Using the Mouse Controls in SudokuXP

The mouse controls in SudokuXP let you complete puzzles without using the keyboard. They also provide handy shortcuts for maintaining candidate lists, and an auto-crosshatching feature that draws the crosshatch lines for you.

Switching mouse controls on
The mouse control pads
Updating squares
Updating candidate lists
Colouring squares
Flagging Candidates


Switching mouse controls on

When SudokuXP opens, it's in Keyboard mode:

Keyboard mode works like previous versions of the page - you can type and edit the values and candidates in non-fixed squares (i.e. those that aren't in black type) by clicking on them and typing.

To switch to mouse mode, click on the word 'Mouse':

This makes the mouse controls pads visible.

(Note - you can make the page open in mouse mode automatically by choosing 'Change my settings' from the SudokuXP main menu, then chooing 'Mouse' in the 'Page preferences' section.)

In mouse mode, you work by choosing a value and an action, then clicking on the puzzle square where you want it to happen. For example, to place a 9 in the top-left square of the puzzle, you'd click '9' and 'Update square', then click on the top-left square. 

Once you've switched to mouse mode, clicking on a puzzle square will perform an action, such as placing a value in it.

You can switch back to keyboard mode by clicking 'Keyboard'. You can switch between keyboard and mouse modes at any time.

The mouse control pads

The mouse control area contains two types of button:

The areas shaded red in this picture are the action buttons. These make things happen, for example updating a square's value or showing a flag in all squares with a particular candidate.

Some actions need you to choose a value from the values pad (the area shaded green). For example, to place a value in a square, you click Update Square, then click the value you want to place in the square (the value button you've chosen changes to the same colour as the Update Square button). Then you go to the puzzle grid and click on the square in which you want to place the value.

Other actions don't need a value - for example Clear (empty) square, and the four square-colouring buttons (Blue, Green, Yellow and Red). It doesn't matter which value button is highlighted when you use these actions.  

If you choose a different set of symbols, the values pad changes to show those symbols, like this:


Updating squares

The first two actions fill and empty puzzle squares. They have no effect if you click on a fixed (black-type) square.

This places the selected value in any non-fixed puzzle square you click on. For example, to place a 9 in the top-left puzzle square, you'd set the mouse controls like this:

Then click on the top-left square of the puzzle, like this:


This empties any square you click on, returning it to a blank value. For example, to clear the top-left square, set the mouse controls like  like this:

(Note - there's no need to select a value - any will do.)

Now click on the top-left square, like this:


You can undo changes you've made to square values by pressing the undo button:

Pressing the button multiple times will undo multiple changes, back to the start of the puzzle.


Updating candidate lists

The next two actions are useful if you're maintaining your own candidate lists (manual candidates).

This adds the selected value to the candidate lists of any squares you click on. For example, to add 4 to a square's candidate list, set the mouse controls like this:

Then click anywhere on the square, like this:


Note that you don't have to click in the candidate list itself - anywhere on the square will do. The value is automatically placed in its correct position among the other candidates.

It doesn't matter if you click on a square that already has this candidate value - the click will just be ignored.

This removes the selected value from the candidate lists of any squares you click on. For example, to remove 4 from a square's candidate list, set the mouse controls like this:

Then click anywhere on the square, like this:


You don't have to click in the candidate list itself - anywhere on the square will do.

It doesn't matter if you click on a square that doesn't have this candidate value - the click will just be ignored.

Tip - to maintain manual candidate lists, when you've put a value in a square, choose (leaving the same value selected) then click on any squares in the same row, column and box which contain that candidate value.



Crosshatching is a technique for finding which square within a box (3X3 or 4X4 squares) should contain a particular value. For a full description of crosshatching, see How to solve sudoku.

This action crosshatches any box you click on for the currently selected value. For example, to crosshatch a box for 4, set the mouse controls like this:

then click on any square in the box, like this:

Crosshatching lines are drawn to show you which squares in the box are prevented from containing the selected value, because that value already appears in the same row or column:


Tips for using crosshatching with other actions.


Colouring squares

SudokuXP's Hint system uses coloured square backgrounds to show patterns of squares such as pairs, X-Wings and Forcing Chains. You can also give squares coloured backgrounds manually, to help you find patterns yourself.  SudokuXP provides four colours - Blue, Green Yellow and Red.

To colour a square, click on a colour button:


There's no need to select a value.

Now click on the square you want to colour:

  Clicking again removes the colour from the square...

Clicking the Clear Highlights action button will remove colour from all the squares in the puzzle.


Flagging Candidates

Many advanced solving patterns (for example X-Wings and Conjugate Pair Chains) depend on finding squares in the puzzle that contain a particular candidate (possible) number. SudokuXP will show you all the squares that contain a particular candidate by placing a flag image in each one.

To flag a candidate:

Click the candidate value you want to flag, then click Flag Candidate (you can do this in either order).

In the puzzle grid, squares with your chosen candidate now contain a flag. To flag a different candidate, click a different value in the values pad, and the flags will change.

Candidate flags work with auto and manual candidates, and adjust automatically as your candidate lists change (so if you fill a square with the currently flagged value and the value is removed from candidate lists in related squares, their flags will disappear too). If you're using manual candidates, the flags will represent whatever's in your lists - if you've got your lists wrong, the flags will be wrong too.

If you like, you can have flagged squares coloured green as well, to add to their visibility. To do this, choose this option from the Puzzle Info button menus:


Flagged squares now have green backgrounds, like this:

To switch background colouring off, choose the same menu option again.

If you're using coloured backgrounds for flagged squares, avoid using green as a manually-applied background colour (see 'Colouring Squares' above). If you give a square a non-green colour, then this colour will not be overwritten by the candidate flagging system.

An example of using candidate flagging and square colouring to find an advanced pattern.

To find a Conjugate Pair Chain, you need to find rows, columns and boxes in which a particular candidate occurs in exactly two squares. These two squares are called a 'conjugate pair'. In the extreme puzzle (id 3503) we've flagged candidate 2:

To build a chain of conjugate pairs, you need to find places where one of the squares in a conjugate pair is also half of a conjugate pair on the same candidate in one (or both) of its other two areas. As you build the chain, give the squares alternating coloured backgrounds, like this (we've used blue and red):

The first conjugate pair (in the same box) is r5c1-r4c3 . r4c3 is also in conjugate pairs with r4c6 and r7c3 (i.e. row 4 and column 3 each contain exactly two flags). r4c6 is also in a conjugate pair with r5c4, which in turn is in a conjugate pair with r7c4. Applying alternating blue and red backgrounds at each step in the chains leaves us with two blue squares in row 7. This means that blue is the 'wrong' colour, and all red squares can be set to 2.


Paul Stephens, 2005. All rights reserved.