Is this expected behavior from a Powershell for loop

Question

A little background: I am writing a short method to transform SomethingLikeThis into something_like_this. I was using a foreach loop on the .ToCharArray of $name, but that has not worked out as expected for input such as BIOS and PCIDevices. I opted to go for a for loop so I can check the previous character. To be clear, I do not need help on that portion of the code... Please read on.

function Get-FileName {
    param([string] $name)
    $out = ''
    $chars = $name.ToCharArray()

    for ($i = 0; $i -lt $chars.Length; $i++, $c = $chars[$i]) {
        if ($c -ge 'A' -and $c -le 'Z' -and -not $out.Length -eq 0) {
            $out += '_'
        }

        Write-Host $c
        $out += $c.ToString().ToLower()
    }

    return $out;
}

Get-FileName "BIOS"

The error this gives me is:

Invalid assignment expression. The left hand side of an assignment operator needs to be something that can be assigned to like a variable or a property.

Ideally I would have for ($i = 0; $i -lt $chars.Length; $c = $chars[$i++])

But that causes this to happen

You cannot call a method on a null-valued expression.
At C:\Users\kylestev\Desktop\test.ps1:145 char:32
+             $out += $c.ToString <<<< ().ToLower()
B
I
O
b_i_o

I come from a Java and C# background so this not working is kind of a let down. Anyone know of a workaround for making this work as a one liner? I would rather not have to assign a value to $c in the body of the loop, but I can if there is no workaround.

Cole9350 · Accepted Answer · 2013-07-26 19:33:49Z

2

Its only null because you never declared it.

$c= ''
for ($i = 0; $i -lt $chars.Length; $i++,($c = $chars[$i])) {

Just like Java and C#, declaring variables and parenthesis still apply.

answered Jul 26, 2013 at 19:33

Cole9350

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5 Comments

kstev Over a year ago

Why is $c = $chars[$i] not a declaration where $i = 0 is?

Cole9350 Over a year ago

That statement says "Index into chars array at the value of $i, and then assign the value to C" The script errors only on the first iteration of the loop. Because, like other languages, the order of operations for a for loop is for(do first;condition;execute after). So you try to do a $c.tostring before c=$chars[$i++]

Cole9350 Over a year ago

So to answer your question directly. They are both declarations! you just needed to declare it earlier

kstev Over a year ago

Ah... Totally forgot that it executes after the loop has completed. Thanks!

Cole9350 Over a year ago

No problem man. The sooner you start treating powershell like any other Object Oriented language, the easier it gets.IMO

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