Looping through a hash, or using an array in PowerShell

Question

I'm using this (simplified) chunk of code to extract a set of tables from SQL Server with BCP.

$OutputDirectory = "c:\junk\"
$ServerOption =   "-SServerName"
$TargetDatabase = "Content.dbo."

$ExtractTables = @(
    "Page"
    , "ChecklistItemCategory"
    , "ChecklistItem"
)

for ($i=0; $i -le $ExtractTables.Length – 1; $i++)  {
    $InputFullTableName = "$TargetDatabase$($ExtractTables[$i])"
    $OutputFullFileName = "$OutputDirectory$($ExtractTables[$i])"
    bcp $InputFullTableName out $OutputFullFileName -T -c $ServerOption
}

It works great, but now some of the tables need to be extracted via views, and some don't. So I need a data structure something like this:

"Page"                      "vExtractPage"
, "ChecklistItemCategory"   "ChecklistItemCategory"
, "ChecklistItem"           "vExtractChecklistItem"

I was looking at hashes, but I'm not finding anything on how to loop through a hash. What would be the right thing to do here? Perhaps just use an array, but with both values, separated by space?

Or am I missing something obvious?

Answer 1

Shorthand is not preferred for scripts; it is less readable. The %{} operator is considered shorthand. Here's how it should be done in a script for readability and reusability:

Variable Setup

PS> $hash = @{
    a = 1
    b = 2
    c = 3
}
PS> $hash

Name                           Value
----                           -----
c                              3
b                              2
a                              1

Option 1: GetEnumerator()

Note: personal preference; syntax is easier to read

The GetEnumerator() method would be done as shown:

foreach ($h in $hash.GetEnumerator()) {
    Write-Host "$($h.Name): $($h.Value)"
}

Output:

c: 3
b: 2
a: 1

Option 2: Keys

The Keys method would be done as shown:

foreach ($h in $hash.Keys) {
    Write-Host "${h}: $($hash.Item($h))"
}

Output:

c: 3
b: 2
a: 1

Additional information

Be careful sorting your hashtable...

Sort-Object may change it to an array:

PS> $hash.GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     Hashtable                                System.Object


PS> $hash = $hash.GetEnumerator() | Sort-Object Name
PS> $hash.GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     Object[]                                 System.Array

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This and other PowerShell looping are available on my blog.