# Nested array

In nested array theory, a **nested array** is an array of depth greater than one, that is, an array that contains at least one element which is not a simple scalar. It is roughly equivalent to a boxed array in flat array theory; more precisely, non-mixed nested arrays correspond exactly to boxed arrays. Depending on language, an empty array may never be considered nested, or it may be considered nested if its prototype is nested.

Scalar functions descend into nested arrays one element at a time. Arrays can be nested at an arbitrary depth so this descent constitutes a traversal. Nested arrays form a tree structure with some additional information (the shape) at each node.

A nested array is a kind of pointer array and is almost always stored using pointers.

The result of Enclose on an array other than a simple scalar is always a nested array. The Dyalog APL primitive Nest (`⊆`

), which encloses a non-simple argument, is named for this property: it applies the smallest transformation to turn the argument (unless it is a simple scalar) into a nested array.

APL features [edit]
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Built-ins | Primitive function ∙ Primitive operator ∙ Quad name |

Array model | Shape ∙ Rank ∙ Depth ∙ Bound ∙ Index ∙ Axis ∙ Ravel ∙ Ravel order ∙ Element ∙ Scalar ∙ Vector ∙ Matrix ∙ Simple scalar ∙ Simple array ∙ Nested array ∙ Box ∙ Cell ∙ Major cell ∙ Subarray ∙ Empty array ∙ Prototype |

Concepts and paradigms | Leading axis theory ∙ Scalar extension ∙ Conformability ∙ Scalar function ∙ Pervasion ∙ Glyph ∙ Identity element ∙ Total array ordering |

Errors | LIMIT ERROR ∙ RANK ERROR ∙ SYNTAX ERROR ∙ DOMAIN ERROR ∙ LENGTH ERROR ∙ INDEX ERROR ∙ VALUE ERROR |