Indexlist

Indexlist is a term used in computing to describe a sequence of indices that refer to elements in another list or array. An indexlist is typically an array or list of integers, though some languages support non-numeric index keys. It is used to select, reorder, or subset data, enabling operations such as gathering (extracting elements at the specified positions) and scattering (placing elements into positions indicated by the indexlist).

In one-dimensional contexts, an indexlist like [2, 4, 0] applied to [a, b, c, d, e] with zero-based indexing yields [c, e, a]. In multi-dimensional arrays, a separate indexlist per axis can implement advanced indexing, allowing source data to be rearranged along each dimension. Many programming languages implement this as vectorized or “fancy” indexing, often returning a new array rather than modifying the original in place.

Implementation and considerations vary by language. Valid indices are required; duplicates are typically allowed; out-of-range indices raise errors or are handled according to language rules. Performance depends on representation and the operation; some implementations perform gathering in linear time with respect to the size of the indexlist, while others may incur additional overhead for alignment or memory.

Applications of indexlists include data extraction, feature reordering in datasets, construction of permutation maps, and aligning data across different sources. They are commonly used in numerical computing, data processing, and array-oriented programming.

See also: permutation, fancy indexing, gather/scatter, indexing by arrays.