PowerShell Performance: += Operator versus ArrayList to add Elements to an Array

Sometimes you need to add elements to an array in PowerShell within a loop without knowing beforehand how many elements you are going to add.

In the most cases using the += operator make it all work easily.
Since the += operator actually…

  • …creates a new array in the memory with a length of the current array +1
  • copies all the elements of the current array into the new array
  • adds the new element at the end of the new array
  • deletes the current array from memory…

…the whole process is extremely time consuming (or expensive) if your array grows larger and larger.
On the other hand, using the .Add() function at a System.Collections.ArrayList just adds a new element at the end of the array without copying it first, which is much faster.

Therefore, I would suggest using the .NET System.Collections.ArrayList instead as shown below.

To demonstrate the massive difference in performance between the += operator and the System.Collections.ArrayList, I wrote two short scripts to demonstrates the effect.

Using the += operator to fill an array

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Measure-Command {
 
    $myArrayList = @()
    for($i = 1; $i -le 100000; $i++){
        $myArrayList += "another array element"
    }
}

As you can see, filling the array with 100’000 elements took a bit over 3 minutes.

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Days              : 0
Hours             : 0
Minutes           : 3
Seconds           : 12
Milliseconds      : 236
Ticks             : 1922365557
TotalDays         : 0.00222496013541667
TotalHours        : 0.05339904325
TotalMinutes      : 3.203942595
TotalSeconds      : 192.2365557
TotalMilliseconds : 192236.5557

Using the .NET System.Collections.ArrayList to fill an array

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Measure-Command {
 
    $myArrayList = New-Object System.Collections.ArrayList
    for($i = 1; $i -le 100000; $i++){
        $myArrayList.add("another array element")
    }
}

Filling the array with 100’000 elements using the System.Collections.ArrayList only took 148 milliseconds and was therefore staggering 1’295.8 times faster!

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Days              : 0
Hours             : 0
Minutes           : 0
Seconds           : 0
Milliseconds      : 148
Ticks             : 1483534
TotalDays         : 1.71705324074074E-06
TotalHours        : 4.12092777777778E-05
TotalMinutes      : 0.00247255666666667
TotalSeconds      : 0.1483534
TotalMilliseconds : 148.3534

The difference will grow even bigger as the array gets bigger.

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