Longest common substring for more than two strings in PowerShell? - powershell

Question

how can I find the matching strings in an array of strings in PowerShell:
example:
$Arr = "1 string first",
"2 string second",
"3 string third",
"4 string fourth"
Using this example, I want this returned:
" string "
I want to use this to find matching parts of file names and then remove that part of the file name (like removing the artist's name from a set of mp3 files for example), without having to specify which part of the file name should be replaced manually.

Answer 1

$arr = "qdfbsqds", "fbsqdt", "bsqda"
$arr | %{
$substr = for ($s = 0; $s -lt $_.length; $s++) {
for ($l = 1; $l -le ($_.length - $s); $l++) {
$_.substring($s, $l);
}
}
$substr | %{$_.toLower()} | select -unique
} | group | ?{$_.count -eq $arr.length} | sort {$_.name.length} | select -expand name -l 1
# returns bsqd
produce a list of all the unique substrings of the inputstrings
filter for substrings that occur #inputstrings times (i.e. in all input strings)
sort these filtered substrings based on the length of the substring
return the last (i.e. longest) of this list

Answer 2

If it ( the artist name etc) is only going to be a single word:
$Arr = "1 string first", "2 string second", "3 string third", "4 string fourth"
$common = $Arr | %{ $_.split() } | group | sort -property count | select -last 1 | select -expand name
$common = " {0} " -f $common
Update:
Implementation that seems to work for multiple words ( finding the longest common substring of words):
$arr = "1 string a first", "2 string a second", "3 string a third", "4 string a fourth"
$common = $arr | %{
$words = $_.split()
$noOfWords = $words.length
for($i=0;$i -lt $noOfWords;$i++){
for($j=$i;$j -lt $noOfWords;$j++){
$words[$i..$j] -join " "
}
}
} | group | sort -property count,name | select -last 1 | select -expand name
$common = " {0} " -f $common
$common

Answer 3

Here is a "Longest Common Substring" function for two strings in PowerShell (based on wikibooks C# example):
Function get-LongestCommonSubstring
{
Param(
[string]$String1,
[string]$String2
)
if((!$String1) -or (!$String2)){Break}
# .Net Two dimensional Array:
$Num = New-Object 'object[,]' $String1.Length, $String2.Length
[int]$maxlen = 0
[int]$lastSubsBegin = 0
$sequenceBuilder = New-Object -TypeName "System.Text.StringBuilder"
for ([int]$i = 0; $i -lt $String1.Length; $i++)
{
for ([int]$j = 0; $j -lt $String2.Length; $j++)
{
if ($String1[$i] -ne $String2[$j])
{
$Num[$i, $j] = 0
}else{
if (($i -eq 0) -or ($j -eq 0))
{
$Num[$i, $j] = 1
}else{
$Num[$i, $j] = 1 + $Num[($i - 1), ($j - 1)]
}
if ($Num[$i, $j] -gt $maxlen)
{
$maxlen = $Num[$i, $j]
[int]$thisSubsBegin = $i - $Num[$i, $j] + 1
if($lastSubsBegin -eq $thisSubsBegin)
{#if the current LCS is the same as the last time this block ran
[void]$sequenceBuilder.Append($String1[$i]);
}else{ #this block resets the string builder if a different LCS is found
$lastSubsBegin = $thisSubsBegin
$sequenceBuilder.Length = 0 #clear it
[void]$sequenceBuilder.Append($String1.Substring($lastSubsBegin, (($i + 1) - $lastSubsBegin)))
}
}
}
}
}
return $sequenceBuilder.ToString()
}
To use this for more than two strings, use it like this:
Function get-LongestCommonSubstringArray
{
Param(
[Parameter(Position=0, Mandatory=$True)][Array]$Array
)
$PreviousSubString = $Null
$LongestCommonSubstring = $Null
foreach($SubString in $Array)
{
if($LongestCommonSubstring)
{
$LongestCommonSubstring = get-LongestCommonSubstring $SubString $LongestCommonSubstring
write-verbose "Consequtive diff: $LongestCommonSubstring"
}else{
if($PreviousSubString)
{
$LongestCommonSubstring = get-LongestCommonSubstring $SubString $PreviousSubString
write-verbose "first one diff: $LongestCommonSubstring"
}else{
$PreviousSubString = $SubString
write-verbose "No PreviousSubstring yet, setting it to: $PreviousSubString"
}
}
}
Return $LongestCommonSubstring
}
get-LongestCommonSubstringArray $Arr -verbose

Answer 4

If I understand your question:
$Arr = "1 string first", "2 string second", "3 string third", "4 string fourth"
$Arr -match " string " | foreach {$_ -replace " string ", " "}