Begin With the Bypass

Sysudoku Basic starts with the slink marking bypass, so called because it defers even the marking of strong link partners to following Basic stages. Slinks are used extensively in the bypass, but not pencil marked. Subsets alone are pencil marked, because they reserve cells for subset values.

The bypass is a systematic process, allowing you to concentrate on simple tasks at the right times.  Most of the resources of the bypass you will discover by your own experience with it, but you can accelerate the process by reading some of the detailed traces on puzzles reviewed on this blog.  This page takes you through a detailed bypass of a 5-star Dave Green puzzle, to its collapse point. The walkthrough explains how you read a bypass trace, by showing how to write one.  For that we a briefing on the grid geography according to Sysudoku.

Here is how boxes and lines are named in Sysudoku. Box names are compass directions from a center box C. Rows are numbered North to South, and columns, West to East.

The bypass works by discovering where outside clues restrict  placements inside a box to one cell.  A box can contain only one clue of each value. Clues outside the box “sweep” the value from cells in the same line of the box. Outside clues on different lines can  coordinate to force a clue in the box, in two ways:

beginners-dublexOne bread-and-butter move of the bypass is the double line exclusion, or dublex. In the dublex, the same value sweeps two lines of two boxes in the same direction, forcing a placement in the third line  of the box they cross.  

Of course, new clues join right in, eliminating clues in other boxes. In all Sysudoku diagrams, added clues and pencil marks are in a script font, like the 1 above. Formal script numbers, like the other clues above, are givens, which define the puzzle.

beginners-crosshatchThe other bypass mainstay move is the crosshatch. Clues of the same value in two crossing lines  simultaneously sweep a box. Here,  E (East) and SW (yes, SouthWest)  1’s  sweep the W box, leaving only one cell for the W 1-clue, or as we name it in Sysudoku traces, W1. Where is W1?

Did I mention traces? A trace is a record of the solving actions. These pages will show you many advantages of tracing, when dealing with hard puzzles.  In Sysudoku traces, space is saved by not saying why or exactly where in the box the change occurs. Instead, the trace readers know, or figure out, why the change occurs. They post each placement to the grid as they read. In this case W1 goes into r6c1. On partial grids, you have to look where it is cut off. Was that your answer?

OK, let’s do the bypass with a trace on a 5-star puzzle. This one was offered in Dave Green’s the Akron Beacon Journal on December 10, 2017, a Sunday.

We sweep all boxes of each value 1 through 9 not already containing the value. When there are more than two clues of a number on the grid, we do this box by box, going West to East in each bank, North bank through South bank. With only two candidates there can only be one dublex or two crosshatches.

Starting with 1, we look at the crosshatch targets NE and S. Just this once, sweeps are denoted by arrows. Four NE cells can contain 1. In the S box, two candidates form a 1 -slink. Since one of these candidates is true, the aligned slink joins c4 in a dublex confining C1 to the intersection Cc6. Three cells, and we move on. We’ll write in the slinks in box marking.

Now check for clues and slinks for the value 2. There’s more than two, so you check the incoming on every box not containing 2. Cover this and compare your results with NE:  2, W: 3, C:2, SW: 4, SE3.  That’s the order of the scan for Sysudoku traces.

We have better luck with the 3’s, hitting NW3 immediately. Before looking at more boxes, however, we look for the effect of the newest clue.  It so happens that NW3 generates NE3 and W3, both by dublex.

We can start the bypass trace.


NW3 is a cause, and indented below are listed its immediate effects. We continue by treating each listed effect as a cause. Each event is described as a clue value in a box. Trace readers  post events to the grid as they read, keeping the grid up to date. Beyond that, the reader supplies the where and why. It’s a very compact record. Reading traces Sysudoku traces will make you a Sysudoku Basic expert. Writing them will enable you to track down and correct your mistakes.

The effects list should include any effects on earlier values, in this case, values 1 and 2. Effects on later values are usually ignored, because they show up in later lists. No need to check for the same event twice.

Note that W3 is posted above in pencil mark size. That marks it as an effect not yet used as a cause.

What are the NE3 effects? The dublex with E3 leaves two free cells. Two aligned free cells can be significant, but not here.  But looking in the other direction, there is something. NE3 leaves three free cells in r1, for three missing values. In Sysudoku it’s called a 3-fill.

A 3-fill is a box or line having only three cells free for placements. Here is the plaque on the Sysudoku office wall governing 3-fills.

Let’s call the first condition, the two-value rule; the second, the two-cell rule. These conditions produce surprising clues. Convince yourself that they work every time.

The 3-fill rule was adopted into Sysudoku basic in 2016, five years into the blog. Why do it? The 3-fill gets to the free cell squeeze sooner. One free cell means a clue, two free cells, a pair of cells reserved for two numbers. The 3-fill was moved into the bypass from the third basic stage, line marking.

You can extend the squeeze further with a 4-cell rule, with a three-number rule and a three-cell rule. That would require much more searching, for much less frequent clues. I think this puzzle tells us that we have the right balance.

In this case, the missing numbers are 4, 8 and 9. Neither condition is met, but what we can do is put a string of the missing numbers next to r1, the 3-fill unit, to alert us to any action that satisfies the 3-fill condition. It is represented in the trace by a 3-fill marker showing the unit and the missing numbers.

Still on the grid above, we turn now to W3, promote it to a cause, and find it has no immediate effects. Continuing the box scan, two free cells are left in C, two in S, and two in SE. On the 4’s, the single W4 has a chance because of the SE “wall”, SEr9 . W4 leaves an unwritten slink in  SE. The hidden dublex leaves another in SW.

Going on to the 5’s, a crosshatch N5 leaves a 3-fill in N. Missing numbers are 4, 8 and 9 again. Then as the box scan reaches SW, the SE wall’s unwritten aligned triple dublex/NW5 crosshatch produces SW5, taking one of those free celsl for SE4. You could interpret it as SW5 creating a 3-fill r9[148], and the given 4 seeing two of its cells.  We jump on the two remaining free cells of r9, where N1 places the missing 1.

Here is trace to this point.  How would you continue it?  Where does the S1 and S8 go? Who is the cause of the two-cell squeeze that now produces S7 and S3?

It will help if you write your own trace as well as maintaining your own grid. Sysudoku Basic is easy to do but is too organized and graphic to be easily conveyed in words.

Now S1 is considered the cause of S7 and S3, because C8 is already posted as an effect when S1 becomes a cause, S3 forms two dublexes for C3 and SE3.

Notice how the trace at this point becomes a guide to the next marking action, in a way that avoids overlooking any effects.

When the C3, SE3 and then S7 bringing no clues, we step back up to the last effect listed, but not examined as a cause. We pull it to the right, where there is space for an effects list, and continue. In this case, S8 is the next cause, and the c4 3-fill is noted, with a fill string on the grid, and a bracketed version in the trace. Since the 3-fill has no immediate effect, the “step up” is actually repeated, with the search for the next “pull out” reaching the top line of the trace, where original causes are added, and we are directed to look to see if SE5 is there yet. Follow that verbiage with your trace extension.  Check yourself out on the next full trace panel.

This exercise might have illustrated the advantages of solving and tracing with an office tool like ©PowerPoint. This advantage is even more striking when we get to doing the Advanced graphics you have been enjoying in the blog.

Going to the 6’s, place NW6 in the grid above, and you see three immediate effects,  SW6 and 3-fills in NW and NE.  The clue of the same number comes first. To list the effects first, then treat them in turn as causes, we list the 3-fills  in unexpanded form:

SW6 completes the 6’s and has no effect.

In the NW 3-fill, the first condition of the two-number rule applies, for NW1.

It’s a subset known as a naked pair. The two cells are reserved for 4 and 8 and these candidates will be the only two candidates of these two values in the box. Pencil marks are added at the top of the two cells because we have two box slinks.

In the trace, the products of a 3-fill resolution are listed as its effects. From this list, W1 produces a 3-fill in c3, and NW48 resolves the N box 3-fill, for five clues. Imagine all that, then check yourself below.

The N9 effects list ends with a NEr3 3-fill, actually a 3-fill in both NE and r3.  






Here is the trace, which you can now follow to the next grid.

We not finished exploring  the effects of the SE5 list, and have left behind many unexplored branches of the depth first tree.

As often happens, the 3-fill lists have stretched the trace across the page. Ordinarily, we could abandon the trace and fill in the remaining cells freestyle, but we one this one we may have trouble bridging the blank space in the middle bank.

The trace is continued here to the solution. Bypass generation of causes and effects continues in the same manner, clearly in collapse mode. You can continue solving and tracing from the diagram above, or continue by reading the trace as you post to the grid above.

This Dave Green 5-star featured a box configuration that is helpful to know about – the wall. Sometimes a number builds is own wall by sweeping the single cell missing from it.

As you read Sysudoku basic traces, you’ll become familiar with many more of these memorable box configurations. Here are two more.

A 2 by 2 square of clues creates slinks or clues from sweeps from either direction. Look along the passing row and column for clues or marks of numbers missing from the square. The square may include a pair or triple subset.





A sweep across a four-corner box of clues creates a slink in the other direction.  



The Sysudoku Basic bypass is enough to handle newspaper and magazine puzzles, up to a 5-star level. In harder puzzles, you are using bypass skills to follow up every removal or confirmation.  Next on the Guide path to advanced class puzzles are the pages on box marking and line marking.


12 Responses to Begin With the Bypass

  1. Roger says:

    Hi, I’m definatly an amateur when I comes to Suduko. I approach the puzzles with a different goal in mind – use an Ink pen and don’t put anything down on paper but the answer. While this eliminates me from the more advanced puzzles, it has helped me to exersize my mind and memory – which was my primary goal.

    • Sudent says:

      Thanks for the comment, Roger. You would enjoy The Nikoli puzzle books distributed in U.S. by Workman Publishers NY. Your strategy is probably one that computers use on all puzzles, backtracking trial and error. You go further than most of us can, and it does sharpen the mind. Let me know if you ever get pulled into sysudokie land, the subject of my blog. It’s a different world, equally fasinating.

  2. Prakash says:

    Since I just met you, I guess two months isn’t long to wait for a good secert revelation.I read a little of your blog and I love the excitement that you infuse regarding your book. It really must be a great time in your life.

    • Sudent says:

      Yes, Prakash, I’m having a great time. The only secret I am aware of is what the Sudoku experts that must be reading my blog by now make of it. If they have an audience, it’s past time to register an opinion.

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      Jeffrey, my theme is the WordPress default theme Twenty Ten. Most of the graphics come from the templates described on the Solving Templates page, with PowerPoint graphics shapes and tables added. My header is a slice from my travel photos. I’m happy you like the effect.

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    • Sudent says:

      Thanks, Estelle. Having good content does take much work, but if you use WordPress and stick with simple themes, posting is easy and programming knowledge is not required. Cost is minimal as well. So organize for a long posting narrative and go for it.

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