This post demonstrates how, in line marking, the partially line marked grid invites the error of using an incompletely marked cell as a completely marked one.
Every Beware in this review of Will Shortz’ Hard Sudoku v.1 highlights a different basic challenge. This illustrates why treating the basic stage seriously is a source of enjoyment not to be missed.
In Beware 155 most placements occur on a grid with five marked and four unmarked lines in each direction. Here is the grid as a long collapse begins with a hidden single N7 in c6.
In conventional basic, we would finish the line marking before seeking clues.
But its Sysudoku Basic, which continues from N5 as reported in this basic trace.
Now let’s examine the grid where the c6 naked pair 69 confirms N8. It does because c6 is line marked and we know the entire contents of r3c6. Looking over at r3c1, we might think we have a clue 8r2c2. But we don’t, because c1 is not marked, and we don’t know there is not an 8 candidate in r2c1.
We are not digital computers. Such a mistake is based on the type of assumption our brain makes every waking moment, just to navigate our clamorous world. The fill strings along the edges are a record of what is line marked and what is not. As we line mark, we must stay aware of the fill string record.
Moving down the trace a little further, we reach the point where the contents of r3c5 is settled, and the hidden single C8 can be declared.
But we trace it as c5hs8 because column 5 is the unit in which the value 8 has only one place.
Beware 155 is solved without marking another line via fill string.
Next comes Beware! Very Challenging 165 in the Will Shortz Hard Sudoku v1 review. Yes, another challenge of your line marking concentration. Or, if you’re a skeptic, try it your way and compare, with data.