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`null`

Generate the void object of type DOM_NULL

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Syntax

```null()
```

Description

`null()` returns the void object of domain type `DOM_NULL`. It represents an empty sequence of MuPAD® expressions or statements.

The void object does not produce any output on the screen.

Various systems functions such as `print` or `reset` return the void object.

The void object is removed from sequences (“flattening”). It can be used to remove elements from lists or sets. Cf. Example 2.

Examples

Example 1

`null()` returns the void object which does not produce any screen output:

`null()`

The resulting object is of domain type `DOM_NULL`:

`domtype(null())`

This object represents the empty expression sequence and the empty statement sequence:

`domtype(_exprseq()), domtype(_stmtseq())`

Some system functions such as `print` return the void object:

`print("Hello world!"):`

`domtype(%)`

Example 2

The void object is removed from lists, sets, and expression sequences:

```[null(), a, b, null(), c], {null(), a, b, null(), c}, f(null(), a, b, null(), c)```

`a + null() + b = _plus(a, null(), b)`

`subsop([a, x, b], 2 = null()), subs({a, x, b}, x = null())`

However, `null()` is a valid entry in arrays and tables:

`a := array(1..2): a[1] := 1: a[2] := null(): a`

`domtype(a[1]), domtype(a[2])`

`t := table(null() = "void", 1 = 2.5, b = null())`

`domtype(t[b]), t[]`

`delete a, t:`

Example 3

The void object remains if you delete all elements from an expression sequence:

`a := (1, b): delete a[1]: delete a[1]: domtype(a)`

The operand function `op` returns the void object when applied to an object with no operands:

`domtype(op([])), domtype(op({})), domtype(op(f()))`

`delete a:`

Return Values

Void object of domain type `DOM_NULL`.