Powershell Adjusting Column Values across multiple text files - powershell

Question

New to Powershell and looking for some help. I have multiple xyz files that are currently displaying a z value with positive up and I need to make all the values negative (or z-positive down). So far my best attempt has been to try to cobble together what I know from other lines of code, but I'm still far from a solution.
$data = Get-ChildItem "C:\Users\dwilson\Desktop\test" -Recurse |
foreach ($item in $data) {$item.'Col 3' = 0 - $item.'Col 3'}
Any help would be greatly appreciated!

Answer 1

Somewhat of an aside, but you're using mixed syntax here.
foreach ($i in $set) { ... }
This is the Foreach statement. It does not accept input from the pipeline, nor will it automatically send output down the pipeline. See also Get-Help about_Foreach.
Get-ChildItem | ForEach-Object { ... }
This is the ForEach-Object command. It accepts input from the pipeline and sends output down the pipeline. Critically, this command also has a default alias of foreach. See also Get-Help ForEach-Object.
The help for the Foreach statement explains how PowerShell decides what foreach means (emphasis mine):
When Foreach appears in a command pipeline, PowerShell uses the foreach alias, which calls the ForEach-Object command. When you use the foreach alias in a command pipeline, you do not include the ($<item> in $<collection>) syntax as you do with the Foreach statement. This is because the prior command in the pipeline provides this information.

Answer 2

Using the looping over the files returned by Get-ChildItem you can import each with Import-CSV using the -Header parameter to assign the property name for each column. Then we can update the information for that property then export it. Using the ConvertTo-CSV cmdlet and then using Select-Object -Skip 1 we can drop the header off the CSV before exporting it.
$Files = Get-ChildItem "C:\Users\dwilson\Desktop\test" -Recurse
foreach ($File in $Files) {
$Data = Import-CSV $File -Header 'Col1','Col2','Col3'
$newData = ForEach ($Row in $Data) {
$Row.'Col3' = 0 - $Row.'Col3'
$Row
}
$newData |
Convertto-CSV -NoTypeInformation |
Select-Object -Skip 1 |
Out-File "$($File.Fullname).new)"
}