# Simple array

In the nested array model, a **simple** or **flat array** is an array whose elements are all simple scalars. A non-empty array is simple if and only if it has a depth of 0 (which makes it a scalar) or 1. Empty arrays are considered simple in most contexts, although they may have depth greater than 1 in languages that use the prototype to determine depth.

The simple arrays are a superset of the arrays allowed in flat array theory without boxes (or those which have depth 0 in flat array theory): they include all arrays of a single type, as well as arrays which mix types like numbers and characters. Arrays which would not be representable in flat array theory—those which contain a mixture of simple scalar types, or contain both simple scalars and other arrays—are called mixed.

The term "simple" is not typically used in flat APLs. On the APL Wiki, it may be used to mean a non-boxed array.

APL features [edit]
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Built-ins | Primitives (functions, operators) ∙ Quad name |

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

Data types | Number (Boolean, Complex number) ∙ Character (String) ∙ Box ∙ Namespace ∙ Function array |

Concepts and paradigms | Conformability (Scalar extension, Leading axis agreement) ∙ Scalar function (Pervasion) ∙ Identity element ∙ Complex floor ∙ Total array ordering ∙ Tacit programming (Function composition, Close composition) ∙ Glyph |

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