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lhs=.

removes any rules defined for lhs.

Details
Details and Options Details and Options
Examples  
Basic Examples  
Scope  
Properties & Relations  
Possible Issues  
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Tech Notes
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Unset

lhs=.

removes any rules defined for lhs.

Details

  • Rules are removed only when their left‐hand sides are identical to lhs, up to renaming of patterns.
  • Unset[Once[expr]] removes any stored value for Once[expr].

Examples

open all close all

Basic Examples  (2)

Define a value:

Wolfram Language code: x = 5;

Undefine it:

Wolfram Language code: x=.

Check that the definition was cleared:

Wolfram Language code: x

Undefine a function:

Wolfram Language code: f[x_] := x ^ 2
Wolfram Language code: f[x_]=.

Scope  (2)

Set the value of a variable:

Wolfram Language code: x = 5; x

Clear it:

Wolfram Language code: x=. x

Create a downvalue for f:

Wolfram Language code: f[x_] := x ^ 2

Clear functions (downvalues):

Wolfram Language code: f[x_]=.

Check that it was cleared:

Wolfram Language code: f[5]

Properties & Relations  (2)

Create definitions for a factorial computation:

Wolfram Language code: fact[1] = 1; fact[n_] := n fact[n - 1]

Use =. to clear definitions with a particular left-hand side:

Wolfram Language code: fact[1]=. Definition[fact]

Clear all definitions:

Wolfram Language code: Clear[fact] Definition[fact]

Use Once[expr] to evaluate an expression just one time:

Wolfram Language code: Once[Print["again"];RandomReal[]]

Reevaluating shows that it does not change:

Wolfram Language code: Once[Print["again"];RandomReal[]]

Use Unset[Once[expr]] to clear a remembered value of Once:

Wolfram Language code: Unset[Once[Print["again"];RandomReal[]]] Once[Print["again"];RandomReal[]]

Possible Issues  (1)

Define an upvalue:

Wolfram Language code: h/:f[h[x_]] := fh[x]

To clear an upvalue, the tag has to be given:

Wolfram Language code: h/:f[h[x_]]=.

Check that it was cleared:

Wolfram Language code: Definition[h]
Wolfram Research (1988), Unset, Wolfram Language function, https://reference.wolfram.com/language/ref/Unset.html (updated 2016).

Text

Wolfram Research (1988), Unset, Wolfram Language function, https://reference.wolfram.com/language/ref/Unset.html (updated 2016).

CMS

Wolfram Language. 1988. "Unset." Wolfram Language & System Documentation Center. Wolfram Research. Last Modified 2016. https://reference.wolfram.com/language/ref/Unset.html.

APA

Wolfram Language. (1988). Unset. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/Unset.html

BibTeX

@misc{reference.wolfram_2026_unset, author="Wolfram Research", title="{Unset}", year="2016", howpublished="\url{https://reference.wolfram.com/language/ref/Unset.html}", note=[Accessed: 12-June-2026]}

BibLaTeX

@online{reference.wolfram_2026_unset, organization={Wolfram Research}, title={Unset}, year={2016}, url={https://reference.wolfram.com/language/ref/Unset.html}, note=[Accessed: 12-June-2026]}