[powershell] Looping through a hash, or using an array in PowerShell

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?

Here is another quick way, just using the key as an index into the hash table to get the value:

$hash = @{
    'a' = 1;
    'b' = 2;
    'c' = 3
};

foreach($key in $hash.keys) {
    Write-Host ("Key = " + $key + " and Value = " + $hash[$key]);
}

I prefer this variant on the enumerator method with a pipeline, because you don't have to refer to the hash table in the foreach (tested in PowerShell 5):

$hash = @{
    'a' = 3
    'b' = 2
    'c' = 1
}
$hash.getEnumerator() | foreach {
    Write-Host ("Key = " + $_.key + " and Value = " + $_.value);
}

Output:

Key = c and Value = 1
Key = b and Value = 2
Key = a and Value = 3

Now, this has not been deliberately sorted on value, the enumerator simply returns the objects in reverse order.

But since this is a pipeline, I now can sort the objects received from the enumerator on value:

$hash.getEnumerator() | sort-object -Property value -Desc | foreach {
  Write-Host ("Key = " + $_.key + " and Value = " + $_.value);
}

Output:

Key = a and Value = 3
Key = b and Value = 2
Key = c and Value = 1

About looping through a hash:

$Q = @{"ONE"="1";"TWO"="2";"THREE"="3"}
$Q.GETENUMERATOR() | % { $_.VALUE }
1
3
2

$Q.GETENUMERATOR() | % { $_.key }
ONE
THREE
TWO

A short traverse could be given too using the sub-expression operator $( ), which returns the result of one or more statements.

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

forEach($y in $hash.Keys){
    Write-Host "$y -> $($hash[$y])"
}

Result:

a -> 1
b -> 2
c -> 3

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

This and other PowerShell looping are available on my blog.

You can also do this without a variable

@{
  'foo' = 222
  'bar' = 333
  'baz' = 444
  'qux' = 555
} | % getEnumerator | % {
  $_.key
  $_.value
}

If you're using PowerShell v3, you can use JSON instead of a hashtable, and convert it to an object with Convert-FromJson:

@'
[
    {
        FileName = "Page";
        ObjectName = "vExtractPage";
    },
    {
        ObjectName = "ChecklistItemCategory";
    },
    {
        ObjectName = "ChecklistItem";
    },
]
'@ | 
    Convert-FromJson |
    ForEach-Object {
        $InputFullTableName = '{0}{1}' -f $TargetDatabase,$_.ObjectName

        # In strict mode, you can't reference a property that doesn't exist, 
        #so check if it has an explicit filename firest.
        $outputFileName = $_.ObjectName
        if( $_ | Get-Member FileName )
        {
            $outputFileName = $_.FileName
        }
        $OutputFullFileName = Join-Path $OutputDirectory $outputFileName

        bcp $InputFullTableName out $OutputFullFileName -T -c $ServerOption
    }