The case notes
Reading a 1-5-9 index
A digit in an indexed cell tells where that cell’s column number appears in the same row. In a 159 index, columns 1, 5 and 9 are marked. Watch a worked example, then try the idea in a puzzle.
Put it into practice
Indexed columns: 1 · 5 · 9
These shaded cells are an index. In this puzzle they sit in columns one, five and nine.
r2c5 = 3 → r2c3 = 5
Look at the five-column. It contains a three. That says: find the number five in column three of this same row.
r2c1 = 7 → r2c7 = 1
The same trick works in the one-column. Its seven points us to column seven, where we put one.
r2c9 = 2 → r2c2 = 9
And the nine-column holds two, so nine belongs in column two. The marked column and its digit trade places.
Marked column c holds v → column v holds c
The special part of a 1-5-9 index is where the marks sit. The indexing rule itself works in any marked column.
Read the transcript
These shaded cells are an index. In this puzzle they sit in columns one, five and nine. Look at the five-column. It contains a three. That says: find the number five in column three of this same row. The same trick works in the one-column. Its seven points us to column seven, where we put one. And the nine-column holds two, so nine belongs in column two. The marked column and its digit trade places. The special part of a 1-5-9 index is where the marks sit. The indexing rule itself works in any marked column.
