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CountDistinctBy[{e1,e2,…},f]

gives the number of distinct values of f[ei] that occur.

CountDistinctBy[f]

represents an operator form of CountDistinctBy that can be applied to an expression.

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CountDistinctBy

CountDistinctBy[{e1,e2,…},f]

gives the number of distinct values of f[ei] that occur.

CountDistinctBy[f]

represents an operator form of CountDistinctBy that can be applied to an expression.

Details

Examples

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Basic Examples  (2)

Count the number of distinct first parts:

Wolfram Language code: CountDistinctBy[{{a, x}, {a, y}, {b, z}}, First]

Count the number of distinct first parts of the values of an association:

Wolfram Language code: CountDistinctBy[<|a -> {a, x}, b -> {a, y}, c -> {b, z}|>, First]

Use the operator form of CountDistinctBy:

Wolfram Language code: CountDistinctBy[First] @ {{a, x}, {a, y}, {b, z}}

Applications  (1)

Extract the words that occur in the text of Alice in Wonderland:

Wolfram Language code: words = StringSplit @ ExampleData[{"Text", "AliceInWonderland"}]; Length[words]

Count the number of distinct words, ignoring case:

Wolfram Language code: CountDistinctBy[words, ToLowerCase]

Properties & Relations  (2)

CountDistinctBy[list,f] gives the same result as CountDistinct[f/@list]:

Wolfram Language code: CountDistinctBy[{1, 2, 3, -2, -3}, Abs]
Wolfram Language code: CountDistinct[Abs /@ {1, 2, 3, -2, -3}]

CountDistinctBy[list,f] applies the transformation f to each element, then compares the results:

Wolfram Language code: CountDistinctBy[{{a, x}, {a, y}, {b, z}}, First]

CountDistinct[list,test] applies test to two elements to determine if they are equivalent:

Wolfram Language code: CountDistinct[{{a, x}, {a, y}, {b, z}}, First[#] == First[#2]&]
Wolfram Research (2014), CountDistinctBy, Wolfram Language function, https://reference.wolfram.com/language/ref/CountDistinctBy.html.

Text

Wolfram Research (2014), CountDistinctBy, Wolfram Language function, https://reference.wolfram.com/language/ref/CountDistinctBy.html.

CMS

Wolfram Language. 2014. "CountDistinctBy." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/CountDistinctBy.html.

APA

Wolfram Language. (2014). CountDistinctBy. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/CountDistinctBy.html

BibTeX

@misc{reference.wolfram_2026_countdistinctby, author="Wolfram Research", title="{CountDistinctBy}", year="2014", howpublished="\url{https://reference.wolfram.com/language/ref/CountDistinctBy.html}", note=[Accessed: 12-June-2026]}

BibLaTeX

@online{reference.wolfram_2026_countdistinctby, organization={Wolfram Research}, title={CountDistinctBy}, year={2014}, url={https://reference.wolfram.com/language/ref/CountDistinctBy.html}, note=[Accessed: 12-June-2026]}