powershell foreach - changes to arrayitems don't update array

Question

I'm currently writing a PowerShell script to help out a user.

I am reading a file which is generated by a program into an array.

I iterate through it with a foreach and make changes to it as necessary.

Then I want to write the text, including changes, into a new file.

Param(
    [Parameter(Mandatory=$true, Position=0, HelpMessage="pulse?")]
    [string]$pulse,

    [Parameter(Mandatory=$true, Position=1, HelpMessage="milimeter?")]
    [string]$milimeter
)

$textfile = Get-Content C:\11111_O.jbi

foreach($string in $textfile) {

    $string -match '(EC\d*=)'

    if($matches) {
        [string]$regex = $matches[1]
        [string]$replacement = ($regex + $pulse + ',')
        $string = $string -replace '(EC\d*=)', "$replacement"
    }
}

$textfile | Out-File -FilePath C:\new_file.jbi

But even though i have checked the code inside the foreach multiple times (it does what it's supposed to do to $string). The output of $textfile always stays the same.

How can I get $textfile to update and reflect the changes I want to do to it in my foreach?

arco444 · Accepted Answer · 2017-02-13 15:40:06Z

5

Because you're using foreach, you get a copy of each line into the $string variable - that is what you modify on each iteration, so the contents of $textfile itself are never changed.

You can just use a for loop instead:

for ($i = 0; $i -lt $textfile.count; $i++) {
  $textfile[$i] -match '(EC\d*=)'
  if($matches) {
    [string]$regex = $matches[1]
    [string]$replacement = ($regex + $pulse + ',')
    $textfile[$i] = $textfile[$i] -replace '(EC\d*=)', "$replacement"
  }
}

$textfile | out-file -filepath C:\new_file.jbi

answered Feb 13, 2017 at 15:40

arco444

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briantist · Accepted Answer · 2017-02-13 19:00:11Z

2

The $string variable in the foreach statement refers to the "current" item in the collection, but it's a copy, not a reference to the original collection.

You should make a new array or make it a pipeline with the ForEach-Object cmdlet:

$textfile | ForEach-Object -Process {
    $string = $_
    if($string -match '(EC\d*=)')
    { 
        [string]$regex = $matches[1]
        [string]$replacement = ($regex + $pulse + ',')
        $string = $string -replace '(EC\d*=)', "$replacement"
    }
    $string    
} | out-file -filepath C:\new_file.jbi

Your regex stuff is also unnecessarily complex. You can just do the replace, using a backreference to refer to the matched part, so the whole thing can be further simplified to this:

$textfile | ForEach-Object -Process {
    $_ -replace '(EC\d*=)', "`${1}$pulse,"
} | out-file -filepath C:\new_file.jbi

Having done that, you no longer need ForEach-Object because -replace can work on arrays:

$textfile -replace '(EC\d*=)', "`${1}$pulse," | 
   out-file -filepath C:\new_file.jbi

edited Feb 13, 2017 at 19:00

answered Feb 13, 2017 at 15:40

briantist

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6 Comments

mjolinor Over a year ago

I can't get that second example to work. I can get -replace '(EC\d*=)', "`$1$pulse," to work, but it fails if $pulse starts with a digit. Also, I think you need an -append on that Out-File, or trade it for Add-Content.

briantist Over a year ago

@mjolinor you're right, I updated the code so it works correctly with the backreference, but I don't think you'd want to append in this case. The output of the ForEach-Object call will all be piped into Out-File, so it will contain the full content, unless I'm missing something.

mjolinor Over a year ago

You're right about the -append, but I'd still consider that replace potentially buggy. Also, the Foreach isn't really necessary. -Replace will work as an array operator.

briantist Over a year ago

@mjolinor true, -replace as array operator is even better, but why would the replace be buggy?

mjolinor Over a year ago

As I said earlier, it seems to fail if $pulse starts with a digit. I think that confuses the parsing of the backreference.

|

Mike Garuccio · Accepted Answer · 2017-02-13 15:39:14Z

0

If you really need to update $textfile in place you would need to use a for loop, however if you are just outputting to a file there is no need to manipulate the array, you can just dump the manipulated contents straight to the file.

foreach($string in $textfile) {
  $string -match '(EC\d*=)'
  if($matches) { 
    [string]$regex = $matches[1]
    [string]$replacement = ($regex + $pulse + ',')
    $string -replace '(EC\d*=)', "$replacement"
  }
} | out-file -filepath C:\new_file.jbi

answered Feb 13, 2017 at 15:39

Mike Garuccio

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