Friday Fun: Number Crunching

Earlier this week I was looking at the GoFundMe website in the midst of debating a new project. One of the considerations I have for sites like this is the expense involved. Certainly I don't expect this type of service to be free. But I started wondering about what net donations might look like. According to GoFundMe, assuming I am interpreting this correctly, the site takes 5% of the donation right off the top. In addition to that there is a processing fee of 2.9% of the donation plus 30 cents. At least for the US. So for a $10 donation the net donation would be $8.91.

10 - (10*.05) - (10*.029 +.30)

Alright then.  This should be simple enough to turn into a PowerShell function.

Function FundIt {
[cmdletbinding()]

Param(
[Parameter(Position=0,Mandatory,ValueFromPipeline)]
[double]$Donation,
[alias("currency")]
[Switch]$UseCurrency
)

Begin {
    Write-Verbose "Starting: $($MyInvocation.Mycommand)" 
     
    #define rates
    $GFMRate = .05
    $WePay = .029
    $PerTransaction = .30

} #begin

Process {
    Write-Verbose "Calculating donation of `$$Donation"
    
    $GFM = $donation * $GFMRate
    $Processing = [math]::Round(($Donation * $WePay) + $PerTransaction,2)
    $NetDonation = [math]::Round( $Donation - $GFM - $Processing,2)

    if ($UseCurrency) {
        #Imporant: the values will be treated as strings
        [pscustomobject]@{
          Donation = "{0:c2}" -f $Donation
          Fees = "{0:c2}" -f  ($GFM + $Processing)
          NetDonation = "{0:c2}" -f $NetDonation
        }
    }
    else {
        [pscustomobject]@{
          Donation = $Donation
          Fees = $GFM + $Processing
          NetDonation = $NetDonation
        }
    }

} #process

End {
    Write-Verbose "Ending: $($MyInvocation.Mycommand)"
} #end

} #close function

This also should make for a fun learning opportunity.

The function takes a parameter for the donation amount. I cast it as a [double] in the event someone might donate $10.25. If used [int] PowerShell would turn it into $10.  I'll come back to the other parameter in a bit.

I define the rates from GoFundMe as variables.  This makes it easier to calculate some values.

$GFM = $donation * $GFMRate
$Processing = [math]::Round(($Donation * $WePay) + $PerTransaction,2)
$NetDonation = [math]::Round( $Donation - $GFM - $Processing,2)

You'll note that I'm using the Round() method of the .NET Math class. This is so that the result is formatted to 2 decimal points.  This is what I want to see, not necessarily how the site operates. With these values I can create a custom object.

[pscustomobject]@{
    Donation = $Donation
    Fees = $GFM + $Processing
    NetDonation = $NetDonation
}

Here's a simple demonstration, and yes I know I'm not using a standard function name but this is for fun.

I also wrote the function as an advanced function so that I can pipe values into it. If you look at the parameter definition for $Donation you'll see that I have a setting for ValueFromPipeline.

This makes it pretty clear about what happens with each donation.  But what did I end up with?  My function writes objects to the pipeline so that I can use other cmdlets, like Measure-Object.

Here's the fun part: I can measure multiple properties.

10,5,2,5,5,15 | fundit | 
measure-object -property Donation,Fees,netdonation -sum | 
Select Property,Sum,Count

I thought that was pretty handy.

I know that the values are in dollars.  But let's say I was creating a report and I wanted to make it pretty and include the $ sign, or whatever my currency symbol might be.

If you recall, I included a switch parameter called UseCurrency along with a parameter alias of currency.

[alias("currency")]
[Switch]$UseCurrency

If I run the function with –UseCurrency, then this parameter will have a value of $True. That is how a Switch parameter works. In my function I can test the parameter value and it it is true, then I'll create the same custom object, except that I will use the –F operator to format the value as a string using the currency symbol.

if ($UseCurrency) {
    #Imporant: the values will be treated as strings
    [pscustomobject]@{
        Donation = "{0:c2}" -f $Donation
        Fees = "{0:c2}" -f  ($GFM + $Processing)
        NetDonation = "{0:c2}" -f $NetDonation
    }
}

This operator is used to format strings in the .NET Framework. You can read more about this online at http://go.microsoft.com/fwlink/?LinkID=166450. But in short, the {0} on the left side of the operator is a numbered place holder. The c is the modifier which in this case indicates to use a currency format string and I'm limiting it to 2 places.  I probably don't need that since I'm already rounding but I left it in for the sake of education.  On the right side of the –f operator is a comma separated list of values that will "plug in" to the place holders. The net result is this:

That looks pretty, but be aware that these values are strings.

This means my previous method of getting a sum will fail.  Instead I need to use numbers and then format the result.

10,5,2,5,5,15 | fundit | 
measure-object -property Donation,Fees,netdonation -sum | 
Select Property,@{Name="Total";Expression={ "{0:c}" -f $_.sum}},Count

Now I have the best of everything. Although if I truly want to make a pretty report, I can use Format-Table so that I can specify an alignment on the custom property.

10,5,2,5,5,15 | fundit | 
measure-object -property Donation,Fees,netdonation -sum | 
Format-Table Property,
@{Name="Total";Expression={ "{0:c}" -f $_.sum};Align="Right"},Count

I hope you found this fun and informative.  If you have any questions about what I did or why, please drop them in the comments.

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