Count-Object

With all of the great built-in commands for processing pipelines the absence of a good command to count the number of elements in a pipeline seems to stand out.  The best built-in way to count the number of objects in a pipeline is to convert the value into an array and then take the length. For instance take the following script which looks for the word “test” in all of the .ps1 scripts in or below the current directory

 $PS> gci –re –in *.ps1 | %{ ss test $_.FullName } 

Now if i want to count that i must convert it into an array and then take the length.  Simple enough right?

 $PS> @(gci –re –in *.ps1 | %{ ss test $_.FullName } ).Length

This approach works but has a couple of issues associated with it.

The first is a bit whiney I’ll admit.  I often use powershell scripts in an incremental fashion.  I write a search/expression, decide it includes to much data and refine it.  With any decent command shell this is a pretty simple operation, hit up and add another element to the powershell pipeline and you’re in business. 

The array method of counting though requires me to add data on both sides of the expression.  So it’s a choice of holding down left and waiting for the cursor to get to the right position or leaving the keyboard and opting for the mouse.  I don’t like either solution because it takes too long or gets my hand off of the keyboard.  Did I mention this was whiney?

The second is that it forces you to allocate a contiguous block of memory to examine the  length.  While this is usually not a big concern, it can cause noticeable performance issues if you are processing a lot of data.  This is especially true if the length is taken on inner expressions.

A better solution is using a filter to count the elements.  Filters integrate into the powershell pipeline and process data a single element at a time.  This solves both problems above.  The first is that it will smoothly integrate onto the end of an existing pipeline.  Also because it is a filter it processes each element individually preventing a huge array allocation. 

 function Count-Object() {
    begin {
        $count = 0
    }
    process {
        $count += 1
    }
    end {
        $count
    }
}

Now lets get back to the original problem.

 $PS> gci –re –in *.ps1 | %{ ss test $_.FullName } | count-object 

Much nicer

Comments

• Anonymous
  December 19, 2008
  I like the filter, but you could also have used the built in measure-object. PS>dir | measure-object Count    : 2749 Average  : Sum      : Maximum  : Minimum  : Property : It has a few other capabilities as well as you can see from the properties of the object returned. PS>dir | measure-object Length -sum -minimum -maximum Count    : 2652 Average  : Sum      : 1126972010 Maximum  : 17318336 Minimum  : 35 Property : Length