Subassignment

Subassignment is a term used in programming and computer science to describe an assignment operation where the target is a subcomponent of a larger data structure rather than the entire variable. This typically involves selecting a part of a composite object—such as a subarray, a slice, or a field within a record or object—and assigning new values to that selected portion.

Contexts and forms

Subassignment appears in languages that support indexing, slicing, or field access. Common forms include assigning to a range of elements in an array or matrix (subarrays or submatrices), assigning to a segment of a string, or updating a specific field of a structured data type. In some languages, subassignment can also apply to nested or multi-dimensional structures, enabling changes deep within a data hierarchy without altering the whole object. The left-hand side of a subassignment typically involves an index, range, or path that specifies exactly which subcomponent will receive the new value.

Semantics and implications

Subassignment updates only the targeted portion while leaving the rest of the data structure intact. Depending on the language, subassignment can:

- extend or shrink the structure if the operation targets a dynamic subcomponent (for example, adding elements to a sparse array),

- enforce bounds or require compatible shapes or types for the assigned values,

- be in-place, returning a reference to the modified structure or the subcomponent, or produce a new object depending on mutability rules.

Examples

In many languages, subassignment appears as array or matrix slicing, such as assigning to a range of elements rather than the entire array, or setting a nested field within an object. For instance, a[2:5] = b[2:5] or obj.field.subfield = value demonstrates targeting a portion of a larger data structure.

See also

Subscript, slice, field access, assignment operator.