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Basic Examples

Get the longest common prefix of strings:
In[]:=
[◼]
LongestCommonPrefix
[{"the cat in the hat","the cat insists"}]
Out[]=
the cat in
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[◼]
LongestCommonPrefix
[{"the cat in the hat","the cat insists","the catapult is inferior to the trebuchet"}]
Out[]=
the cat
Get the longest common prefix of lists:
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[◼]
LongestCommonPrefix
[{{1,2,3},{1,2,x}}]
Out[]=
{1,2}
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[◼]
LongestCommonPrefix
[{{a,b,c},{a,b,1,2},{a,b,"hello"}}]
Out[]=
{a,b}
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[◼]
LongestCommonPrefix
[{Fibonacci[Range[3,10]],Prime[Range[10]]}]
Out[]=
{2,3,5}
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Scope

Multiple arguments will be interpreted as a list:
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[◼]
LongestCommonPrefix
["the cat in the hat","the cat insists","the catastrophe"]
Out[]=
the cat
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[◼]
LongestCommonPrefix
[{1,2,3},{1,2,x}]
Out[]=
{1,2}
If given an
Association
,
LongestCommonPrefix
uses the values:
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[◼]
LongestCommonPrefix
[<|"a"{1,2,3},"b"{1,2,x}|>]
Out[]=
{1,2}
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Options

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IgnoreCase

By default,
LongestCommonPrefix
is case sensitive for strings:
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[◼]
LongestCommonPrefix
["Testing","TEST STRING"]
Out[]=
T
Set
IgnoreCase

True
to treat uppercase and lowercase letters as equivalent:
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[◼]
LongestCommonPrefix
["Testing","TEST STRING",IgnoreCaseTrue]
Out[]=
Test
When ignoring case, the result will match the case of the first string:
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[◼]
LongestCommonPrefix
["TEST STRING","Testing","tested",IgnoreCaseTrue]
Out[]=
TEST
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[◼]
LongestCommonPrefix
["tested","TEST STRING","Testing",IgnoreCaseTrue]
Out[]=
test
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[◼]
LongestCommonPrefix
["Testing","tested","TEST STRING",IgnoreCaseTrue]
Out[]=
Test

Method

Specify that a linear search algorithm should be used:
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[◼]
LongestCommonPrefix
[{"abc","abx"},Method"LinearSearch"]
Out[]=
ab
Use a binary search:
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[◼]
LongestCommonPrefix
[{"abc","abx"},Method"BinarySearch"]
Out[]=
ab
Choose a search algorithm automatically:
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[◼]
LongestCommonPrefix
[{"abc","abx"},MethodAutomatic]
Out[]=
ab
If prefixes are likely to be very short relative to the sequence length, a linear search can be a little faster:
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Short[strings="abc"<>#&/@RandomChoice[Alphabet[],{10000,1000}]]
Out[]//Short=
{abcswzzmxuyoucnzhhhijmu…qemiurscovcjabbseusynbzf,9998,…}
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AbsoluteTiming
[◼]
LongestCommonPrefix
[strings,Method"LinearSearch"]
Out[]=
{0.0055724,abc}
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AbsoluteTiming
[◼]
LongestCommonPrefix
[strings,Method"BinarySearch"]
Out[]=
{0.01693,abc}
Using
Automatic
resorts to a linear search in this case:
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AbsoluteTiming
[◼]
LongestCommonPrefix
[strings,MethodAutomatic]
Out[]=
{0.0060141,abc}
The worse-case performance of a linear search is much worse than a binary search however:
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prefix=StringJoin[ConstantArray[Alphabet[],50]];​​Short[strings=prefix<>#&/@RandomChoice[Alphabet[],{10000,1000}]]
Out[]//Short=
{abcdefghijklmnopqrstuvw…kfamtctcamhvqwtqbipjmrru,9998,…}
In[]:=
AbsoluteTiming
[◼]
LongestCommonPrefix
[strings,Method"LinearSearch"]//Short
Out[]//Short=
{2.10399,abcdefghijklmnopqrstu…fghijklmnopqrstuvwxyz}
In[]:=
AbsoluteTiming
[◼]
LongestCommonPrefix
[strings,Method"BinarySearch"]//Short
Out[]//Short=
{0.141262,abcdefghijklmnopqrst…fghijklmnopqrstuvwxyz}
Using
Automatic
resorts to a binary search in this case:
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AbsoluteTiming
[◼]
LongestCommonPrefix
[strings,MethodAutomatic]//Short
Out[]//Short=
{0.14541,abcdefghijklmnopqrst…fghijklmnopqrstuvwxyz}
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Applications

Find the common root directory for a list of files:
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files=RandomSample[FileNames["*",$InstallationDirectory,3],10]
Out[]=
{C:\Program Files\Wolfram Research\Mathematica\12.0\SystemFiles\Kernel\TextResources,C:\Program Files\Wolfram Research\Mathematica\12.0\SystemFiles\Formats\UnsignedInteger16,C:\Program Files\Wolfram Research\Mathematica\12.0\SystemFiles\Formats\ArcGRID,C:\Program Files\Wolfram Research\Mathematica\12.0\SystemFiles\CharacterEncodings\ISO8859-10.m,C:\Program Files\Wolfram Research\Mathematica\12.0\SystemFiles\Formats\Graph6,C:\Program Files\Wolfram Research\Mathematica\12.0\SystemFiles\Formats\MPS,C:\Program Files\Wolfram Research\Mathematica\12.0\SystemFiles\CharacterEncodings\EUC-JP.m,C:\Program Files\Wolfram Research\Mathematica\12.0\SystemFiles\Components\ArXiv,C:\Program Files\Wolfram Research\Mathematica\12.0\SystemFiles\Formats\UnsignedInteger32,C:\Program Files\Wolfram Research\Mathematica\12.0\SystemFiles\Formats\NEXUS}
In[]:=
root=
[◼]
LongestCommonPrefix
[files]
Out[]=
C:\Program Files\Wolfram Research\Mathematica\12.0\SystemFiles\
Show relative file names:
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StringDelete[files,StartOfString~~root]//Sort//Column
Find the common parent directory for a list of cloud objects:
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Properties and Relations

For a single string or list, the longest common prefix is simply itself:
For strings, an empty string is returned if no prefix is found:
For lists, an empty list is returned if no prefix is found:
If an empty string appears in the list of strings, the output will always be an empty string:
If an empty list appears in the list of lists, the output will always be an empty list:
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Possible Issues

Lists and strings cannot be mixed:
Since LongestCommonPrefix can return a list or string depending on the input, at least one element must be given to resolve ambiguity:
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Neat Examples

Find the lengths of longest common prefixes in random binary sequences of length 100:
Find words that share a prefix with “BirdSay”:
BirdSay the results: