Set (data structure) in PowerShell

Question

Is there a way the define a Set data-structure in PowerShell?

In computer science, a set is an abstract data type that can store certain values, without any particular order, and no repeated values. It is a computer implementation of the mathematical concept of a finite set. Unlike most other collection types, rather than retrieving a specific element from a set, one typically tests a value for membership in a set.

I need to use a data structure as keystore that:

assures no-repetitions;
minimizes the computational effort in order to retrieve and remove an element.

user2555451 · Accepted Answer · 2015-03-18 16:11:04Z

76

You can use the .NET HashSet class that is found under System.Collections.Generic:

$set = New-Object System.Collections.Generic.HashSet[int]

The collection guarantees unique items and the Add, Remove, and Contains methods all operate with O(1) complexity on average.

answered Mar 18, 2015 at 16:11

user2555451

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Ohad Schneider · Accepted Answer · 2019-04-05 08:31:10Z

32

If you prefer to stick with native PowerShell types, you can use HashTable and just ignore the key values:

# Initialize the set
$set = @{}

# Add an item
$set.Add("foo", $true)

# Or, if you prefer add/update semantics
$set["foo"] = $true

# Check if item exists
if ($set.Contains("foo"))
{
    echo "exists"
}

# Remove item
$set.Remove("foo")

For more information see: https://powershellexplained.com/2016-11-06-powershell-hashtable-everything-you-wanted-to-know-about/#removing-and-clearing-keys

edited Apr 5, 2019 at 8:31

answered Jan 5, 2019 at 15:50

Ohad Schneider

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4 Comments

K. Frank Over a year ago

At least with powershell core $set.Add("foo", $true) throws an exception if the key already exists. Therefore $set["foo"] = $true should be used instead, as it not only allows creating new keys but also to update existing ones.

Ohad Schneider Over a year ago

@K.Frank that really depends on the behavior you're after - you might very well want that exception is certain scenarios. I will conceded that it's more consistent with the Remove behavior (which doesn't throw if the item doesn't exist), so I'll add it to my answer - thanks!

Timo Over a year ago

So HashTable and HashSet is similar in PowerShell: No duplicate keys?

Ohad Schneider Over a year ago

@Timo PowerShell types are .NET types. Since they are similar in that regard (no duplicate keys) in .NET, this similarity is the same in PowerShell: learn.microsoft.com/en-us/dotnet/api/…

Ghan · Accepted Answer · 2020-06-09 11:56:43Z

10

Hashset is what you are looking for if you want to store only unique values in an array with relatively faster add, remove and find operations. It can be created as -

$set = [System.Collections.Generic.HashSet[int]]@()

answered Jun 9, 2020 at 11:56

Ghan

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1 Comment

dan-gph Over a year ago

Can shorten slightly by omitting the System namespace: $set = [Collections.Generic.HashSet[int]]@() .

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