How do I populate an array of unknown length in Powershell?

Question

How do you work with dynamic-length arrays (ArrayLists / Lists) in Powershell? I basically want a 2D-array where the length of the outermost index is unknown.

I tried initializing an array with $array = @(), but would get index out of range exceptions when addressing anything in this. Then I tried using the += operand, as I read in an article, but that would result in string concatenation and not element addition.

Example:

$array = @()
$array += @("Elem1x", "Elem1y")
$array += @("Elem2x", "Elem2y")
Echo $array[0][0]

Output: "E" instead of "Elem1x";

Keith Hill · Accepted Answer · 2012-10-12 16:23:45Z

12

Christian's answer is the PowerShell way to go and works great for most cases (small to medium size arrays). If your array is large then you may want to consider using ArrayList for performance reasons. That is, every time you use += with an array, PowerShell has to create a new array and copy the old contents into the new array and assign the new array to the variable. That's because .NET arrays are fixed size. Here's how you could do this using an ArrayList:

$list = new-object system.collections.arraylist
$list.Add(("Elem1x", "Elem1y", "Elem1z")) > $null
$list.Add(("Elem2x", "Elem2y")) > $null
$list[0][0]

BTW what the operator += does depends on the type of the object on the left hand side of the operator. If it is a string, then you get string concatenation. If the object is an array, then the right hand side is appended to the array (via create new array/copy).

edited Oct 12, 2012 at 16:23

answered Oct 12, 2012 at 15:04

Keith Hill

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1 Comment

EBGreen Over a year ago

I will also add that in my experience, a multi dimensional array is almost always inferior to some other data structure such as suggested here an arraylist of arrays. But I may just be prejudiced against multi dimensional arrays :)

CB. · Accepted Answer · 2012-10-12 14:36:57Z

10

Try this way:

$array = @()
$array += ,@("Elem1x", "Elem1y")
$array += ,@("Elem2x", "Elem2y")
$array[0][0]

edited Oct 12, 2012 at 14:36

answered Oct 12, 2012 at 14:11

CB.

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3 Comments

Nilzor Over a year ago

Can anyone explain what the comma actually does?

CB. Over a year ago

@Nilzor read here: blogs.msdn.com/b/powershell/archive/2007/01/23/…

Trike Over a year ago

The link is dead and I can't find any other references. Anyone has a relevant link nowadays? Found: In expression mode, as a unary operator, the comma creates an array with just one member. Place the comma before the member.

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