How can I repeat an iteration in a PowerShell foreach loop?

Question

I frequently have to loop through sets of data (SQL results, lists of computers, etc.) in PowerShell, performing the same operation (e.g function X) on each time. I use foreach loops almost exclusively as they are simple, effective and easily understood by others who may need to follow my code.

I would like to make some of my code more robust, in the sense of retrying operations that fail (up to Y times). There's more than one way to achieve this, for example within the foreach loop, wrapping function X in a do{} while() loop. In this example, assume that function X only returns non-null results when it is "successful":

foreach($dataitem in $dataset){
  $Result = $null
  $Attempts = 0
  do{
    $Attempts++
    $Result = <call function X on $dataitem>
  } while(($Attempts -lt 3) -and (-not $Result))
}

I was wondering whether there was any way to flatten this logic a bit, i.e. a more advanced way of using foreach loops, so I can do away with the do{} while() loop.

I have encountered the opposite of what I want, namely using $foreach.MoveNext() to skip forwards in the loop, but haven't found anything that (dangerously?) would keep foreach processing the same item.

Essentially: Can a foreach loop be made to re-do an iteration?.

MisterSeajay · Accepted Answer · 2016-04-07 16:14:38Z

2

While I agree with Jisaak, to answer your question you could achieve this using something like the following

foreach ($i in 1..10) {
    $i
    if ($limit++ -gt 10) { break } # just to stop infinite loop

    if ($i -eq 4) {
        foreach.Reset()
        1..($i-1) | Foreach-Object {
               [void]$foreach.MoveNext()
        }
    }
}

Of course I wouldn't do anything this silly outside of an exercise. The mix of foreach() {} and Foreach-Object is done to simplify the scope of the automatic IEnumeration object $foreach.

Edit

I thought it useful to include a link to a page that describes the difference between foreach as a keyword and as a cmdlet:

edited Apr 7, 2016 at 16:14

MisterSeajay

4,7891 gold badge28 silver badges53 bronze badges

answered Apr 7, 2016 at 13:37

Djarid

5144 silver badges11 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

MisterSeajay Over a year ago

Interesting; so it is possible to .Reset the foreach loop? And then the foreach "cursor" is made to catch up to the previous item before this iteration ends and, hey-presto, you are now re-processing the same item you were just on.

MisterSeajay Over a year ago

And I think I need to think harder about the difference between foreach and Foreach-Object...

MisterSeajay Over a year ago

OK, so I don't like this answer in the sense of ever planning to use it, but it does tackle what I was wondering, i.e. how to control the flow of foreach.

Martin Brandl · Accepted Answer · 2016-04-07 11:24:48Z

2

I wouldn't try to re-do an iteration of a foreach loop. That doesn't feels right to me. Instead, I would create a function that implement something like a retry-logic pattern:

function Retry-Process()
{
    Param(
        [scriptblock]$action,
        [scriptBlock]$validator,
        [int]$retryCount
    )

    1 .. $retryCount | % {  
        $result = & $action
        if (& $validator ($result))
        {
            $result
            break
        }    
    }
}

Example call:

Retry-Process -action { '3' } -validator { Param($a)  $a -eq '3'} -retryCount 5

And within your foreach loop:

foreach($dataitem in $dataset) {
  $Result = Retry-Process `
    -action { <call function X on $dataitem> } `
    -validator { Param($returnValue)  $returnValue } ` # Validate return value != null
    -retryCount 3
}

answered Apr 7, 2016 at 11:24

Martin Brandl

59.4k16 gold badges151 silver badges190 bronze badges

2 Comments

MisterSeajay Over a year ago

I like the proposal, hence upvote, but ultimately I'm wondering if there is a way to control the way that foreach iterates through the array.

Martin Brandl Over a year ago

Well, you could do such things with a for loop, for example for ($i = 1; $i -lt $dataset.count; $i++) where you just decrement $i if its not the expected result. Anway - what you need is a implementation of the well known retry-logic pattern. I woudln`t try to implement that using foreach manipulations when you can define a reusable function.

Collectives™ on Stack Overflow

How can I repeat an iteration in a PowerShell foreach loop?

2 Answers 2

Edit

3 Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Edit

3 Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related