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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.

Details
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Examples  
Basic Examples  
Scope  
Generalizations & Extensions  
Applications  
Properties & Relations  
Possible Issues  
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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:

Scope  (1)

Elements of the list can be any expression:

Generalizations & Extensions  (1)

Use on an expression with head f:

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

Applications  (1)

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

Properties & Relations  (3)

LengthWhile gives the length of the result from TakeWhile:

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

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

Possible Issues  (1)

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

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_2025_lengthwhile, author="Wolfram Research", title="{LengthWhile}", year="2008", howpublished="\url{https://reference.wolfram.com/language/ref/LengthWhile.html}", note=[Accessed: 01-February-2026]}

BibLaTeX

@online{reference.wolfram_2025_lengthwhile, organization={Wolfram Research}, title={LengthWhile}, year={2008}, url={https://reference.wolfram.com/language/ref/LengthWhile.html}, note=[Accessed: 01-February-2026]}