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LengthWhile[list,crit]

gives the number of contiguous elements ei starting at the beginning of list for which crit[ei] is True.

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Basic Examples  
Scope  
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LengthWhile

LengthWhile[list,crit]

gives the number of contiguous elements ei starting at the beginning of list for which crit[ei] is True.

Details

  • LengthWhile can be used on an object with any head, not necessarily List.

Examples

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Basic Examples  (1)

Find the length of the run of elements below 10:

Wolfram Language code: LengthWhile[{1, 1, 2, 3, 5, 8, 13, 21}, # < 10&]

Scope  (1)

Elements of the list can be any expression:

Wolfram Language code: LengthWhile[{a, Pi, 3, 2, {1, 2, 3}, {a, b}, 10}, Head[#] =!= List&]

Generalizations & Extensions  (1)

Use on an expression with head f:

Wolfram Language code: LengthWhile[f[1, 1, 2, 3, a, 8, 13, 21], IntegerQ]

The result is the same as for a list of the values:

Wolfram Language code: LengthWhile[{1, 1, 2, 3, a, 8, 13, 21}, IntegerQ]

Applications  (1)

Find the length of the run of digits in before a 7 occurs:

Wolfram Language code: LengthWhile[First[RealDigits[Pi, 10, 100]], # != 7&]

Properties & Relations  (3)

LengthWhile gives the length of the result from TakeWhile:

Wolfram Language code: TakeWhile[{1, 1, 2, 3, 5, 8, 13, 21}, # < 10&]
Wolfram Language code: LengthWhile[{1, 1, 2, 3, 5, 8, 13, 21}, # < 10&]

LengthWhile is equal to Length if crit[ei] is True for all elements:

Wolfram Language code: LengthWhile[{1, 1, 2, 3, 5, 8, 13, 21}, NumberQ]
Wolfram Language code: Length[{1, 1, 2, 3, 5, 8, 13, 21}]

Otherwise it is 1 less than the first position for which crit[ei] is not True:

Wolfram Language code: data = {1, 1, 2, 3, 5, 8, 13, 21}
Wolfram Language code: LengthWhile[data, OddQ]
Wolfram Language code: Position[Map[OddQ, data], _ ? (Not[TrueQ[#]]&), Heads -> False][[1, 1]] - 1

Possible Issues  (1)

The count terminates for any value of crit[ei] that is not True:

Wolfram Language code: LengthWhile[{E, 8, a, b, 20, 1.4}, If[NumericQ[#], True, #]&]
Wolfram Language code: Map[If[NumericQ[#], True, #]&, {E, 8, a, b, 20, 1.4}]
Wolfram Research (2007), LengthWhile, Wolfram Language function, https://reference.wolfram.com/language/ref/LengthWhile.html (updated 2008).

Text

Wolfram Research (2007), LengthWhile, Wolfram Language function, https://reference.wolfram.com/language/ref/LengthWhile.html (updated 2008).

CMS

Wolfram Language. 2007. "LengthWhile." Wolfram Language & System Documentation Center. Wolfram Research. Last Modified 2008. https://reference.wolfram.com/language/ref/LengthWhile.html.

APA

Wolfram Language. (2007). LengthWhile. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/LengthWhile.html

BibTeX

@misc{reference.wolfram_2026_lengthwhile, author="Wolfram Research", title="{LengthWhile}", year="2008", howpublished="\url{https://reference.wolfram.com/language/ref/LengthWhile.html}", note=[Accessed: 12-June-2026]}

BibLaTeX

@online{reference.wolfram_2026_lengthwhile, organization={Wolfram Research}, title={LengthWhile}, year={2008}, url={https://reference.wolfram.com/language/ref/LengthWhile.html}, note=[Accessed: 12-June-2026]}