbers or tick marks that are placed in the corners of a
cell to indicate that it is still open as a possible answer for that
To use your own Marks in Snow Day Sudoku first make sure that Show My Marks has been turned on inside of Marks Options
When you Select an empty cell, if you have Show My Marks turned on you'll see the Marks Entry Box:
Think of the Marks entry box as a ZOOM-IN or closeup of the selected
cell. It shows a close up of cell so you can see the marks for
that cell more easily. By tapping on the numbers you will toggle
that mark on and off. There are also 3 icons below the marks
- The first one (all black) turns on all 9 marks in a single tap.
- The next one (all white) turns off all 9 marks.
- The last button with the "A" will automatically select whatever
marks are left for this cell given the current state of the row, column
and block. Note that these automatic marks are only going to be
as accurate as the other answers you've already placed in the
surrounding cells. If you have a mistake somewhere near this
cell, the automatic marks function will obey that mistake in it's
logic. Also important to note is that there is no point penalty
for using THIS automatic marks feature because it is simply automating
a process that you can do manually in about 3 seconds.
For an example of how to use My Marks lets look at the following single row:
Knowing only this information about the empty cells we can deduce that
the missing numbers on the line are 1, 4, and 9. But for now we don't
know which cell contains which number. So each number is a candidate
for each cell. Marks help you keep of these candidates. As we solve
other cells that relate to these, we get more information about what
CAN NOT be in these cells, and we can remove them as candidates by
removing or turning off the marks.
To begin with we will put those 3 marks in those 3 cells:
Of course a real sudoku game also has colums and blocks and you use all
these related cells to gather information. If we expand our view to
include all 3 blocks surrounding this row we can see other numbers in
the mix and remove some of the candidates:
Remember that there can only be one instance of each number in each row
column and block. The 4 in the first block removes it as a
candidate for all empty cells in that block and therefore leaves only
the 1 and 9. In the next block the 1 removes it as a
candidate. And in the last cell the 9 and the 4 are removed as
candidates. When only one candidate remains it must be the answer
for that cell.
In this example we now can solve the whole line. The 1 is the only
remaining candidate for the last spot. That is called a Single
because there is only one candidate left. Once we solve the 1,
the 1 can be removed as a candidate for the first spot leaving only the
9, and in turn this leaves only the 4 for the middle spot.
When using My Marks you have to do all of the adding and removing of
marks yourself. If the computer did it for you it would be a
serious advantage. However, that's exactly what True Marks are.
What True Marks do is actively update marks across the whole game board
as you enter values in related cells. In the example above the
True Marks would automatically remove the 1 from that first spot when
you entered one as an answer to the last spot. The only caveat with
True Marks is that if you enter in a wrong answer the marks will follow
that. They don't guarentee that you're answering cells correctly.
If you ever see True Marks show NO markes in a cell, you've answered
something incorrectly somewhere and you should use Get Hint
to find out where.
Be aware that using True Marks will negatively effect your score. Using My Marks does not.
One thing you can't do with True Marks is edit them. Why would you want
to edit them if they're right? True Marks only takes the basic approach
of looking at all 3 units to remove candidates for a cell. This
method will help solve Easy Puzzles but to solve harder puzzles you
have to use more advanced techniques to narrow down your candidates.
So we included an option to copy the True Marks to My Marks so they can be edited.
the Marks Options icon to bring up the Marks Option card. From there
you can switch between True Marks and My Marks, and do mass
modification to your marks. Here is the card and what each option
Marks Display Modes
- The marks
icons will be darker when they are being displayed and lighter when
they are turned off. Only one set of marks can be displayed at a
time. Turning on one set will turn off the other. Remember
that any time you manually enter a My Mark it will turn them on so you
can see the mark you just entered.
Reveal / Hide True Marks
Turns on the True Marks display and turns off the My Marks display if
they were on. Turning on True Marks will cause a penalty in the
score for the next cell you answer and will reduce your bonus points
when you solve the puzzle.
Show / Hide My Marks
- Turns on and off the custom marks. There is no penalty for using My Marks.
Copy True Marks to My Marks
takes a snapshot of the current state of the True Marks and copies them
to your My Marks. This will erase any previous marks you had
made. This option will cause a one time penaly in the score for
the next cell you answer and will reduce your bonus points when you
solve the puzzle.
Clear All My Marks
- resets all of your custom My Marks by turning them all off.
Set All My Marks On
- turns all
9 marks on for all empty cells. You might want to do this to
start with them all on and remove them as you go.
compass icon is the Get Hint feature. Get Hint does exactly that, it
gives you a hint without giving you any answers. To understand the
hints you will need to understand a couple of basic solving
techniques. These are explained in the Solving Sudoku
section. Get Hint offers three different kinds of hints: Found A
Mistake, Found A Single, and Found A Hidden Single. The hints are
tiered, meaning that you'll only get a hidden single if there are no
singles to be found, and only get a single if there are no
mistakes. Each use of Get Hint only gives you one hint for one
cell. Repeated use can result in the same hint being given over
and over, or cycle through several hints, depending on the state of the
board. Using Get Hint results in a penalty for the next cell you
Found A Mistake
(Red Highlight) - Will show a cell that contains a wrong answer by highlighting it Red.
Found A Single
(Green Highlight) - Will show a cell that contains a naked single by highlighting it Green.
Found A Hidden Single
(Teal Highlight) - Shows a cell that contains a hidden single by highlighting it a Teal Color.
Snow Day Sudoku has several game options to customize your playing
experince. To access the Game Options Card tap the Game Options Icon.
- Whether or not
to play sounds during the game.
Line Completion Sparkle
units (rows, columns, or blocks) are completed a sparkle animates
across them. This options customizes the conditions for the sparkle to
- The sparkle animates
when ever all 9 cells are filled in, whether the answers are correct or
not. In "Always" even if you put the same number in each cell it
will still animate.
- The sparkle animates only
if you don't have any repeated numbers. You have to have each of
the numbers 1 through 9. However, in this mode the sparkle will
still animate if you have 2 of the numbers swapped.
Only If Correct
- The sparkle animates only if all 9 numbers are in the correct spots.
Why so much attention to the sparkle animation? The 3 different
options offer different levels of verification that your answers are
correct. Some players don't like any spoilers. If you have the
options set to Only If Correct
and you fill in a row but there's no sparkle, you would know that you
had made a mistake. If you don't want this kind of information we
provide the option to set it to Always or 1-9.
Timer Clock Display
Personally I don't like playing against a clock, so we put in the
option to not see the clock until you've completed your game.
- This options
also falls in the "spoilers" category. Since the score is
negative when you answer the cell incorrectly, the score is actually a
huge hint. If you don't want to see these hints, or if you just
don't like the score you have the option of turning it to only show the
Score when you've solved the puzzle. FYI - It is harder to get a
good score with it turned off.
Game Center Alias
- The alias or name for your currently logged in Game Center user.
Snow Day Sudoku employes the same method of scoring games as Big Bad Sudoku Book. The
idea behind our scoring system is that you get points for each cell
that you answer, plus you get a bonus for correctly completing units,
and you get a bonus for completing the puzzle. The score for
answering a cell is determined by the difficulty of the answer and by
how many candidates were left for the cell when you answered it.
The math behind the scoring system is a little involved because it
considers previous wrong answers, the type of method required to solve
the cell, and the number of candidates. It also consideres if the
player used any hints or true marks. Here are the basics.
- An incorrect answer is -18 points.
- If the cell was a Sinlge it's +21 points.
- If the cell was a Hidden Singel it's +36 points, plus 4 points for each true candidate.
- If the cell requires a more advanced technique it's worth +72 points plus 7 points for each true candidate.
points are additive. For example each wrong answer will be -18 points
which will go against the positive score you get once you finally
answer it correctly.
If True Marks or the double long tap
were used to help answer the cell there is a 9 point penalty.
Line Completion Bonus Points
A line completion is an easy way to say a unit completion - so it's
awarded when a row, column or block is completely filled in
correctly. Multiple units completed at the same time multiplies
- A single line is worth +9 Points.
- The second line is worth +18 Points.
- The third line is worth +21 Points.
You only get the line completion bonus if you completed the line
correctly the first time. An incorrect answer in the line forfits the
bonus for that line. You can only get the bonus once per unit.
Puzzle Completion Bonus
There is a potiential of 18 bonus points per cell. Using any hints, true marks, or double long tap
highlighting (cheats) or getting a wrong answer wipes the bonus for that cell.
At the end of the game the bonus is tallied. 10% is taken off for each
hint that was used during the game. Another 25% is removed if
true marks were ever used during the game. It is possible to lose
your entire puzzle completion bonus by using too many hints and true
Snow Day Sudoku uses Game Center to track Leaderboards and
Achievements. Game Center requires iOS 4.2 or later, an internet
connection, and a Game Center account.
The easiest way to use Game Center is to log in using the Game Center
app, then launch Snow Day Sudoku. You should only ever have
to log in to Game Center once (or once in a while) on each device.
If you're device supports Game Center and you launch Snow Day Sudoku
without first logging in to Game Center, you will be asked to log
in. At this point if you hit "Cancel" Sudoku Book will assume
that you don't wish to use Game Center and it won't bother you with the
log in screen again. After that if you wish to use Game Center
with Snow Day Sudoku you'll have to log in via the Game Center app.
The two icons above are the Leaderboards icon and the Achievements
Icon. When you are logged in to Game Center these icons will be
fully black (active). When you are logged out of Game Center they will
be grey or dehighlighted.
When they are active if you tap them you'll be taken to Apple's Game
Center interface where you can browse the Leaderboard or see your
Achievements. When they are dehighlighted tapping one of the
icons will give you the opportunity to log back in.
Note that our implimentation of Game Center is broken right now and we expect it to be functioning correctly in a day or two.
Snow Day Sudoku keeps track of 7 Leaderboards using Game Center. They are:
- Total of All Scores, All Games, All Levels
- Total Scores from Easy Puzzles
- Total Scores from Medium Puzzles
- Total Scores from Hard Puzzles
- Total Scores from Expert Puzzles
- Total Scores from Insane Puzzles
- Total Number of Games Played
Snow Day Sudoku has 9 Achievements that you can earn. Some
of them are hidden until you complete other Achievements (and we won't
spoil them here). The initial ones are:
- Easy Peasy - Earn 10,000 combined points on Easy puzzles.
- Medium Well - Score 30,000 combined points on Medium puzzles.
- Hard Ways - Score a total of 50,000 combined points on Hard puzzles.
- Expert Witness - Score a total of 70,000 combined points on Expert puzzles.
- Insane Brain - Score a total of 100,000 combined points on Insane puzzles.
- Big Winner - Play a puzzle challenge that a friend shared with you and beat their score.
- Devoted - Score a total of 250,000 points across all puzzles.
In the Using Marks
section of this guide we went
over the basic idea of keeping track of all the possible candidates for
each cell by using My Marks. This technique is critical to solving
harder puzzles and is a prerequsite for many of the solving methods we
will discuss here. It's a good idea to read over that section
before jumping into this one.
There are many different ways to approach solving a sudoku
puzzle. Here we will go over a few popular methods as breifly as
possible and also describe some of the solving terms such as Single,
Hidden Single, Naked Pairs, Hidden Pairs, and others.
A great first concept to grasp is the idea of the Unit. A Unit is either a row, a column, or a 3x3 block. Check out the Glossary
for more in depth descriptions of each of those.
Any rule or solving concept that can be applied to one type of unit can
be applied to another type. For example, the basic rule of Sudoku is
that each unit must contain exactly one instance of each of the numbers
1-9. If you look at any of the 81 cells on the board you can see that
it is a member of 3 units. It is in a row, it is in a column, and
it is in a block. So when considering any particular cell and
what number could go in that cell, you have to look at each of the 3
units that cell is in and eliminate any number as a possibilty that is
already being used somewhere else in any related unit.
That might be a complicated way of saying what can easily be shown by long tapping an empty cell:
Each of the highlighted cells above are related to the selected
one. Any number that is in any of the highlighted cells can not
be the answer for the selected cell.
In the example above, it is clear that the only remaining candidate is
a 7. When a cell only has one remaining possible candidate it is
called a Single. It is sometimes referred to as a Naked Single
because the candidate is sitting out there all by itself.
The easiest way to find singles without using True Marks is to use the
long tap to highlight the units related to a cell and cross-scan for
A hidden single is when a particular candidate can only be found in one
cell of a unit. For example if a row has several empty cells but
only one of the cells has a 1 as a candidate, the 1 must go there. To
demonstrate the hidden single concept let's look at a fresh Insane
In the puzzle above a cross-scan reveals that the selected cell has 3
candidates: 5, 6 and 7. Looking first at only the row you'll find that
all 3 of the candidates can be found as candidates in other cells. Next
look at the column. Again the 5, 6 and 7 are repeated several times as
candidates in other cells in the column. However if you look at only
the block, the 6 and 7 appear in other cells but the 5 is exclusive to
that cell. This exclusitivity makes it a Hidden Single and 5 has
to be the answer for that cell.
Looking at all the highlighted cells above do you see any other hidden
singles? The sixth cell over in the row has the candidates 1 and
7. Since that's the only cell in the row that can be a 1 it must
be a 1.
Another way to find hidden singles is to use the double long tap
Advanced Solving Techniques
Singles and Hidden Singles are concepts that lead to a direct answer
for a cell. If the cell contains a single or hidden single then that's
the answer. Advanced solving techniques are ways of using logic to
reduce candidates even further. The more you reduce candidates across
the puzzle, the more singles and hidden singles you'll find.
Also, any time you solve a cell you can remove the answer as a
candidate for all of its related cells. Solving Sudoku is as simple as
repeating the cycle of finding singles and hidden singles, reducing
candidates, and finding singles and hidden singles again, and repeat. This is how computers solve sudoku.
Here are a few advanced techniques.
If you have two cells in a unit that each contain an identical pair of
candidates and only those two candidates, then no other cells in that
unit can be those two values.
Think of it this way: if A has to be either a 1 or a 2 and B also has
to be either a 1 or a 2, then one of them has to be a 1 and the other
has to be a 2. You have two slots for two numbers. Therefore no
other cells in that unit could be 1 or 2 because they're already taken
up by those two cells. Let's visualize using a block from the same
In this example every cell has 7 as a candidate. But the bottom two
cells form a naked pair because of ther matching candidates. Both have
7 and 2. No matter what's going on in the other cells we know that one
of those two cells must be a 7 and the other one must be a 2.
Therefore no other cell in the group can be a 2 or a 7. We can
eliminate the 7 as a candidate for the other two cells. When we do this
it leaves only 6 as a Single in the top cell. This revelation
further reduces the candidates in the middle cell only leaving the 5 -
which is the same answer we reached in the Hidden Single exercise above.
The same concept can be applied to triples and quads. If you have
3 cells each with only the same 3 candidates, those 3 candidates have
to go in those 3 cells and therefore can't go anywhere else in the unit.
Here, from the same Insane puzzle, we have 3 cells each with only the
candidates 4, 6, and 7. These 3 cells must contain these 3
numbers, although at this point we don't know which cell contains which
number. But the remaining 3 cells are instantly solved by this
method. The 7 must be removed as a possibility from the rest of
the block. This leaves the 1, 9 and 8 as Singles.
When you get into triples and quads the rule even gets relaxed a little
bit. As long as the 3 cells don't contain any extra candidates, they
don't all 3 have to contain all 3. Because at the end of the day you
still have 3 cells which have to contain 3 candidates, so all 3 of
those candidates can be removed from the rest of the unit. Look
at this example:
Three cells contain the combined candidates of 3, 4 and 7. Those 3 candidates can be removed from the rest of the row:
In this case reducing the candidates didn't directly reveal any
singles. But it did reveal another Naked Pair. The 2-5 2-5 is a
pair which removes the 2 from the 2-9 leaving only the 9 as a
Single. Another way to look at it is that the candidate reduction
revealed the 9 as a new Hidden Single.
The hidden pair takes the logic of a hidden single and applies it to
pairs. If two cells in a unit contain a pair of candidates, hidden
among other candidates, that are not found in any other cells in the
unit, the two candidates have to go in those two cells. This one
is certainly easier to show:
The candidates in this block appear to be all over the place. But on
close inspection you'll notice that the 7 and 9 only appear in two
cells. This means that the 7 is in one of those two cells and the 9 is
also in one of those to cells. So again you have two cells that must
contain the two candidates. It's still a pair but it was a little
harder to find. You can remove the rest of the candidates from
these two cells.
What does this candidate reduction reveal about the rest of the cells
in the block? Well, it allows you to easily solve the two
highlighted cells (above). Choose your solving method. The cell on the
right is left with a 6 as a hidden single. The 2-5 2-5 is another
naked pair which eliminates the 5 from the cell on the left, leaving
only the 3 as a single there.
When a particular candidate only occurs in one row within a block, it
can be eliminated from the rest of the row. Same for when a
candidate only occurs in one column within a block, it can be
eliminated from the rest of the column.
In the example above the two highlighted cells to the right are the
only two cells in that block that contain the canidate 5. The 5 has to
be in one of those two cells. Since those two cells are in the same row
inside that block, the 5 can't be anywhere else in that row and
therefore can't be in the highlighted cell on the left. Eliminating 5
as a candidate from the cell on the left reveals a hidden single in the
5 above it.
More Solving Methods
If you're looking for more techniques google "Sudoku Box Line Reduction" "Sudoku X-Wing" and "Sudoku Chains".
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This guide is current for version 1.0.1 of Snow Day Sudoku. There are no known issues at this time.