setindex

Setindex is a programming concept and, in some languages, a specific function name for assigning a value to a particular position within an indexed collection. The operation is defined for data structures that support indexing, such as arrays, lists, vectors, or other sequence types. In mutable containers, setindex typically updates the element in place; in immutable containers, the operation may return a new container with the updated element.

In practice, the mechanics vary by language. In Julia, the canonical form is setindex!(collection, value, index), which mutates the collection. The familiar syntax collection[index] = value is a shorthand that calls setindex! under the hood. In languages with strict immutability, such as some functional languages, a non-mutating variant may exist that returns a new collection with the specified element replaced, leaving the original unchanged. Other languages expose indexing assignment through operator or method forms, such as x[index] = value in Python or JavaScript, where the operation is typically implemented directly by the language’s runtime rather than a named function like setindex.

A related limitation is that setindex presumes the container has a defined notion of positions. For mathematical sets, which are unordered, indexing is not defined, and setindex has no meaning unless a wrapper or sequencing is imposed.

Common considerations include performance, with direct indexed updates often O(1) for arrays and O(n) or higher for more complex structures, and immutability implications, where updates produce new structures rather than altering existing ones. See also indexing, getindex, and setitem in language-specific contexts.