Locked Single & Hidden Locked Single

This page deals with two completely different things —

• Locked Single

• Hidden Locked Single

I'm treating them together to emphasize the difference between them.

Locked Single

External synonyms:  naked locked single, naked single.

As a result of the constant updating of Candie Markup, it often occurs that there is (now) some Cell whose candies have all been killed except for one:  so you can crown that candie (promote it to a BigNum).

Why this sole remaining candie is called a Locked Single will make more sense to you after you've read about Locked Sets.

Examples:

Locked Single (two examples). The yellow Cell in Row 2 contains only one candie; so you can enter a BigNum 3 in that Cell. In the Middle Right Block, some preceding move has killed the 2s in the two indicated Cells, and the yellow Cell now contains only one candie (a 5):  so you can promote that 5 to a BigNum in that Cell.

In both the Locked Single examples above, as soon as the new BigNum is placed, its Lookalikes have to be killed in all the Cell's Buddies; that additional step is not explicitly shown.

This Tactic is perfectly obvious, and it occurs dozens of times as you work your way through a Sudoku.

(Sometimes people call a Locked Single a Naked Locked Single, when they want to emphasize the contrast with a Hidden Locked Single.)

 

Hidden Locked Single

External synonym:  hidden single.

It occurs frequently that a Cell has a bunch of live candies in it, but, in some specific House, it is the only Cell that will accept a 9:  so you are forced to ignore the other candies in that Cell and to enter a BigNum 9 in the Cell. (Similarly for an 8, or a 7, or a 6, . . .)

Why this you-need-me candie is called a Hidden Locked Single will make more sense to you after you've read about Hidden Locked Sets.

Examples:

Hidden Locked Single (two examples). In Column 2, the yellow Cell is the only Cell that contains a 6-candie. So that Column's 6 must be in this Cell, and you can crown that 6 (promote it to a BigNum). The other two candies in the Cell are irrelevant. In the Middle Right Block, the yellow Cell is the only one that contains a 1-candie, so you can fill this Cell with a 1.

In the first of these examples, when the 6 is entered as a BigNum in the yellow Cell of Column 2, all the other 6-candies in the Upper Left Block and in Row 2 have to be killed; this additional step is not explicitly shown. Similar remarks apply to the placement of the BigNum 1 in the yellow Cell of the Middle Right Block.

Hidden Locked Singles occur quite frequently, but it is easy to overlook them because the unique candie occurs buried in a bunch of other candies. If you are ever completely stuck for a move, check all the Rows, Columns, and Blocks for the presence of a Hidden Locked Single.

Since it took me awhile to realize just how important this Tactic is, I will take the liberty of nagging you about it from time to time in the discussion of other Tactics.