[System.Array] Archives - tommymaynard.com

After reading a short post on the PowerShell subreddit recently, I was compelled to write a quick post here about arrays and hash tables. The concepts are similar, but the two are different, meaning that — interruption! There have now been two posts where the OP didn’t know one versus the other. It is basic stuff, sure, but it is important stuff.

Anyway, talking about one when you mean the other isn’t going to be okay any longer — it really never was. While someone mentioned the incorrect usage in the thread, writing, for me, still felt necessary and potentially beneficial.

Instead of writing my own definitions, let’s use Microsoft’s. We’ll begin with an array: “Data structures designed to store collections of items.“ If you think of a variable, back when you learned about those the first time, you probably learn how to store a single value in your variable. Okay good. This time we’re going to store multiple values in a single variable. That is what this data structure provides.

We are beginning with a simple variable that contains a simple, single value. Not much to it.

[PS7.2.0][C:\] $SingleValue = 'One thing'
[PS7.2.0][C:\] $SingleValue
One thing
[PS7.2.0][C:\] $SingleValue.GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     String                                   System.Object

Now, let’s work with arrays. We’re still going to use a single variable, we’re just going to load it up with a different data structure that’s going to let us store multiple values in it. We actually don’t have to do much to make this happen, so here we go.

In this example, we’re creating a variable named $Array01. We’re storing the numeric values of 1, 2, and 3 inside the variable as an array.

[PS7.2.0][C:\] $Array01 = 1,2,3
[PS7.2.0][C:\] $Array01
1
2
3
[PS7.2.0][C:\] $Array01.GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     Object[]                                 System.Array

Well, that was easy. This next example is the same as above, however, it includes @() around our numeric values. This operator — the array sub-expression operator — takes whatever is inside it and makes it into an array.

[PS7.2.0][C:\] $Array02 = @(4,5,6)
[PS7.2.0][C:\] $Array02
4
5
6
[PS7.2.0][C:\] $Array02.GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     Object[]                                 System.Array

The array sub-expression operator will even make an array of nothing.

[PS7.2.0][C:\] $Array03 = @()
[PS7.2.0][C:\] $Array03
[PS7.2.0][C:\] $Array03.GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     Object[]                                 System.Array

Here is one you probably do not see often. The comma in front of the 7 will indicate to PowerShell to create an array that contains a single value, which we will prove when we start returning the count of our various arrays.

[PS7.2.0][C:\] $Array04 = ,7
[PS7.2.0][C:\] $Array04
7
[PS7.2.0][C:\] $Array04.GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     Object[]                                 System.Array

[PS7.2.0][C:\] $Array04.Count
1
[PS7.2.0][C:\] $Array03.Count
0
[PS7.2.0][C:\] $Array02.Count
3
[PS7.2.0][C:\] $Array01.Count
3

Do you get it? It is a data structure designed to accommodate multiple values. You did not see it here yet, but arrays can hold strings (words or sentences), just as easily as numbers. You can even mix them in the same array and store them in a single variable.

This array, stored in the $Array05 variable contains five different colors.

[PS7.2.0][C:\] $Array05 = 'black','red','green','blue','brown'
[PS7.2.0][C:\] $Array05
black
red
green
blue
brown

Each item in an array can be referenced by an index — a number that represents its location. It is a positional place in line. The first element, or item in the array — black in our case — is in index zero. This is because arrays are always zero-based. This means red would be in index 1, green in index 2, blue in 3, and brown in 4. Zero through four is five, as in five total elements, or values, in our array. Let’s verify.

[PS7.2.0][C:\] $Array05[0]
black
[PS7.2.0][C:\] $Array05[1]
red
[PS7.2.0][C:\] $Array05[2]
green
[PS7.2.0][C:\] $Array05[3]
blue
[PS7.2.0][C:\] $Array05[4]
brown
[PS7.2.0][C:\]

Let’s try some other things! While not often used in my day-to-day, we can perform arithmetic on the indexes. We can even use the range operator (..) and go forward and backward through our elements. What else… Oh, let’s try some negative indexes numbers, too. Those work in reverse from the end of the array to the beginning.

[PS7.2.0][C:\] $Array05[3-3]
black
[PS7.2.0][C:\] $Array05[2+1]
blue
[PS7.2.0][C:\] $Array05[0..4]
black
red
green
blue
brown
[PS7.2.0][C:\] $Array05[4..0]
brown
blue
green
red
black
[PS7.2.0][C:\] $Array05[-1]
brown
[PS7.2.0][C:\] $Array05[-2]
blue
[PS7.2.0][C:\] $Array05[-3]
green
[PS7.2.0][C:\] $Array05[-4]
red
[PS7.2.0][C:\] $Array05[-5]
black

Hash tables have a good number of similarities to arrays, and perhaps that is the reason why there is some confusion. Microsoft even uses the word array to describe them: “A hash table, also known as a dictionary or associative array, is a compact data structure that stores one or more key/value pairs.” I don’t use the words associative array often enough, but I should; I like it.

To the person on Reddit that called hash tables, hashmaps, and just maps: No. I believe that term is a carryover from maybe Java. In my experience, I have not heard it in the PowerShell community.

Anyway, arrays and associative arrays are both data structures. They can both hold multiple values, and they can both be stored in a single variable. The difference is keys and values. Arrays hold single items in each index, while a hash table holds a key-value pair in each.

Before we fill this up, let’s create an empty hash table. Notice the difference between this and the array. If it is used for an array, the operator will always be an instant giveaway: @() is for arrays, and @{} is for hash tables. Notice my liberal use of the GetType() method in this post; it’ll help you make object determinations, so you can always know which you’re working with, without having to view, or see, the data stored in the data structure.

[PS7.2.0][C:\] $Hash01 = @{}
[PS7.2.0][C:\] $Hash01
[PS7.2.0][C:\] $Hash01.Count
0
[PS7.2.0][C:\] $Hash01.GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     Hashtable                                System.Object

Now, let’s populate a hash table… in VS Code. Somethings I would just rather not demonstrate in the console.

$Hash02 = @{
    Dad = 'David'
    Mom = 'Betty'
    Daughter = 'Janice'
    Son = 'Bryan'
}
$Hash02
Name                           Value
----                           -----
Son                            Bryan
Dad                            David
Mom                            Betty
Daughter                       Janice

This also could have been written in a couple of other ways.

$Hash02 = @{
    Dad = 'David'; Mom = 'Betty'; Daughter = 'Janice'; Son = 'Bryan'
}
$Hash02 = @{
    Dad      = 'David'
    Mom      = 'Betty'
    Daughter = 'Janice'
    Son      = 'Bryan'
}

While hash tables have indexes, they’re non-numeric. Actually, I am not even sure I like the term index when working with hash tables. I suggest we call it what they are: keys. Maybe you noticed, but the order in which we created our hash table was not the order in which it was returned. A numeric value here would not help us anyway. Instead, we’re going to use the keys as our indexes to return our values.

$Hash02['Dad']
David
$Hash02['Daughter']
Janice

There is a good possibility that you are probably going to want to loop through and display all the values in your hash table. It’s a little out of place, but we’ll do that with one of our arrays here, too. Using the GetEnumerator() method allows us to get to the key and value, values out of our hash table. In this first, hash table example we are using the key and associated value in a string. This requires the use of the subexpression operator ($()), so that we can display our values within the string.

foreach ($Person in $Hash02.GetEnumerator()) {
    "The $($Person.Key) is $($Person.Value)."
}

The Son is Bryan
The Dad is David
The Mom is Betty
The Daughter is Janice

We are using the subexpression operator in the string created by looping over our array, too.

$Array05 = 'black','red','green','blue','brown'
foreach ($Color in $Array05) {
	"Is your favorite color $($Color)?"
}
Is your favorite color black?
Is your favorite color red?
Is your favorite color green?
Is your favorite color blue?
Is your favorite color brown?

This has been a lengthy post — who knew this would happen!? There is one last thing I want to mention, and that is about creating an ordered hash table, or dictionary. We tend to use the dictionary term when we order a hash table.

$Hash03 = [ordered]@{
    Monday    = 'Chipotle'
    Tuesday   = 'Pizza'
    Wednesday = 'Mahi Mahi'
    Thursday  = 'Ravioli'
    Friday    = 'Orange Chicken'
}
$Hash03

Name                           Value
----                           -----
Monday                         Chipotle
Tuesday                        Pizza
Wednesday                      Mahi Mahi
Thursday                       Ravioli
Friday                         Orange Chicken

Because this is ordered, we can use numeric indexes again. Keep in mind, however, that doing this is only going to return the values and not the keys, as well. It may even momentarily confuse you into thinking this is an array, not a hash table, or an associative array. Maybe stick to using a foreach at this point. Do not forget the GetType() method to ensure you know your data structures.

$Hash03[0..4]
Chipotle      
Pizza
Mahi Mahi     
Ravioli       
Orange Chicken

$Hash03.GetType()
IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------     
True     True     OrderedDictionary                        System.Object

Okay, I am stopping here. There’s plenty to know about these two data structures, and at this point, it is more than I’m willing to share. Even so, this should provide a decent breakdown to go along with the Microsoft documentation and other information that’s available to consume on this topic. Know what you are talking about.

Take a look at this post’s title. I got that as a question, from a coworker yesterday. I didn’t know the answer right away, but I took some time last night and a half hour or so today to put it to bed. As of now, I am able to remove a hash table from an array of hash tables. Ugh.

The below array is the one you get when you don’t cast the variable. That’s to say the $x variable doesn’t need [System.Array] in front of it. It effectively does nothing different to that variable. The problem with this type of array is that it’s partially immutable. We can’t remove values from the array.

[System.Array]$x = 'one','two','three'

Therefore, we either need to create a new array to hold the hash tables we still want (as we are removing one of them), or cast our array as [System.Collections.ArrayList]. A variable cast as such allows us to remove values from it.

[System.Collections.ArrayList]$y = 'four','five','six'

Use this next information as a key, or legend, for the further below code. The entire code can be copied into the ISE, or Visual Studio Code, and run.

1. Removes the user-defined variables used in the below code.
2. Creates three hash tables that each include a User key, a Number key, and a Color key.
3. Prompts user to enter 1 or 2 whether they want to use the same array, or a new one.*

* The same array indicates we’re casting our variable as [System.Collections.ArrayList] and using the same variable. The new array indicates we’re casting a new variable as [System.Array], which again is the default and doesn’t actually require a cast.

4. Creates an array of hash tables based on the value 1 or 2.
5. Displays the current array of hash tables (before any changes).
6. Loops through the values in the array and uses the same array (if 1 was chosen), or creates a new array (if 2 was chosen).
7. Displays the updated array of hash tables (removes the hash table that includes the User “bsmith”).

#1 Remove variables (not using function/function scope).
Clear-Host
Remove-Variable -Name Hash1,Hash2,Hash3,Option,HashtableArray,i,HashtableArrayNew -ErrorAction SilentlyContinue 

#2 Create hash tables.
$Hash1 = @{User = 'landrews';Number = 1;Color = 'Red'}
$Hash2 = @{User = 'bsmith';Number = 2; Color = 'Blue'}
$Hash3 = @{User = 'sjackson';Number = 3;Color = 'Yellow'}

#3 Set SameArray vs. NewArray.
Write-Output -InputObject 'This function is hard coded to remove the hash table that include "bsmith" from an array.'
Do {
    $Option = Read-Host -Prompt 'Press 1 to use the same array, or 2 to create a new array'
} Until ($Option -eq 1 -or $Option -eq 2)

#4 Create array of hash tables.
Switch ($Option) {
    '1' {[System.Collections.ArrayList]$HashtableArray = $Hash1,$Hash2,$Hash3; break}
    '2' {[System.Array]$HashtableArray = $Hash1,$Hash2,$Hash3}
}

#5 Display unmodified hash table.
###################################
$HashtableArray
"'''''''^ Array of hash tables bfore ^''''''"
"'''''''v Array of hash tables after v''''''"
###################################

#6 Loop through array of hash tables.
For ($i = 0; $i -le $HashtableArray.Count - 1; $i++) {
    Switch ($Option) {
        '1' {
            If ($HashtableArray[$i].Values -contains 'bsmith') {
                $HashtableArray.Remove($HashtableArray[$i])
            }
        }
        '2' {
            If (-Not($HashtableArray[$i].Values -contains 'bsmith')) {
                [System.Array]$HashtableArrayNew += $HashtableArray[$i]
            }
        }
    }
}

#7 Display updated array.
Switch ($Option) {
    '1' {$HashtableArray}
    '2' {$HashtableArrayNew}
}

The below results show the exact same thing when run either by entering 1 or 2. The difference is the variable that’s displayed. You can see the above variable that’s returned based on 1 or 2. If it’s 1, we display the $HashtableArray variable (the one we created initially). If it’s 2, we display the $HashtableArrayNew variable. That’s the one we create, since we can’t modify the existing $HashtableArray variable when it’s cast as [System.Array].

This function is hard coded to remove the hash table that includes "bsmith" from an array.
Press 1 to use the same array, or 2 to create a new array: 2

Name                           Value
----                           -----
Color                          Red
Number                         1
User                           landrews
Color                          Blue
Number                         2
User                           bsmith
Color                          Yellow
Number                         3
User                           sjackson
'''''''^ Array of hash tables bfore ^''''''
'''''''v Array of hash tables after v''''''
Color                          Red
Number                         1
User                           landrews
Color                          Yellow
Number                         3
User                           sjackson

And with that, I’m done here. Fun stuff. Mostly.

tommymaynard.com

A PowerShell Resource. Since June 2014.

Tag Archives: [System.Array]

There is a Difference: Arrays Versus Hash Tables

Remove a Hash Table from an Array of Hash Tables