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.