Welcome to SudokuAssistant, designed to provide assistance when you’re solving Sudokus and to provide you with Sudokus to solve. The assistance is at varying levels, ranging from analysis of possibilities, through the provision of hints, to the automatic solution of a Sudoku. It can help you to understand how to solve a Sudoku if you’re inexperienced, or just help you solve them more quickly if you’re more experienced.

Its key features are:

  • it can analyse all the cells in a Sudoku and inform you which numbers are possible in the cells;
  • it can fill in all the cells that have only one possible number based on this analysis, so saving you time and effort;
  • it can provide you with a clue - which cell to look at for your next move
  • it can provide you with a hint on what to do next - and then do it for you if you request it to do so;
  • it keeps a record of what you’re doing, and lets you go back a step at a time if you think you’ve made a mistake, or go right back to the beginning;
  • it can check and highlight errors;
  • it can solve the Sudoku for you automatically if you just want the solution for some reason.

The hints and the automatic solution can handle any 9x9 Sudoku that has a solution, even those that require you to make guesses and backtrack if the guesses prove wrong. It has a Set Mark and a Go back to Mark feature that should prove useful if you’re trying to solve such Sudokus unaided.

SudokuAssistant also has many built-in Sudokus for you to practise on.

How to solve a Sudoku

A traditional Sudoku puzzle is built around a 9x9 grid with 81 cells. This grid consists of 27 groups - the 9 rows, the 9 columns and the 9 3x3 squares (shown shaded in the grid below). 9 numbers, 1-9, must be placed on the grid such that each number occurs once and once only in each group (i.e. in each row, each column and each 3x3 square).

A Sudoku puzzle has the symbols in some of the cells already defined, so the puzzler must try to place the numbers 1-9 in the blank cells so that they all satisfy the constraint of appearing exactly once in each group.

Technique 1 - Last unoccupied cell in a group

By far the easiest technique used in solving any Sudoku is looking for a group that has eight occupied cells. The value that must go in the only unoccupied cell is then obvious: the number from 1-9 that isn't already in the group.

For example, in the following partially-completed Sudoku, look at column 3. This has eight occupied cells, so that the only unoccupied cell (in row 4) must be a 3.

Technique 2 - Only cell in a square that can take a particular value

Technique 1 normally cannot be used when you start to solve a Sudoku. Instead, most solvers of Sudokus would start off by looking to see whether there is an easily identifiable cell that must take a certain value because it is the only one in a square that could take this value. This involves looking at the 3 rows and 3 columns that intersect with this square. For example, take a look at the top-middle square of the Sudoku below.

This currently doesn't have a 1, so where could the 1 be placed in the top-middle square? It cannot go in the first column, as there's already a 1 in that column (in the square below). It cannot go in the second column, as there's already a 1 in that column further down (in the bottom-middle square). It must therefore go in the third column. Within that column, it cannot go in the third row as this already contains a 4. It cannot go in the second row as there's already a 1 in that row (in the top-left square). This means that the 1 MUST go in the top row of its third column, as there's nowhere else available.

Technique 3 - Only value a cell can take

With Technique 1, we looked for the last unoccupied cell in a group. This is a special, easy case of the situation where there is only one value that a cell can take. More generally, every cell is a member of a row, a column and a group. If we look at these and find that the row, column and group together contain eight different numbers, then this cell must contain the ninth.

For example, look at the cell at row 6, column 4, in the following partially-completed Sudoku:

Row 6 and column 4 together contain eight different numbers, meaning that this cell must contain the ninth number, i.e. 6.

Technique 4 - Only cell in a row or column that can take a particular value

This is very similar to Technique 2 but is generally harder to spot. Instead of looking at 3 rows and 3 columns, we need potentially to look at the 9 rows or columns that cross this one, and the three groups it crosses.

For example, look at row 1 in the following partially-completed Sudoku:

This must contain a 2. Now, there's a 2 in the top-left square already, so there cannot be a 2 in the first two empty cells of the row. The 4th cell cannot contain a 2 as there's already one in that column. Likewise, there are already 2s in columns 8 and 9, so these cells cannot take a 2. This leaves only cell 7 of the row that can take a 2.

Making use of analysis

Techniques 3 and 4 can be employed by simply looking at the Sudoku, but their use can be hard to spot. So, for many Sudokus, solvers may need at some point to carry out a simple analysis of remaining blank cells to see which numbers are possible for it in order to satisfy the constraints.

Now, each cell is a member of three groups: a row, a column and a square. For example, the top-left cell is a member of the top row, the leftmost column and the top-left square. Essentially, the analysis involves checking the row, the column and the square associated with the cell to see which of the 9 numbers do not already exist in one of these, as these are the only numbers that could possibly be placed in the cell.

So, for the following Sudoku:

if we look at the top-left cell, we can see that because the top row already contains 6 and 7, the leftmost column contains 7, 5, 2 and 8 and the top-left square contains 6, 1, 7, 3, and 5, the only values that it could possibly contain are 4 or 9.

If we continue this analysis, we can see that the cells of the top-left square can be analysed to yield the following results, where defined values are shown in large, black bold text, while possible values are shown in small, green italics, where 2489 means that the cell could contain only a 2, a 4, an 8, or a 9.

If we focus on this top-left square, we can see an example of the easiest step to take when solving a Sudoku, once you have carried out such an analysis: the cell in the second row and the first column can take only a 4. This must therefore be a 4, so a 4 can be entered into the cell. This is an example of Technique 3. A lone possibility would also result with Technique 1.

The top-left square also contains an example of a step to take that is less easy to spot from the analysis. The cell in the top row and second column can take the values 2, 4, 8 or 9. However, if we look at the rest of the cells in this square, we can see that this is the only cell that can take an 8. This therefore means that this cell MUST contain the 8, as the 8 must appear somewhere in this square. This is an example of Technique 2, but similar reasoning can be used for rows and columns to help spot examples of Technique 4.

Given that we can then fix these two cells, we can quickly see that the cells of the top-left square can be completed as follows:

An analysis of the complete grid should then enable the solver to begin filling in more of the values until it is complete.

Many Sudokus can be solved using only the four techniques already described, either with or without the help of detailed analysis.

However, with more difficult Sudokus, these four techniques may not be enough and other techniques may be required. For example, suppose we reach a position where no values can be defined using these four techniques, but find that there is a square (similarly, for a row or column) that contains the following defined values and possible values:

Note that there are two cells containing only the same two possible values: 3 and 9. Although we can’t at this stage determine which is the 3 and which is the 9, we do know that these two cells between them MUST contain the 3 and the 9 since neither can take any other value. Given that, we know that the 3 and the 9 cannot appear in any other cell of this group. We can therefore simplify some of the other cells in the group by eliminating these possible values from them. This results in the following situation:

In this case, we can see that this leaves the central cell that has only one possible value, 4, so we can fix this, and then the analysis will show that the bottom-left cell must be a 6, so we can fix that:

This is an example of the fifth technique that can be used:

Technique 5: Look for 2 cells in a particular group that have exactly 2 possible values, the same for each cell. If any other cells of the group have any of these 2 values as possibilities, these values may be eliminated as possibilities for those other cells. More generally, look for n cells in a particular group that have exactly n possible values, the same for each cell. If any other cells of the group have any of these n values as possibilities, these values may be eliminated as possibilities for those other cells.

Another technique that is available for helping you to solve a Sudoku can be demonstrated if a situation is reached where a particular square (similarly, for a row or column) contains the following defined values and possible values:

In this case, note that there are two cells containing the same possible values, 1 or 2, as well as other possible values, but that no other cells have 1 or 2 as possible values. Similar to Technique 5, we can say that therefore these two cells must between them contain the values 1 and 2. This means that these two cells cannot possibly take any other value, so the other values can be eliminated as possibilities:

Having simplified the possibilities in this way, we find that the cell in the leftmost column and second row down (possible values 7 or 8) is now the only one that can take a 7, so this must be 7.

This is an example of the sixth technique that can be used:

Technique 6: Look for 2 cells in a particular group that share the same 2 values as possibilities and are the only ones in the group to have either of these values as possibilities. If either of these 2 cells have any other values as possibilities, these can be eliminated. More generally, look for n cells in a particular group that share the same n values as possibilities and are the only ones in the group to have any of these values as possibilities. If any of these n cells have any other values as possibilities, these can be eliminated.

Most Sudokus require solvers to use only these six techniques (and the easiest ones require solvers to use only the first two). However, some puzzles are created that can not be solved using just these techniques. Now, more advanced logic can sometimes be employed in such situations, but this will not be discussed here. Instead, when a situation is reached where none of the these six techniques can be applied, solvers can simply select a cell that has a certain number of possibilities (preferably only two), guess which one may be correct, then continue to see whether this results in a valid solution or whether it results in an inconsistency that shows the guess was wrong, in which case another guess must be made.

So, the seventh technique that might be required is:

Technique 7: Select any of the possible values for a cell and set the cell to be this value. Continue trying to solve the puzzle. If this results in an inconsistency, so showing that the value set was wrong, go back and undo the value that was guessed and also any values set after this guess was made as they are dependent on this invalid guess. Then choose any other possible value not yet selected instead and see how this goes.

Note: some puzzle setters may force the solver to make guesses in more than one place, so the guessing and backtracking can be very complex.

Entering your Sudoku

To solve or obtain assistance with a Sudoku you are trying to solve, maybe one from a newspaper, magazine, or book, you first need to load it into SudokuAssistant.

To do this, first press the New button, which clears any numbers from the main 9x9 grid. Then, just enter the values in the following way on the grid:

  • Select the value to be entered by pressing the relevant number with a green background at the top or on the left. Note that once selected, the background becomes darker.
  • Press the cell in which you wish this value to go. When you press on a cell, the number with the dark green background is put into this cell (removing any existing number).
If you make an error entering the numbers and need to remove one, you can first press on the empty green cell in the top-left corner, then press on the number you wish to remove. Alternatively, if you just wish to remove the last number you entered just press the < button to remove the last number entered.

Hint: to avoid having to press the green numbers before entering each number, try entering all the 1s, then all the 2s, and so on.

Once you have entered the puzzle numbers, and are ready to try to solve it, press the Base button. This tells the system to treat the numbers entered as the Base Sudoku, i.e. the one to be solved. The main Sudoku screen disappears for a short time, while the system checks that there is a valid solution and records it, and informs you if there is not. The concept of a Base Sudoku and its solution is important for some of the functionality, e.g. error-checking.

Loading a built-in Sudoku

If you don’t have a puzzle to solve, you might like to try one of the built-in ones. These are graded from level 1 (Easiest) to level 3 (Hardest). When you first open SudokuAssistant, one of the built-in Sudokus is selected randomly and automatically loaded.

To load a puzzle of a particular level, first press on the More button and select one of the levels, 1-3. Then, press the Back button, and press Load. A new Base Sudoku is then loaded and displayed (you don't need to press the Base button). As with pressing the Base button, the main screen disappears for a short time, while the system checks determines and records the solution.

As well as the levels 1-3, you can also select '?' as the level. This means that when Load is pressed, a Sudoku of any level is selected and loaded. Note: this is the default, so you don't need to change the setting if you want a random-level Sudoku to be loaded, just press Load.

The gradings are as follows, and are based on solving each of them and recording the techniques used to solve them.

  1. Techniques 1 and 2 only

  2. Techniques 3 and/or 4 needed in addition

  3. Techniques 5 and/or 6 needed in addition

Note: No built-in Sudokus require the use of Technique 7 (guessing).

Note: You may find that you can choose to use a higher-level technique than is strictly necessary through the puzzle level.

There are 300 fundamentally different built-in Sudokus (100 of each level). However, these are transformed in various ways before being presented to you, so that it may appear that there are millions of different built-in Sudokus.

Solving a Sudoku manually

The addition or removal of numbers from cells is carried out in exactly the same way as for initially entering the Sudoku numbers manually.

Note: if you try to fix a cell to a value that can already be found in one of the groups associated with the cell, an error message will be displayed in the output box beneath the buttons as SudokuAssistant will not let you do this.

The following buttons may prove useful when you are trying to solve a Sudoku manually without assistance:

  • The < button takes you back one step. You can repeatedly press this to go back additional steps, until you reach the start of the puzzle.
  • The |<< button takes you back to the beginning of the puzzle
  • The Mark button is useful if you find you need to guess a value (which you shouldn't need to do with our built-in Sudokus or most others). Press this before you make a guess, which sets a mark (a position you may wish to return to). If you later find that the guess you made was wrong, press the M < button and you will be taken back to the display as it was when you pressed the Mark button, so that you can then guess a different value (our advice is to guess the values that are possible in numerical order). Note: You can set more than one mark. Pressing the M < button takes you back to the last one; pressing it again takes you back to the one before that, and so on.
  • The Simp button may be used to record the simplifications of Techniques 5 and 6. Details of this are given under Obtaining assistance, as it is best used when the Analysis Display is turned on.

Once you've solved the puzzle, press New to clear the grid, or Load to load a built-in one.

Checking a Sudoku

You can check whether you have made any errors in solving the Sudoku by pressing the Check button. This checking can only be done once the system has determined and recorded the valid solution (either through the Load functionality or by pressing the Base button).

If the system finds that the solution you have entered is different from the solution it determined and recorded, errors are highlighted in yellow text on the main grid. Note that if your Sudoku has more than one valid solution, unlikely but possible, your solution might be shown as an error if it's not the one the system finds. However, most commercially-produced Sudokus have only one solution.

Obtaining assistance

Analysis Display

A simple analysis of which values look possible for any blank cells lies at the heart of solving any Sudoku. SudokuAssistant carries out such an analysis with every move you make. It records as possible values for a cell any value that is not obviously impossible due to the cell's membership of a group that already contains this value. You may choose to display the results of the analysis on the grid at any time when you are solving a Sudoku.

To switch on the display of the analysis, press the A+/- button. This is actually a toggle, so you can switch it off by pressing it again.

When the Analysis Display is on, the possible values for a cell are indicated by small green squares shown in a 3x3 sub-grid within the cell, representing the possible values as follows:

So, a grid with the display:


means that this cell could take the values: 1, 5, 8 and 9. All other values are impossible.

As you make changes to the grid, the Analysis Display is updated automatically.

You can also choose to see the Analysis Display in a different format, in which small red squares are used to indicate which number are impossible. So, the above situation would be shown as:


For a display on screen, this is probably less useful than the display of what is possible. However, when solving Sudokus manually on paper, many people use an approach similar to this for their analysis. It has the great benefit that you don't have to remove dots already added, since we gradually eliminate possibilities.

The selection of the two formats of Analysis Display is made by first pressing the More button, and then selecting either 'Possible=green' or 'Impossible=red' option. Press Back to return to the main screen.

Note: if at some point you see that the Analysis Display shows that no values are possible for a cell, you know that you have made a mistake.

Note: Once switched on, the Analysis Display stays on until the A+/-, New, Load or Base button is pressed

Fixing cells that can take only one value

Once you see the analysis, it may be clear that there are several cells that can take only one value. Instead of laboriously having to fix each such cell in turn, you may press the Lone button, which fixes all these cells to their single possible value at once (the number of cells fixed is shown in the output box). This lets you focus on the more challenging techniques.

Simplifying

Techniques 5 and 6 both involve simplification of the possibilities in a group, based on 2 or more cells that share the same possible values. The Simp button allows you to specify such simplification and have it displayed on the Analysis Display.

If you see an opportunity to use Technique 5 or 6, first press the Simp button. Since simplification always relates to a specific group, you first need to specify the group concerned. The system will ask you to press on the first (top-left) cell of the group. For a square, this is, of course, the actual top-left cell of the group; for a row, it is the leftmost cell, while for a column it is the top cell. It will then ask you to press on the last (bottom-right) cell of the group. For a square this is the bottom-right cell of the group, while for a row it is the rightmost cell and for a column it is the bottom cell.

Once the group has been specified, you will then be asked to press on one of the cells containing the same possible values as one or more other cells. This means you should press on one of the cells on which the simplification is to be based.

The system will then check to see if a simplification is possible in this group, based on the specified cell. If so, it carries out the simplification and changes the Analysis Display accordingly. If such a simplification is not possible, a message to this effect is shown in the output box at the bottom.

The system starts off by checking for simplifications involving 2 cells, but then looks for those involving 3, 4, 5, and so on.

Note: Like the fixing of values to cells, simplifications can also be undone with the < button.

Note: Although it is easier to see the potential for simplifications, and the result of such simplifications, with the Analysis Display turned on, the functionality may also be used when the Analysis Display is not on.

Hints and Clues

An outstanding feature of SudokuAssistant is its Hints and Clues functionality, useful not only for inexperienced puzzlers, who can see how to go about tackling a Sudoku, but also for experienced puzzlers who may have got stuck.

SudokuAssistant can provide simple clues or detailed hints.

Note: With both simple clues and detailed hints, individual cells are referred to by their Row and Column numbers, where these correspond to the green boxes. So, R2C3 means the cell at the intersection of Row 2 & Column 3. For groups, rows and columns and referred to just by their associated number, while the 3x3 squares are referred to as:

Simple clues

At any point, you may press the Clue button. The system then looks to see what your next move could be. If the next move is to fix a cell to a value based on Techniques 1-4, the clue given in the text box below the button is the cell reference. Likewise, if the next move is to simplify the possible values for the cells of a group based on Techniques 5 or 6, the clue given in the text box is the cell reference of one of the cells that share the same possible values.

This simple clue should provide you with a focus, and help you to determine the next move. However, if you still cannot see the next move, you may press on the H button, which then prompts the system to give you a detailed hint (see Detailed hints).

If the suggested next move is anything but one of the simple moves based on Techniques 1-6, it provides a detailed hint (see next).

Detailed hints

If you always wish to see detailed hints instead of clues, you may press on the Hint button.

For a detailed hint, an action is suggested, together with the reasoning behind this, in the text box. In all cases, you can then choose either to carry out the proposed action yourself, or to press the Y button, in which case the system carries out the action for you.

The Hints functionality can cope with all 7 techniques, but it will always look for and suggest the easier ones first, so you should never be asked to guess unless the puzzle setter has set a puzzle that requires guessing. When looking for simplifications (Techniques 5 and 6), it will first look to see whether there are 2 cells where Technique 5 can be used, then for 2 cells where Technique 6 can be used, then for 3 cells where Technique 5 can be used, then for 3 cells where Technique 6 can be used, and so on.

Automatic puzzle solving

If, for some reason, you just wish to see the solution to a Sudoku without solving it yourself, just press the Auto button. You will see the display change as the solution progresses a step at a time. You may use this with the Analysis Display turned on or off. As with Hints, all Techniques can be handled, and the easier Techniques will be used first.

Once solved, the word Solved is shown in the output box. If the solution necessitated making guesses and setting marks, you will be informed of the number of marks set in the output box also.

If the system finds that no solution is possible (it finds a cell that has no possible values), the message 'No solution available' is displayed.

Note: The More and Check buttons act as a Stop button for the Auto functionality, while the Back button on these screens stops the Check function.

Terms of use

This software may be used for personal purposes only.  It may not be used for commercial purposes.

The software, code and script of this application may not be copied or be distributed to others.

The software, code and script may not be altered in any way.

Copyright of the software, code and script, and its help documentation, is held by TopAccolades Limited.  No part of the software, code, script or documentation may be copied or used in another application or document.

Although every effort has been made to ensure that this software functions as intended, TopAccolades Limited provides no warranty of merchantability, or fitness for a particular purpose, in relation to this application or its support.

TopAccolades Limited shall not be liable for any damages whatsoever arising out of the use of or inability to use this application or the provision of or failure to provide support.

Copyright TopAccolades Limited
www.topaccolades.com

Version: 3.0 (20170327)
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