I often see the usage of # symbol in Powershell coding. Want to ask what is it used for? Example as below
$DistributionPointGroups = #("London")
#() is the array operator, which makes sure that even a single item (or zero) is returned as an array.
THE ARRAY SUB-EXPRESSION OPERATOR
The array sub-expression operator creates an array, even if it
contains zero or one object.
The syntax of the array operator is as follows:
#( ... )
You can use the array operator to create an array of zero or
one object.
PS C:\>$a = #("One")
PS C:\>$a.Count
1
PS C:\>$b = #()
PS C:\>$b.Count
0
Source: about_Arrays
Related
Can an array be used as the key in a hashtable? How can I reference the hashtable item with an array key?
PS C:\> $h = #{}
PS C:\> $h[#(1,2)] = 'a'
PS C:\> $h
Name Value
---- -----
{1, 2} a # looks like the key is a hash
PS C:\> $h[#(1,2)] # no hash entry
PS C:\> $h.Keys #
1
2
PS C:\> $h[#(1,2)] -eq 'a'
PS C:\> $h[#(1,2)] -eq 'b'
PS C:\> foreach ($key in $h.Keys) { $key.GetType() } # this is promising
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True Object[] System.Array
PS C:\> $PSVersionTable.PSVersion.ToString()
7.1.4
While you can use arrays as hashtable keys, doing so is impractical:
Update: There is a way to make arrays work as hashtable keys, but it requires nontrivial effort during construction of the hashtable - see this answer.
You'll have the use the very same array instances as both the keys and for later lookups.
The reason is that arrays, which are instances of .NET reference types (as opposed to value types such as integers), use the default implementation of the .GetHashCode() method to return a hash code (as used in hashtables), and this default implementation returns a different code for each instance - even for two array instances that one would intuitively think of as "the same".
In other words: you'll run into the same problem trying to use instances of any such .NET reference type as hashtable keys, including other collection types - unless a given type happens to have a custom .GetHashCode() implementation that explicitly considers distinct instances equal based on their content.
Additionally, it makes use of PowerShell's indexer syntax ([...]) awkward, because the array instance must be nested, with the unary form of ,, the array constructor operator. However, dot notation (property access) works as usual.
$h = #{}
# The array-valued key.
$key = 1, 2
$h[$key] = 'a'
# IMPORTANT:
# The following lookups work, but only because
# the *very same array instance* is used for the lookup.
# Nesting required so that PowerShell doesn't think that
# *multiple* keys are being looked up.
$h[, $key]
# Dot notation works normally.
$h.$key
# Does NOT work, because a *different array instance* is used.
$h.#(1,2)
A simple test for whether a given expression results in the same hashtable lookup every time and is therefore suitable as a key is to call the .GetHashCode() method on it repeatedly; only if the same number is returned every time (in a given session) can the expression be used:
# Returns *different* numbers.
#(1, 2).GetHashCode()
#(1, 2).GetHashCode()
To inspect a given object or type for whether it is (an instance of) a .NET reference type vs. value type:
# $false is returned in both cases, confirming that the .NET array
# type is a *reference type*
#(1, 2).GetType().IsValueType
[Array].IsValueType
Workaround:
A workaround would be to use string representations of arrays, though coming up with unique (enough) ones may be a challenge.
In the simplest case, use PowerShell's string interpolation, which represents arrays as a space-separated list of the elements' (stringified) values; e.g. "$(1, 2)" yields verbatim 1 2:
$h = #{}
# The array to base the key on.
$array = 1, 2
# Use the *stringified* version as the key.
$h["$array"] = 'a'
# Works, because even different array instances with equal-valued
# instances of .NET primitive types stringify the same.
# '1 2'
$h["$(1, 2)"]
iRon points out that this simplistic approach can lead to ambiguity (e.g., a single '1 2' string would result in the same key as array 1, 2) and recommends the following instead:
a more advanced/explicit way for array keys would be:
joining their elements with a non-printable character; e.g.
$key = $array -join [char]27
or, for complex object array elements, serializing the array:
$key = [System.Management.Automation.PSSerializer]::Serialize($array)
Note that even the XML (string)-based serialization provided by the System.Management.Automation.PSSerializer class (used in PowerShell remoting and background jobs for cross-process marshaling) has its limits with respect to reliably distinguishing instances, because its recursion depth is limited - see this answer for more information; you can increase the depth on demand, but doing so can result in very large string representations.
A concrete example:
using namespace System.Management.Automation
$ht = #{}
# Use serialization on an array-valued key.
$ht[[PSSerializer]::Serialize(#(1, 2))] = 'a'
# Despite using a different array instance, this
# lookup succeeds, because the serialized representation is the same.
$ht[[PSSerializer]::Serialize(#(1, 2))] # -> 'a'
The primary cause of your problems here is that PowerShell's index access operator [] supports multi-index access by enumerating any array values passed.
To understand why, let's have a look at how the index accessor [...] actually works in PowerShell. Let's start with a simple hashtable, with 2 entries using scalar keys:
$ht = #{}
$ht['a'] = 'This is value A'
$ht['b'] = 'This is value B'
Now, let's inspect how it behaves!
Passing a scalar argument resolves to the value associated with the key represented by said argument, so far so good:
PS ~> $ht['a']
This is value A
But we can also pass an array argument, and all of a sudden PowerShell will try to resolve all items as individual keys:
PS ~> $ht[#('a', 'b')]
This is value A
This is value B
PS ~> $ht[#('b', 'a')] # let's try in reverse!
This is value B
This is value A
Now, to understand what happens in your example, let's try an add an entry with an array reference as the key, along with two other entries where the key is the individual values fround in the array:
$ht = #{}
$keys = 1,2
$ht[$keys[0]] = 'Value 1'
$ht[$keys[1]] = 'Value 2'
$ht[$keys] = 'Value 1,2'
And when we subsequently try to resolve the last entry using our array reference:
PS ~> $ht[$keys]
Value 1
Value 2
Oops! PowerShell unraveled the $keys array, and never actually attempted to resolve the entry associated with the key corresponding to the array reference in $keys.
In other words: The index accessor cannot be used to resolve dictionary entries by key is the key type is enumerable
So, how does one access an entry by array reference without having PowerShell unravel the array?
Use the IList.Item() parameterized property instead:
PS ~> $ht.Item($keys)
Value 1,2
Thought I have read enough examples here and elsewhere. Still I fail creating arrays in Power Shell.
With that code I hoped to create slices of pair values from an array.
$values = #('hello','world','bonjour','moon','ola','mars')
function slice_array {
param (
[String[]]$Items
)
[int16] $size = 2
$pair = [string[]]::new($size) # size is 2
$returns = [System.Collections.ArrayList]#()
[int16] $_i = 0
foreach($item in $Items){
$pair[$_i] = $Item
$_i++;
if($_i -gt $size - 1){
$_i = 0
[void]$returns.Add($pair)
}
}
return $returns
}
slice_array($values)
the output is
ola
mars
ola
mars
ola
mars
I would hope for
'hello','world'
'bonjour','moon'
'ola','mars'
Is possible to slice that array to an array of arrays with length 2 ?
Any explenation why it doesn't work as expected ?
How should the code be changed ?
Thanks for any hint to properly understand Arrays in PowerShell !
Here's a PowerShell-idiomatic solution (the fix required for your code is in the bottom section):
The function is named Get-Slices to adhere to PowerShell's verb-noun naming convention (see the docs for more information).
Note: Often, the singular form of the noun is used, e.g. Get-Item rather than Get-Items, given that you situationally may get one or multiple output values; however, since the express purpose here is to slice a single object into multiple parts, I've chosen the plural.
The slice size (count of elements per slice) is passed as a parameter.
The function uses .., the range operator, to extract a single slice from an array.
It uses PowerShell's implicit output behavior (no need for return, no need to build up a list of return values explicitly; see this answer for more information).
It shows how to output an array as a whole from a function, which requires wrapping it in an auxiliary single-element array using the unary form of ,, the array constructor operator. Without this auxiliary array, the array's elements would be output individually to the pipeline (which is also used for function / script output; see this answer for more information.
# Note: For brevity, argument validation, pipeline support, error handling, ...
# have been omitted.
function Get-Slices {
param (
[String[]] $Items
,
[int] $Size # The slice size (element count)
)
$sliceCount = [Math]::Ceiling($Items.Count / $Size)
if ($sliceCount -le 1) {
# array is empty or as large as or smaller than a slice? ->
# wrap it *twice* to ensure that the output is *always* an
# *array of arrays*, in this case containing just *one* element
# containing the original array.
,, $Items
}
else {
foreach ($offset in 0..($sliceCount-1)) {
, $Items[($offset * $Size)..(($offset+1) * $Size - 1)] # output this slice
}
}
}
To slice an array into pairs and collect the output in an array of arrays (jagged array):
$arrayOfPairs =
Get-Slices -Items 'hello','world','bonjour','moon','ola','mars' -Size 2
Note:
Shell-like syntax is required when you call functions (commands in general) in PowerShell: arguments are whitespace-separated and not enclosed in (...) (see this answer for more information)
Since a function's declared parameters are positional by default, naming the arguments as I've done above (-Item ..., -Size ...) isn't strictly necessary, but helps readability.
Two sample calls:
"`n-- Get pairs (slice count 2):"
Get-Slices -Items 'hello','world','bonjour','moon','ola','mars' -Size 2 |
ForEach-Object { $_ -join ', ' }
"`n-- Get slices of 3:"
Get-Slices -Items 'hello','world','bonjour','moon','ola','mars' -Size 3 |
ForEach-Object { $_ -join ', ' }
The above yields:
-- Get pairs (slice count 2):
hello, world
bonjour, moon
ola, mars
-- Get slices of 3:
hello, world, bonjour
moon, ola, mars
As for what you tried:
The only problem with your code was that you kept reusing the very same auxiliary array for collecting a pair of elements, so that subsequent iterations replaced the elements of the previous ones, so that, in the end, your array list contained multiple references to the same pair array, reflecting the last pair only.
This behavior occurs, because arrays are instance of reference types rather than value types - see this answer for background information.
The simplest solution is to add a (shallow) clone of your $pair array to your list, which ensures that each list entry is a distinct array:
[void]$returns.Add($pair.Clone())
Why you got 3 equal pairs instead of different pairs:
.Net (powershell based on it) is object-oriented language and it has consept of reference types and value types. Almost all types are reference types.
What happens in your code:
You create $pair = [string[]] object. $pair variable actually stores memory address of (reference to) [string[]] object, because arrays are reference types
You fill $pair array with values
You add (!) $pair to $returns. Remember that $pair is reference to memory block. And when you add it to $returns, it adds memory address of [string[]] you wrote values to.
You repeat step2: You fill $pair array with different values, but address of this array in memory keeps the same. Doing this you actually replace values from step2 with new values in the same $pair object.
= // = step3
= // = step4
= // = step3
As a result: in $returns there are three same memory addresses: [[reference to $pair], [reference to $pair], [reference to $pair]]. And $pair values were overwritten by code with last pair values.
On output it works like this:
Powershell looks at $results which is array.
Powershell looks to $results[0] which reference to $pair
Powershell outputs reference to $pair[0]
Powershell outputs reference to $pair[1]
Powershell looks to $results[1] which reference to $pair
Powershell outputs reference to $pair[0]
Powershell outputs reference to $pair[1]
Powershell looks to $results[1] which reference to $pair
Powershell outputs reference to $pair[0]
Powershell outputs reference to $pair[1]
So you see, you triple output the object from the same memory address. You overwritten it 3 times in slice_array and now it stores only last pair values.
To fix it in your code, you should create a new $pair in memory: add $pair = [string[]]::new($size) just after $returns.Add($pair)
I'm using the S.DS.P PowerShell module in a PowerShell script of mine. In it, I have to create the following object:
$ADUserEntry = #{"distinguishedName"=$null;"objectClass"=$null;"sAMAccountName"=$null;"unicodePwd"=$null;"userAccountControl"=0};
In the documentation of the module, it's stated that I have to do the following assignment to the unicodePwd field of a variable created using this object:
$obj.unicodePwd = ,([System.Text.Encoding]::Unicode.GetBytes("$randomPassword") -as [byte[]]);
Notice how there's a comma before the first parentheses. What is that comma doing there?
As Lee_Dailey has pointed out, what you're seeing is the unary form of the (unfortunately named) "comma operator", i.e., PowerShell's array-construction operator.
The unary form creates a single-element array that wraps its (one and only) operand; the array's type is [object[]], as usual in PowerShell:
$arr = , 'foo' # wrap string 'foo' in a single-element array
$arr.GetType().Name # the array's type -> 'Object[]'
$arr[0].GetType().Name # the type of the array's one and only element -> 'String'
Note that while you can even wrap arrays that way, PowerShell's operator-precedence rules require a literal array operand to be enclosed in (...):
# OK - wraps array 1, 2 in a single-element array.
$arr = , (1, 2)
# !! DOES SOMETHING DIFFERENT:
# Creates a 2-element array whose 1st element is integer 1 wrapped in a
# single-element array
$arr = , 1, 2
The binary form constructs an array from the operands, as expected:
$arr = 1, 2, 3 # 3-element array whose elements are integers 1 and 2 and 3
As an aside, re the specific command shown:
,([System.Text.Encoding]::Unicode.GetBytes("$randomPassword") -as [byte[]])
Neither , nor -as [byte[]] are needed in this scenario, because
[System.Text.Encoding]::Unicode.GetBytes() directly returns a [byte[]] array.
I need to create a ReadOnlyCollection from elements of an array but it seems like ReadOnlyCollection elements can only be defined in declaration of the collection. Is there any other way than listing each element of the array in the declaration of the collection as in the following sample?
[byte[]]$arr=10,20,30
[System.Collections.ObjectModel.ReadOnlyCollection[byte]]$readOnly=
$arr[0],$arr[1],$arr[2]
Thanks
Pass the array to the constructor instead:
$readOnly = New-Object 'System.Collections.ObjectModel.ReadOnlyCollection[byte]' -ArgumentList #(,$arr)
or (PowerShell 5.0 and up):
$readOnly = [System.Collections.ObjectModel.ReadOnlyCollection[byte]]::new($arr)
Now, your question title specifically says copy array elements - beware while you won't be able to modify $readOnly, its contents will still reflect changes to the array that it's wrapping:
PS C:\> $arr[0] = 100
PS C:\> $arr[0]
100
PS C:\> $readOnly[0]
100
If you need a completely separate read-only collection, copy the array to another array first and then overwrite the variable reference with the read-only collection:
$readOnly = [byte[]]::new($arr.Count)
$arr.CopyTo($readOnly, 0)
$readOnly = [System.Collections.ObjectModel.ReadOnlyCollection[byte]]::new($readOnly)
Now you can modify $arr without affecting $readOnly:
PS C:\> $arr[0] = 100
PS C:\> $arr[0]
100
PS C:\> $readOnly[0]
10
Mathias R. Jessen's helpful answer contains good alternatives and background information, but in your case you can simply assign your array to your type-constrained $readOnly variable:
[byte[]] $arr=10,20,30
[System.Collections.ObjectModel.ReadOnlyCollection[byte]] $readOnly = $arr
In PSv3+ you can alternatively use [Array]::AsReadOnly():
[byte[]] $arr=10,20,30
$readOnly = [Array]::AsReadOnly($arr) # shorter, but doesn't lock in the type of $readOnly
As stated, System.Collections.ObjectModel.ReadOnlyCollection<T> only wraps its input array, so later changes to the input array's elements would be reflected in the read-only collection.
However, whether it wraps your actual input array, depends on its specific type, because Powershell may create a new array for you behind the scenes - see this answer for details.
If in doubt, use $arr.Clone() to explicitly create a copy of your array.
How can i delete row from array which is pscustomobject in a loop?
Getting errors if i use this in the loop:
$a = $a | where {condition to remove lines}
Getting the below error
Method invocation failed because [System.Management.Automation.PSObject] does not contain a method named 'op_Addition'.
any suggetion to remove row from array.
Let me make some general points, given the generic title of the question:
Arrays (in .NET, which underlies PowerShell) are fixed-size data structures. As such, you cannot delete elements from them directly.
However, you can create a new array that is a copy of the original one with the unwanted elements omitted, and that is what the pipeline approach facilitates:
# Sample array.
$a = 1, 2, 3
# "Delete" element 2 from the array, which yields #(1, 3).
# #(...) ensures that the result is treated as an array even if only 1 element is returned.
$a = #($a | Where-Object { $_ -ne 2 })
PowerShell automatically captures the output from a pipeline in an array (of type [System.Object[]]) when you assign it to a variable.
However, since PowerShell automatically unwraps a single-element result, you need #(...), the array-subexpression operator to ensure that $a remains an array even if only a single element was returned - the alternative would be to type-constrain the variable as an array:
[array] $a = $a | Where-Object { $_ -ne 2 }
Note that even though the result is assigned back to input variable $a, $a now technically contains a new array (and the old one, if it is not being referenced elsewhere, will eventually be garbage-collected).
As for what you tried:
How can i delete row from array which is pscustomobject
As wOxxOm points out, [pscustomobject] are not arrays, but perhaps you meant to say that you have an array whose elements are custom objects, in which case the above approach applies.
Alternatively, if the array to delete elements from is stored in a property of a custom object, send that property's value through the pipeline instead, and assign the results back to that property.
The error message occurs when you try to use the + operator with a [pscustomobject] instance as the LHS, which is not supported; e.g.:
PS> ([pscustomobject] #{ foo = 'bar' }) + 1
Method invocation failed because [System.Management.Automation.PSObject] does not contain a method named 'op_Addition'.
...
PowerShell doesn't know how to "add" something to a custom object, so it complains.