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I am new to PowerShell, learning it on the fly coming from Python background.

I am pulling data from another tool where the data is retrieved through a REST call.

METERS variable is stored in this format in the source.

{
    "500 HR": 500,
    "1000 HR": 1000,
    "2000 HR": 2000
}

PowerShell code

#REST call to source
$meter=@{}
Foreach ($item in $origresult.Items)  {
$result = (Invoke-RestMethod -Uri $url  -Headers $headers -Method GET -ContentType application/json -ErrorVariable RespErr)
$meter.Add($item.Name,$result.Value)
}
Write-Host ($meter | Out-String) -ForegroundColor Red

This is the Output

Name                           Value
----                           -----
LastUploaded                   2020-12-29T06:38:02
IsEnabled                      1       
METERS                         {...
ORGID                          WHS

How do I retrieve METERS and traverse through the dictionary? I tried this so far. Python spoiled me with its simple data structures, PowerShell is not so straightforward unless there is a simpler way to do this.

$mymeters = $meter.METERS | ConvertFrom-Json
Write-Host ($mymeters | Out-String) -ForegroundColor Yellow

Output

500 HR   : 500
1000 HR  : 1000
2000 HR  : 2000

Here are somethings I have tried so far -

$mymeters = [ordered]@{}
" Here is the item $mymeters.Get_Item(500 HR)" #my silly attempt!
# Looping is a no go either - it says specialized ordered dictionary
foreach ($ind in $mymeters) {
" --> $ind"
}

Output

 Here is the item System.Collections.Specialized.OrderedDictionary.Get_Item(500 HR)
 --> System.Collections.Specialized.OrderedDictionary

I might be missing something real basic but I am unable to figure it out on my own! Any help is really appreciated. All I want is to traverse on the METERS hashtable/dictionary and call a function.

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2 Answers 2

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3

Before getting into the meat of it, let's review a few PowerShell syntax basics and see if we can re-use some of your Pythonic intuition :)

Member access

Just like in Python, you can reference the properties of an object by name using the . member access operator - for non-contiguous names, use quotes:

$mymeters = $meter.METERS | ConvertFrom-Json
$mymeters.'500 HR'  # evaluates to `500`

String expressions

String literals in PowerShell comes in two distinct flavours:

  • Single-quoted strings ('Hello World!')
    • These are verbatim string literals, and the only supported escape sequence is '' (literal single-quote)
  • Double-quoted strings ("Hello World!")
    • These are expandable string literals - automatic interpolation of $variable tokens and $() subexpressions will occur unless explicitly escaped - ` is the escape character, and common sequences you'd expect in most C-like languages (`n, `t, `r, etc.) are natively recognized.

Arbitrary expressions (like $dictionary.get_Item('some key')) will not be evaluated as-is, however.

To get around that, we can either use the -f string format operator:

$mymeters = [ordered]@{}
"Here is item '500 HR': {0}" -f $mymeters['500 HR']

-f should feel familiar if your used to Python3's f strings, but with a caveat - PowerShell's -f operator is a thin wrapper around String.Format(), and String.Format() only supports 0-based placeholders - '{0} {1}' -f 1,2 is valid but '{} {}' -f 1,2 is not.

Another option is to wrap the expression in a $() sub-expression operator inside a double-quoted string literal:

$mymeters = [ordered]@{}
"Here is item '500 HR': $($mymeters['500 HR'])"

Take note that dictionaries in PowerShell supports keyed index access with [] - just like Python :)

Not unlike Python, PowerShell (and .NET in general) has powerful introspection capabilities as well.

Dynamically discovering and iterating the properties of any object is as simple as referencing a special member named psobject:

foreach($propertyMetadataEntry in $someObject.psobject.Properties){
    "Property: {0,-20} = {1}" -f $propertyMetadataEntry.Name,$propertyMetadataEntry.Value
}

Or in your case:

$mymeters = $meter.METERS | ConvertFrom-Json
foreach($meterReading in $mymeters.psobject.Properties){
    "Meter: {0,-20} = {1}" -f $meterReading.Name,$meterReading.Value

    # do whatever else you please with $meterReading here :)
}

This will work for any scalar object (like the object returned by ConvertFrom-Json or Invoke-RestMethod).

For iterating over the entries in a dictionary, you need to explicitly call .GetEnumerator():

$dictionary = [ordered]@{ A = 1; B = 2; C =3 }
foreach($keyValuePair in $dictionary.GetEnumerator()){
    "Dictionary Entry: {0} = {1}" -f $keyValuePair.Key,$keyValuePair.Value
}
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6 Comments

Thank you so much for the basics and the 101, really helped understand the nuances when compared to Python. I still got this when I tried your approach - Meter: Count = 0 Meter: IsReadOnly = False Meter: Keys = System.Collections.Specialized.OrderedDictionary+OrderedDictionaryKeyValueCollection Meter: Values = System.Collections.Specialized.OrderedDictionary+OrderedDictionaryKeyValueCollection Meter: IsFixedSize = False Meter: SyncRoot = System.Object Meter: IsSynchronized = False
Those are the properties of an ordered dictionary object (like the one produced by [ordered]@{}). If you want the entries in a dictionary, you'll need to obtain an enumerator: foreach($kvp in $dictionary.GetEnumerator()){ $kvp.Key,$kvp.Value -join '='}
$mymeters = $meter.METERS | ConvertFrom-Json foreach($kvp in $mymeters.GetEnumerator()) { " --> I got here <-- " $kvp.Key,$kvp.Value -join '=' } I got a null-valued expression error, You cannot call a method on a null-valued expression.
The other way around - ConvertFrom-Json returns a single [PSCustomObject], against which you'll want to iterate over .psobject.Properties. [ordered]@{}, on the other hand, returns an ordered dictionary - and for dictionaries, you need GetEnumerator()
Ah, that did the trick, thank you so much! I used this foreach($meterReading in $mymeters.psobject.Properties) like you suggested.
|
1

Please check my answer below:

1.) I tested the stuff on powershell 5.x. Newer or older Versions of powershell (particulary older) might work slightly different

2.) Invoke-Restmethod does automatically convert the json response into a powershell object, so no further processing needed. Get rid of all the assignment into an hashtable.

$responseJson = '{
    "500 HR": 500,
    "1000 HR": 1000,
    "2000 HR": 2000
}'

$response = $responseJson | ConvertFrom-Json

$nodes = (Get-Member -inputobject $response  -MemberType NoteProperty).Name

$nodes | ForEach-Object {
    echo ("Element: $_ Result: " +$response.$_)
    
}

echo "Another one"

#alternative
foreach ($node in $nodes) {
    echo ("Element: $node Result: " +$response.$node)
}

3.) I think the response is bad formated, so if you have control over the restservice I recommend something like this:

$responseJson = '{
    "hr": [
     500,
    1000,
    2000
    ]
}'

$response = $responseJson | ConvertFrom-Json

$response.hr | ForEach-Object {
    echo ("Element: $_ Result: " +$response.$_)
    
}
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3 Comments

I do have some control over the source, the way METERS has a JSON string can be changed. I need key,value pairs, some edge cases like 12KHR: 12000 and the like. I am able to traverse through the meters with your nodes logic. I got this as output for both options you provided - Ex. Element: 500 HR Result: , can I get key,value output instead? $response.$node or $response.$_ is blank.
Hi, the Key is in $node and the value is in $response.node or what am I missing?
My apologies, variable mess up on my part. This worked too. Thanks again!

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