Richard Siddaway's Blog

This is a very common pattern:

 

$os = Get-CimInstance -ClassName Win32_OperatingSystem
$comp = Get-CimInstance -ClassName Win32_ComputerSystem

$props = @{
OS = $os.Caption
InstallDate = $os.InstallDate
LastBoot = $os.LastBootUpTime
Make = $comp.Manufacturer
Model = $comp.Model
}

New-Object -TypeName PSObject -Property $props

 

Get some data – in this case a couple of WMI classes and create an output object.

Unfortunately, the output looks like this

 

Make        : Microsoft Corporation
Model       : Surface Pro 2
LastBoot    : 30/12/2016 09:41:52
OS          : Microsoft Windows 10 Pro Insider Preview
InstallDate : 08/12/2016 13:20:04

 

The property order is NOT preserved because you’re working with a hash table. If you absolutely have to preserve the property order use an ordered hash table

 

$os = Get-CimInstance -ClassName Win32_OperatingSystem
$comp = Get-CimInstance -ClassName Win32_ComputerSystem

$props = [ordered]@{
OS = $os.Caption
InstallDate = $os.InstallDate
LastBoot = $os.LastBootUpTime
Make = $comp.Manufacturer
Model = $comp.Model
}

New-Object -TypeName PSObject -Property $props

 

All you do is add [ordered] in front of the hashtable definition and your output becomes

 

OS          : Microsoft Windows 10 Pro Insider Preview
InstallDate : 08/12/2016 13:20:04
LastBoot    : 30/12/2016 09:41:52
Make        : Microsoft Corporation
Model       : Surface Pro 2

 

exactly what you defined

You’ve seen how to calculate standard deviation and how to turn that calculation into a PowerShell function. This time we’ll use the calculation to create a class:

class stats {

static [double] StandardDeviation ([double[]]$numbers) {

$mean = $numbers | Measure-Object -Average | select -ExpandProperty Average
$sqdiffs = $numbers | foreach {[math]::Pow(($psitem - $mean), 2)}

$sigma = [math]::Sqrt( ($sqdiffs | Measure-Object -Average | select -ExpandProperty Average) )

return [math]::Round($sigma, 3)

}
}

 

Name the class stats. We’ll create a static method on the class that will calculate the Standard Deviation. The method takes an array of doubles as input – integers are converted automatically to double.

The calculation is the same as before. Just add the return keyword to ensure you actually get the output. Using return is mandatory in PowerShell classes.

Because its a static class you don’t need to create an instance of the class – just use it directly:

$numbers = 1..10
[stats]::StandardDeviation($numbers)
[stats]::StandardDeviation(1..10)


Last time I showed how to calculate the standard deviation of a set of numbers and said the code could easily be turned into a function

function Measure-StandardDeviation {
[CmdletBinding()]
param (
[double[]]$numbers
)
$mean = $numbers | Measure-Object -Average | select -ExpandProperty Average
$sqdiffs = $numbers | foreach {[math]::Pow(($psitem - $mean), 2)}


$sigma = [math]::Sqrt( ($sqdiffs | Measure-Object -Average | select -ExpandProperty Average) )
[math]::Round($sigma, 3)
}

 

I’ve called the function Measure-StandardDeviation based on Measure-Object producing the mean.

The parameter is set to accept an array of doubles – floating point numbers – integers will be automatically converted to doubles.

To use the function

$numbers = 1..10
Measure-StandardDeviation -numbers $numbers

 

or create the input array directly

Measure-StandardDeviation -numbers (1..10)


A while ago I saw something asking about calculating standard deviations for a set of numbers in PowerShell. You can calculate the the mean (average) of a set of numbers using Measure-Object

 

$numbers = 1..10


$mean = $numbers | Measure-Object -Average | select -ExpandProperty Average
$mean
5.5

 

To calculate the standard deviation you:

Subtract the mean of the set from each number in the set and square the difference. Then calculate the mean of those squared differences and finally take the square root of the result.

We already have the mean

so lets calculate the differences and square them

$sqdiffs = $numbers | foreach {[math]::Pow(($psitem - $mean), 2)}

 

The Pow static method of the math class is used to create the squares. The standard deviation (normally referred to as sigma) is calculated:

$sigma = [math]::Sqrt( ($sqdiffs | Measure-Object -Average | select -ExpandProperty Average) )
[math]::Round($sigma, 3)

 

You can calculate the mean of the squares of the differences using measure-object again. I used the Round static method of the math class to give my answer 3 decimal places.

Now that we can calculate the standard deviation we could turn this into a method, or a PowerShell class or even create a proxy function for Measure-Object that enables you to calculate standard deviation.