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

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?.

Answer 1

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
}

Answer 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:
http://www.powershelladmin.com/wiki/PowerShell_foreach_loops_and_ForEach-Object
about_Foreach
Foreach-Object