Get a variable by dynamic variable name

Question

How does one access data imported from a CSV file by using dynamic note property names? That is, one doesn't know the colunm names beforehand. They do match a pattern and are extracted from the CSV file when the script runs.

As for an example, consider a CSV file:

"Header 1","Header A","Header 3","Header B"
0,0,0,0
1,2,3,4
5,6,7,8

I'd like to extract only columns that end with a letter. To do this, I read the header row and extract names with a regex like so,

$reader = new-object IO.StreamReader("C:\tmp\data.csv")
$line = $reader.ReadLine()
$headers = @()

$line.Split(",") | % {
    $m = [regex]::match($_, '("Header [A-Z]")')
    if($m.Success) { $headers += $m.value } }

This will get all the column names I care about:

"Header A"
"Header B"

Now, to access a CSV file I import it like so,

$csvData = import-csv "C:\tmp\data.csv"

Import-CSV will create a custom object that has properties as per the header row. One can access the fields by NoteProperty names like so,

$csvData | % { $_."Header A" } # Works fine

This obviously requires one to know the column name in advance. I'd like to use colunn names I extracted and stored into the $headers. How would I do that?

Some things I've tried so far

$csvData | % { $_.$headers[0] } # Error: Cannot index into a null array.
$csvData | % { $np = $headers[0]; $_.$np } # Doesn't print anything.
$csvData | % { $_.$($headers[0]) } # Doesn't print anything.

I could change the script like so it will write another a script that does know the column names. Is that my only solution?

manojlds · Accepted Answer · 2011-12-15 20:50:25Z

3

I think you want this:

[string[]]$headers = $csvdata | gm -MemberType "noteproperty" | 
                        ?{ $_.Name -match "Header [a-zA-Z]$"} | 
                        select -expand Name
$csvdata | select $headers

Choose the headers that match the condition (in this case, ones ending with characters) and then get the csv data for those headers.

answered Dec 15, 2011 at 20:50

manojlds

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CB. · Accepted Answer · 2011-12-15 20:21:10Z

1

the first thing ( and the only one... sorry) that came in my mind is:

$csvData | % { $_.$(( $csvData | gm | ? { $_.membertype -eq "noteproperty"} )[0].name) }

for get the first's column values and

$csvData | % { $_.$(( $csvData | gm | ? { $_.membertype -eq "noteproperty"} )[1].name) }

for second column and so on....

is this what you need?

answered Dec 15, 2011 at 20:21

CB.

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2 Comments

vonPryz Over a year ago

Almost. It requires one to know the indexes in advance, but can be fixed easily enough by adding a -like search: $csvData | % { $_.$(( $csvData | gm | ? { $_.membertype -eq "noteproperty" -and $_.Name -like $($headers[0])} ).name) }

manojlds Over a year ago

@vonPryz - How is this exactly what you needed? Christian - No offence.

Tomas Panik · Accepted Answer · 2011-12-15 20:51:10Z

0

you can use custom script to parse csv manually:

$content = Get-Content "C:\tmp\data.csv"
$header = $content | Select -first 1
$columns = $header.Split(",")
$indexes = @()
for($i; $i -lt $columns.Count;$i++)
{
    # to verify whether string end with letter matches this regex: "[A-Za-z]$"
    if ($column[$i] -match "[A-Za-z]$")
    {
        $indexes += $i
    }
}

$outputFile = "C:\tmp\outdata.csv"
Remove-Item $outputFile -ea 0
foreach ($line in $content)
{
    $output = ""
    $rowcol = $line.Split(",")
    [string]::Join(",", ($indexes | foreach { $rowcol[$_]  })) | Add-Content $outputFile
}

answered Dec 15, 2011 at 20:51

Tomas Panik

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