Sparse format (metadata)
Beginner
A small way to store a table full of zeros. Keep only the nonzero numbers, plus a note of where each one goes.
Novice
A storage layout that keeps nonzero values and an index (metadata) saying where each sits. Common ones are CSR and CSC (compressed sparse row/column). The index costs memory and lookup time.
Expert
CSR/CSC store a column (or row) index per nonzero plus pointers per row; with 8-bit values and 16-bit indices the metadata is 200% of the value storage. 2:4 needs only a 2-bit position per kept value; block formats need one index per block.
Explained in Sparsity, in-memory and analog compute (Architectures).
See also: Structured sparsity.