PS (2) > $a.length 2 in C#.NET

Painting Code-39 in C#.NET PS (2) > $a.length 2

PS (3) > $a[1] 2 3

However, if we simply run it through Foreach-Object, we ll find that the length of the result is now three, and the second element in the result is simply the number 2 . 172 FLOW CONTROL IN SCRIPTS

PS (4) > $b = $a | foreach { $_ } PS (5) > $b.length 3 PS (6) > $b[2] 2

In effect, the result has been flattened . However, if we use the unary comma operator before the $_ variable, the result has the same structure as the original array.

PS (7) > $b = $a | foreach { , $_ } PS (8) > $b.length 2 PS (9) > $b[1] 2 3

When chaining foreach cmdlets, we need to repeat the pattern at each stage:

PS (7) > $b = $a | foreach { , $_ } | foreach { , $_ } PS (8) > $b.length 2 PS (9) > $b[1] 2 3

Why did we do this Why didn t we just preserve the structure as we pass the elements through instead of unraveling by default Well, both behaviors are, in fact, useful. Consider the follow example, which returns a list of loaded module names:

Here the unraveling is exactly what we want. When we were designing PowerShell, we considered both cases; and in applications, on average, unraveling by default was usually what we needed. Unfortunately, it does present something of a cognitive bump that surprises users learning to use PowerShell. 6.8.2 The Where-Object cmdlet The other flow control cmdlet that is used a lot is Where-Object. This cmdlet is used to select objects from a list, kind of a simple switch cmdlet. It takes each element it receives as input, executes its scriptblock argument, passing in the current pipeline element as $_ and then, if the scriptblock evaluates to true, the element is written to the pipeline. We ll show this with an example that selects the even numbers from a sequence of integers:

PS (4) > 1..10 | where {! ($_ -band 1)} 2 4 6 8 10

The scriptblock enclosed in the braces receives each pipeline element, one after another. If the least significant bit in the element is 1 then the scriptblock returns the logical complement of that value ($false) and that element is discarded. If the least significant bit is zero then the logical complement of that is $true and the element is written to the output pipeline. Notice that the common alias for Where-Object is simply where. And, as with Foreach-Object, because this construction is so commonly used interactively, there is an additional alias, which is simply the question mark ( ). This allows the previous example to be written as:

PS (5) > 1..10| {!($_-band 1)} 2 4 6 8 10

Again this is brief, but it looks like the cat walked across the keyboard (trust me on this one). So while this is fine for interactive use, it is not recommended in scripts because it s hard to understand and maintain. As another, more compelling example of Software by Cats , here s a pathological example that combines elements from the last few chapters type casts, operators, and the flow control cmdlets to generate a list of strings of even-numbered letters in the alphabet, where the length of the string matches the ordinal number in the alphabet ( A is 1, B is 2, and so on).

PS (1) > 1..26| {!($_-band 1)}|%{[string][char]( >> [int][char]'A'+$_-1)*$_} >> BB DDDD FFFFFF HHHHHHHH JJJJJJJJJJ LLLLLLLLLLLL NNNNNNNNNNNNNN PPPPPPPPPPPPPPPP RRRRRRRRRRRRRRRRRR TTTTTTTTTTTTTTTTTTTT VVVVVVVVVVVVVVVVVVVVVV XXXXXXXXXXXXXXXXXXXXXXXX ZZZZZZZZZZZZZZZZZZZZZZZZZZ PS (2) >

The output is fairly self-explanatory, but the code is not. Figuring out how this works is left as an exercise to the reader and as a cautionary tale not to foist this sort of rubbish on unsuspecting coworkers.