delete
, _delete
Delete the value of an identifier
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delete x_{1}, x_{2}, …
_delete(x_{1}, x_{2}, …
)
The statement delete x
deletes the value
of the identifier x
.
For many computations, symbolic variables are needed. E.g., solving an equation for an unknown x
requires
an identifier x
that does not have a value. If x
has
a value, the statement delete x
deletes the value
and x
can be used as a symbolic variable.
The statement delete x1, x2, ...
is equivalent
to the function call _delete(x1, x2, ...)
. The
values of all specified identifiers are deleted.
The statement delete x[j]
deletes the entry j
of
a list, an array,
an hfarray, or a table named x
.
Deletion of elements or entries reduces the size of lists and tables,
respectively.
If x
is an identifier carrying properties set
via assume
, then delete
x
detaches all properties from x
, i.e., delete
x
has the same effect as unassume(x)
.
Cf. Example 3.
The identifiers x
, y
are
assigned values. After deletion, the identifiers have no values any
longer:
x := 42: y := 7: delete x: x, y
delete y: x, y
More than one identifier can be deleted by one call:
a := b := c := 42: a, b, c
delete a, b, c: a, b, c
delete
can also be used to delete specific
elements of lists, arrays, hfarrays, and tables:
L := [7, 13, 42]
delete L[2]: L
A := array(1..3, [7, 13, 42])
delete A[2]: A, A[2]
T := table(1 = 7, 2 = 13, 3 = 42)
delete T[2]: T
Note that delete
does not evaluate the objects
that are to be deleted. In the following, an element of the list U
is
deleted. The original value of U
(the list L
)
is not changed:
U := L: delete U[1]: U, L
Finally, all assigned values are deleted:
delete U, L, A, T: U, L, A, T
delete
can also be used to delete properties
of identifiers set via assume
.
With the assumption “x > 1
”, the
expression ln(x)
has the property “ln(x)
> 0
”, i.e., its sign is 1:
assume(x > 1): sign(ln(x))
Without a property of x
, the function sign
cannot determine
the sign of ln(x)
:
delete x: sign(ln(x))

Void object of type DOM_NULL
.