1

Is there any way of ending a pipe operation while you are in a filter?

A good use would be to get the first n elements of the list, similar to SQL's TOP statement. If we wrote a filter, call it "Top", we could retrieve the first 3 elements like so:

dir | Top 3

Once we have returned the first three, we don't need to process any more elements and in effect we want to be able to tell the pipe

Please don't give me any more elements

I know that Select-Object has the -First argument but that is not the question I am asking.

I have answered the question Powershell equivalent to F# Seq.forall but instead I would rather implement it as a filter which ends pipeline input. If there was an operation called "end-pipeline", I would implement ForAll as:

filter ForAll([scriptblock] $predicate) {
    process {
        if ( @($_ | Where-Object $predicate).Length -eq 0 ) {
            $false;
            end-pipeline;
        }
    }
    end {
        $true;
    }
}
Community
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Tahir Hassan
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2 Answers2

2

you could use "break" statement, it will behave as your "end-pipeline" cmdlet

Jackie
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    It works. However the link http://powershell.com/cs/blogs/tobias/archive/2010/01/01/cancelling-a-pipeline.aspx provided by Christian makes a big deal about ending a pipeline. Is the "break" statement safe to use in all situations? – Tahir Hassan Oct 31 '12 at 11:31
  • generally speaking, it's safe, but it could break the execution of the whole script if you invoke a script file which contains break that is not within some loops like while, for etc. – Jackie Oct 31 '12 at 11:39
  • @TahirHassan - `break` will stop the rest of the script from executing. Try: `0..9 | .{process{if($_ -lt 5){$_} else{break;}}}| measure; write 'end'`. Note that in that example, `measure` does not return anything and "end" does not get printed. You could wrap the whole pipeline in a `try..catch`, but the measure command at the end of the pipeline still would not work. I have been trying to find a good solution to this problem, but haven't yet; see my question at http://stackoverflow.com/q/8749206/291709 – Rynant Nov 01 '12 at 16:04
  • Aware! If you use `break` in a pipeline, which in turn is in some loop, then the pipe will stop, but the loop also! – Davor Josipovic Jun 15 '13 at 17:44
1

Maybe you can add a return if the condition is reached.

Look here for some example

This maybe works:

filter ForAll([scriptblock] $predicate)
{
  process
         {        
            if ( @($input | Where-Object $predicate).length -eq 0 ) 
              {        
                 $false
                 return;
              }
         }
  end    
         {
            $true
         }
}

this works:

filter aaa {    
process 
{      
  if ( ++$i -gt 5 )
  {
      return
  }
  else
  {
      $_
  }
}
}

dir | aaa # return first five element of the pipe...
CB.
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