Xsudo

How to Use Xsudo

This page gives an overview of how Xsudo is used for making and solving Sudoku puzzles. It is also a good starting point to learn where to find the most useful buttons and functions. Links throughout point to further details.

If you new to Sudoku, take a look at Introduction to Sudoku. Put simply, Sudoku is a 9 by 9 grid with 9 rows, 9 columns, and 9 small boxes fit inside the grid. There is only 1 rule. Every row, column, and box must have one of each digit from 1 to 9. You now know everything about Sudoku, except perhaps how to solve it.

Making A Puzzle

Make a New Puzzle

· Press the puzzle maker icon in the top toolbar.

Or a better idea ..........

Use the same puzzle that is used here, which can be found in the puzzle list.

Open the Example Puzzle

· Click the right tab of the puzzle list in the toolbar, which looks like this ...

· Scroll to the top and click on the puzzle called First Puzzle.

· Then click the load puzzle icon to the right.

Solving without Candidates

When solving Sudokus without candidates, filters are often useful. We will use a hatching filter to find a a hidden single in the puzzle. A hidden single occurs when a digit can be placed in only one cell is a row, column, or box. First, we must hide the candidates ....

Hide candidates

· Click the hide/show icon in the toolbar to be with a red border.

Finding a Single

· Select the cell filter in the toolbar filter selector

· Select digit 5 in in the toolbar digit selector, (looks like this) ...

The cell filter shows all cells that have a digit 5 assigned. In Sudoku, another 5 cannot be placed in a row, column, or box that already has a 5. This is the same as saying a 5 cannot be placed in any cell that can see a 5, as shown by the hatching filter.

· Select the hatch filter in the toolbar filter selector

The hatch filter gray shades all cells that can see a digit 5. Box 7 (lower left) has only 2 cells that can't see a 5 and one of these already has the digit 2. Thus, digit 5 can be placed in the other, red cell r9c3.

Digits are placed using the cell menu to the right. Place the digit 5 in r9c3 · Rightclick on cell r9c3 to open the cell menu. · Select [Set Digit] / [Digit 5] to place the digit. |

To learn more about basic Sudoku and filters, see Introduction to Sudoku.

Solving with Candidates

Candidates are small numbers placed in cells to mark where it's possible to place a digit. Candidates are often used in solving puzzles and in almost all Sudoku solving methods. A good introduction and reference for Sudoku methods can be found at Sudopedia.

Here, we describe how candidates are used in Xsudo and just a little about Sudoku methods. See Basic Sudoku Solving Methods for an introduction to Sudoku methods using Xsudo.

Show candidates and highlight cells with the digit 3

· Click the hide/show icon in the toolbar to be without a red border.

· Select digit 3 in in the toolbar digit selector.

· Select the cell filter in the toolbar filter selector

The grid shows all possible candidates and highlights cells where digit 3 is possible.

Box 9 in the lower right has only one highlighted cell, which must therefore be a hidden single. Digit 3 can be placed in the cell.

Place the digit 3 in cell r8c7

· Rightclick cell r8c7 and and select [Set Digit] / [Digit 3] to place the digit.

Change to the candidate filter

· Select the candidate filter in the toolbar filter selector

· Make sure the digit 3 is selected in the digit selector.

The candidate filter can find logic patterns like the Locked Candidates in box 1 below. Row 2 has only two blue 3s one of which must be assigned to the row. Which ever is assigned is also in box 1 thus, the two red 3s in box 1 cannot be in the solution and can be removed. The next section explains how the candidates got colored.

Solving with Logic

Xsudo's specialty is logic, which allows a user to draw any ideas that come to mind and see if they work. Xsudo will understand any logic that can be drawn, even logic that is not part of a known Sudoku method. We will now enter the logic for the locked candidates shown above. First, show the candidates and select the auto link edit mode.

Show all candidates

· Select the cell filter in the toolbar filter selector

· Click digit 3 in the digit selector to deselect it.

Select the auto linking mode

· Select auto link mode [A] in the toolbar's edit mode selector

All candidates of one digit in a row, column, or box is called a truth becasue you know one of them must be true (part of the solution). One way to enter a truth is click on any two candidates one after the other.

Select the digit 3 truth in row 2

· Leftclick on digit 3 in row 2 column 1 (r2c1) to highlight the digit.

· Leftclick on the other digit 3 in row 2 (r2c3) to enter the truth.

The truth is a solid red bar connecting the two digit 3s in row 2. The truth is linked to the other 3s in the box that see it. As described above, if all 3s in the row are also in the box, then other 3s in the box can be eliminated. Candidates that can be eliminated are orange.

Remove the digit 3s in r2c1 and r2c2

· Press the remove candidates icon in the top toolbar .

Tutorial 1 discusses entering logic in more detail, Tutorial 4 is a quick-start tutorial about using the auto-link mode with larger logic like ALS. A good introduction and reference for Sudoku methods can be found at Sudopedia. Beware, there are lots of them.

Using the Solver, Finding an X-Wing

Xsudo can also solve the entire puzzle and show all the logicil steps in the solution. Here, we use the solver to solve the puzzle and find the most difficut step, called an X-Wing. For more details see The Slightly Human Solver. .

Open and run the Solver

· Press the open solver icon in the top toolbar .

· Make sure the Auto button is checked and Traditional is selected.

· Press Solve... to solve the puzzle.

Each line is read: no. of [clues.candidates] step (solutions found) Name T(no. of truths used)

Step 18 in the solver list (highlighted) shows the X-Wing, which requires two truths. To see the X-Wing, select step 18

Select step 18, the X-Wing

· Leftclick on line no.18, X-Wing to display the X-Wing

The X-Wing has two column truths (green) in columns 2 and 7. The two truths are linked together in a loop by red links in rows 1 and 9. The resoning goes this way. The two truths must contain 2 digit 1s, we just don't know which. The only way to arrange this is place one digit 1 in each row, which are links. Thus, the links cannot contain any other 1s and the red 1s can be removed.

You can find more information on X-Wing logic in Basic Sudoku Solving Methods. A good place to learn more about Sudoku solving methods is Sudopedia or Sudoku forums on the internet. Many puzzles from the interent (not all) be copied and pasted into Xsudo, see inputting puzzles and the copy/paste menu help page.

Have fun and, oh by the way, many Sudokus are easy to solve but there are Sudokus so difficult that no human or computer can find it's logicial solution. Here is an example of one such puzzle that was eventually solved with the help of Xsudo, The Illusion of Fata Morgana.