sudokUI › Sudoku techniques › Grouped Nice Loop
Grouped Nice Loop in sudoku
A Nice Loop with at least one group node: two or three cells holding candidate X in one box and one row or column, true if any becomes X.
Why it works
With perfect alternation one end of every weak link is true, eliminating any candidate weakly linked to both its ends. A node where two strong links meet is true: a candidate is placed, a group removes X from outside cells seeing all its cells.
How to spot it
Where a unit's only places for X are a group plus one other cell or group, use that strong link to extend a loop.
Worked example
![Grouped Nice Loop example on a sudoku board. Grouped Nice Loop: the continuous loop 7@r5c9 → 7@r7c9 → 7@r7c7 → 8@r7c7 → 8@[r5c7, r6c7] → 8@r5c9 alternates perfectly; every weak link has exactly one true side, clearing candidates along it. So 1 is removed from r5c9; 8 is removed from r9c7.](/learn/img/grouped-nice-loop.svg)
Grouped Nice Loop: the continuous loop 7@r5c9 → 7@r7c9 → 7@r7c7 → 8@r7c7 → 8@[r5c7, r6c7] → 8@r5c9 alternates perfectly; every weak link has exactly one true side, clearing candidates along it. So 1 is removed from r5c9; 8 is removed from r9c7.
Practise it on a real puzzle. sudokUI generates a puzzle that needs Grouped Nice Loop and takes you straight to the position where it applies. Hints draw the pattern on the board and explain the step.
In a puzzle's rating
Each time the solver needs Grouped Nice Loop, the puzzle's difficulty rating grows by 300. Its class is Unfair. It is needed in 1 in 160 puzzles that sudokUI generates. How the rating works.
More chains and loops
Remote Pair · Chute Remote Pair · X-Chain · X-Cycles (loops) · Grouped X-Cycles · XY-Chain · Twinned XY-Chains · Nice Loop · Alternating Inference Chain · AIC with Groups · AIC with ALSs