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LengthWhile
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LengthWhile

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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

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

Examples

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Basic Examples  (1)Summary of the most common use cases

Find the length of the run of elements below 10:

In[1]:=1
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https://wolfram.com/xid/0i1q71ng2-k02n18
Out[1]=1

Scope  (1)Survey of the scope of standard use cases

Elements of the list can be any expression:

In[1]:=1
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https://wolfram.com/xid/0i1q71ng2-he0846
Out[1]=1

Generalizations & Extensions  (1)Generalized and extended use cases

Use on an expression with head f:

In[1]:=1
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https://wolfram.com/xid/0i1q71ng2-ekj41q
Out[1]=1

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

In[2]:=2
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https://wolfram.com/xid/0i1q71ng2-cq20he
Out[2]=2

Applications  (1)Sample problems that can be solved with this function

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

In[1]:=1
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https://wolfram.com/xid/0i1q71ng2-eqapfd
Out[1]=1

Properties & Relations  (3)Properties of the function, and connections to other functions

LengthWhile gives the length of the result from TakeWhile:

In[1]:=1
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https://wolfram.com/xid/0i1q71ng2-pnmke6
Out[1]=1
In[2]:=2
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https://wolfram.com/xid/0i1q71ng2-jvtj06
Out[2]=2

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

In[1]:=1
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https://wolfram.com/xid/0i1q71ng2-c1hf6y
Out[1]=1
In[2]:=2
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https://wolfram.com/xid/0i1q71ng2-mjko01
Out[2]=2

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

In[1]:=1
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https://wolfram.com/xid/0i1q71ng2-dh9o0n
Out[1]=1
In[2]:=2
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https://wolfram.com/xid/0i1q71ng2-icprgk
Out[2]=2
In[3]:=3
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https://wolfram.com/xid/0i1q71ng2-gr4e2o
Out[3]=3

Possible Issues  (1)Common pitfalls and unexpected behavior

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

In[1]:=1
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https://wolfram.com/xid/0i1q71ng2-cy0dqn
Out[1]=1
In[2]:=2
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https://wolfram.com/xid/0i1q71ng2-cd9si1
Out[2]=2
Wolfram Research (2007), LengthWhile, Wolfram Language function, https://reference.wolfram.com/language/ref/LengthWhile.html (updated 2008).
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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).

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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.

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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

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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: 29-March-2025 ]}

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@misc{reference.wolfram_2025_lengthwhile, author="Wolfram Research", title="{LengthWhile}", year="2008", howpublished="\url{https://reference.wolfram.com/language/ref/LengthWhile.html}", note=[Accessed: 29-March-2025 ]}

BibLaTeX

@online{reference.wolfram_2025_lengthwhile, organization={Wolfram Research}, title={LengthWhile}, year={2008}, url={https://reference.wolfram.com/language/ref/LengthWhile.html}, note=[Accessed: 29-March-2025 ]}

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@online{reference.wolfram_2025_lengthwhile, organization={Wolfram Research}, title={LengthWhile}, year={2008}, url={https://reference.wolfram.com/language/ref/LengthWhile.html}, note=[Accessed: 29-March-2025 ]}