What Does ⍋⍋ Even Mean?

2 days ago

The article explains the meaning of the APL symbol (Grade Down Grade Up).
It defines Grade Up () as indexing items by rank in ascending order.
Grade Down Grade Up () leaves the pair <i, j> unchanged, effectively acting as a Rank function.
The notation <i, j> represents the i-th item having rank j, with negative signs indicating counting from the end.
Four combinations of Grade Up and Grade Down are characterized, showing how they relate to Ascending/Descending Rank and ReverseRank.
An example demonstrates the consistency: Rank + ReverseRank is constant unless items are equal.
The author notes that Paul Mansour exploits these properties in an AverageRank function, highlighting practical utility.

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