Default

Default[f]

gives the default value for arguments of the function f obtained with a _. pattern object.

Default[f,i]

gives the default value to use when _. appears as the i^(th) argument of f.

Default[f,i,n]

gives the default value for the i^(th) argument out of a total of n arguments.

Default[f,]=val

defines default values for arguments of f.

Details

  • _. represents an optional argument to a function, with a default value specified by Default.
  • The necessary values for Default[f] must always be defined before _. is used as an argument of f.
  • Values defined for Default[f] are stored in DefaultValues[f].
  • Default values only affect pattern matching and do not create any evaluation rules for f.

Examples

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Basic Examples  (2)

Define 0 to be the default argument for a function f:

_. represents an argument that can be omitted:

If the second argument is omitted, the default value of 0 is used instead:

If both arguments are omitted, both are taken to have the default value 0:

Define default values for the first and second arguments:

If an argument is omitted, the corresponding default value is used:

Scope  (1)

Define 2 to be the default argument for the pattern f[_.,_]:

Define 3 to be the default argument for the pattern f[_.,_,_]:

With a two-argument pattern, the default value of 2 is used for the omitted argument:

With a three-argument pattern, the default value of 3 is used for the omitted argument instead:

Properties & Relations  (4)

Assigning a value to Default[f] will define default arguments for all positions:

Assigning a value to Default[f] only affects pattern matching for f:

It does not on its own create any transformation rules for f[]:

Use a pattern involving Optional in the definition of f to create a default evaluation:

Defaults can be defined by immediate or delayed assignments:

Use ClearAll to clear default values:

Clear does not clear them:

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

Text

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

CMS

Wolfram Language. 1988. "Default." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/Default.html.

APA

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

BibTeX

@misc{reference.wolfram_2022_default, author="Wolfram Research", title="{Default}", year="1988", howpublished="\url{https://reference.wolfram.com/language/ref/Default.html}", note=[Accessed: 30-September-2022 ]}

BibLaTeX

@online{reference.wolfram_2022_default, organization={Wolfram Research}, title={Default}, year={1988}, url={https://reference.wolfram.com/language/ref/Default.html}, note=[Accessed: 30-September-2022 ]}