null -- generate the void object of
type DOM_NULL
Introductionnull() returns the void object of domain
type DOM_NULL.
Call(s)null()
Returnsthe void object of domain type DOM_NULL.
Related
Functions
Detailsnull() returns the only object of domain
type DOM_NULL. It
represents an empty sequence of MuPAD expressions or
statements.print or reset return the void object.null is a function of the system kernel.
Example
1null() returns the void object
which does not produce any screen output:
>> null()
The resulting object is of domain type DOM_NULL:
>> domtype(null())
DOM_NULL
This object represents the empty expression sequence and the empty statement sequence:
>> domtype(_exprseq()), domtype(_stmtseq())
DOM_NULL, DOM_NULL
Some system functions such as print return the void object:
>> print("Hello world!"):
"Hello world!"
>> domtype(%)
DOM_NULL
Example
2The 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, b, c], {a, b, c}, f(a, b, c)
>> a + null() + b = _plus(a, null(), b)
a + b = a + b
>> subsop([a, x, b], 2 = null()), subs({a, x, b}, x = null())
[a, b], {a, b}
However, null() is a valid
entry in arrays and tables:
>> a := array(1..2): a[1] := 1: a[2] := null(): a
+- -+
| 1, null() |
+- -+
>> domtype(a[1]), domtype(a[2])
DOM_INT, DOM_NULL
>> t := table(null() = "void", 1 = 2.5, b = null())
table(
b = null(),
1 = 2.5,
null() = "void"
)
>> domtype(t[b]), t[]
DOM_NULL, "void"
>> delete a, t:
Example
3The void object remains if you delete all elements from an expression sequence:
>> a := (1, b): delete a[1]: delete a[1]: domtype(a)
DOM_NULL
The operand function op returns the void object when applied
to an object with no operands:
>> domtype(op([])), domtype(op({})), domtype(op(f()))
DOM_NULL, DOM_NULL, DOM_NULL
>> delete a: