Add a column to a csv file and fill up new column based on an existing column powershell - powershell

Question

I have been trying to add a new column to a csv file and populating the new column based on value in an existing column.
I have a table like this:
|name | number | state | desc|
| ---- | ------ |-------|-----|
|a | 1 | n | i |
|b | 2 | n | j |
|c | 3 | l | j |
|d | 4 | m | k |
I want to add a new column data and populate it based on number column matching with an array.
This is my code so far:
$a=("a","b","c")
$b=("p","q","r")
.
.
.
$c= import-csv -Path "C:\..."
$b |where-object {filtered the file based on some criteria}| select-object number, state, desc, #{Name="data"; Expression={Foreach-object {if ($_.number in $a){$_data = "x"}
elseif($_.number in $b){$_.data = "y"}.......} | export-csv -notypeinformation -path "C:\...."
The script runs but do not populate the new column. Please help

Answer 1

You've got the right idea. Import-Csv will produce an array of objects and you can use Select-Object to add calculated properties, then pipe again to Export-Csv. However, it's not exactly clear from the description or the example code what the expression should be. How do you want to define the new "data" property?
For now I'll work with what we have. The array variables $a & $b will never match anything. Also you can't use ForEach-Object like that, nor will assigning to $data work. The returning value of the Expression script block gets assigned to the property you named data. The following example demonstrates the point:
$a = ( "1", "2", "3")
$b = ( "4", "5", "6")
Import-Csv -Path "C:\temp\12-22-20.csv"|
Select-Object number, state, desc,
#{Name = 'Data'; Expression = { If( $_.Number -in $a ){ 'x' } elseif( $_.Number -in $b ){ 'y' } Else { $null }}} |
Export-Csv -Path "C:\temp\12-22-20_New.csv" -NoTypeInformation
The resulting Csv file will look something like:
number state desc Data
------ ----- ---- ----
1 n i x
2 n j x
3 l j x
4 m k y
Update: Example Using Add-Member
You do not need to use a loop to add the property:
$a = ( "1", "2", "3")
$b = ( "4", "5", "6")
Import-Csv -Path "C:\temp\12-22-20.csv" |
Add-Member -MemberType ScriptProperty -Name "data" -Value { If( $this.Number -in $a ){ 'x' } elseif( $this.Number -in $b ){ 'y' } Else { $null }} -PassThru |
Export-Csv -Path C:\temp\12-22-20_New.csv -NoTypeInformation
By using a MemberType of ScriptProperty we can make a slight modification to script block, replacing $_ with $this The pipe is an implicit loop. I'm not sure if there are any detractions to using a ScriptProperty, but this exports as expected. This approach doesn't require storing the output in $c, but -PassThru would facilitate that if preferred.
99% of the time Select-Object is used for this. The only difference I'm aware of it Select-Object converts the objects to PSCustomObjects. Get-Member will preserve the underlying type, however Import-Csv only outputs PSCustomObjects in the first place, so there's no impact here.

Answer 2

Try iterating over the $c array of imported objects and add the new property to all objects. You want to make sure the new column exists in all of the objects. You can either use Select-Object as in your example, or you can use Add-Member to add it to the imported object.
$a=("a","b","c")
$b=("p","q","r")
...
$c = Import-Csv -Path "C:\..."
$c | ForEach-Object {
$value = ""
# custom logic for value of "data"
# if (...) { $value = ... }
$_ | Add-Member -MemberType NoteProperty -Name "data" -Value $value
}
$c | Export-Csv -NoTypeInformation -path "C:\...."