Join every other line in Powershell

Question

I want to combine every other line from the input below. Here is the input.

ALPHA-FETOPROTEIN      ROUTINE    CH 0203 001   02/03/2023@10:45 LIVERF3
     ###-##-####    #######,####        In lab
ALPHA-FETOPROTEIN      ROUTINE    CH 0203 234   02/03/2023@11:05 LIVER
     ###-##-####    ########,########   In lab
ANION GAP              STAT       CH 0203 124   02/03/2023@11:06 DAY
     ###-##-####    ######,##### ####   In lab
BASIC METABOLIC PANE   ROUTINE    CH 0203 001   02/03/2023@10:45 LIVERF3
     ###-##-####    #######,#### ###### In lab

This is the desired output

ALPHA-FETOPROTEIN      ROUTINE    CH 0203 001   02/03/2023@10:45 LIVERF3 ###-##-####    #######,####        In lab
ALPHA-FETOPROTEIN      ROUTINE    CH 0203 234   02/03/2023@11:05 LIVER ###-##-####    ########,########   In lab
ANION GAP              STAT       CH 0203 124   02/03/2023@11:06 DAY ###-##-####    ######,##### ####   In lab
BASIC METABOLIC PANE   ROUTINE    CH 0203 001   02/03/2023@10:45 LIVERF3 ###-##-####    #######,#### ###### In lab

The code that I have tried is

for($i = 0; $i -lt $splitLines.Count; $i += 2){
  $splitLines[$i,($i+1)] -join ' '
}

It came from Joining every two lines in Powershell output. But I can't seem to get it to work for me. I'm not well versed with powershell, but i'm at the mercy of what's available at work.

Edit: Here is the entire code that I am using as requested.

# SET VARIABLES
$inputfile = "C:\Users\Will\Desktop\testfile.txt"
$outputfile = "C:\Users\Will\Desktop\testfileformatted.txt"
$new_output = "C:\Users\Will\Desktop\new_formatted.txt"

# REMOVE EXTRA CHARACTERS
$remove_beginning_capture = "-------------------------------------------------------------------------------"
$remove_end_capture = "==============================================================================="
$remove_line = "------"
$remove_strings_with_spaces = "            \d"
Get-Content $inputfile | Where-Object {$_ -notmatch $remove_beginning_capture} | Where-Object {$_ -notmatch $remove_end_capture} | Where-Object {$_ -notmatch $remove_line} | Where-Object {$_ -notmatch $remove_strings_with_spaces}  | ? {$_.trim() -ne "" } | Set-Content $outputfile

# Measures line length for loop
$file_lines = gc $outputfile | Measure-Object

#Remove Whitespace
# $whitespace_removed = (Get-Content $outputfile -Raw) -replace '\s+', ' '| Set-Content -Path C:\Users\Will\Desktop\new_formatted.csv

# Combine every other line
$lines = Get-Content $outputfile -Raw
$newcontent = $lines.Replace("`n","")
Write-Host "Content: $newcontent"
$newcontent | Set-Content $new_output

for($i = 0; $i -lt $splitLines.Count; $i += 2){
  $splitLines[$i,($i+1)] -join ' '
}

Answer 1

Just read two lines and then print one

$inputFilename = "c:\temp\test.txt"
$outputFilename = "c:\temp\test1.txt"

$reader = [System.IO.StreamReader]::new($inputFilename)
$writer = [System.IO.StreamWriter]::new($outputFilename)
while(($line = $reader.ReadLine()) -ne $null)
{
   $secondLine = ""
   if(!$reader.EndOfStream){ $secondLine = $reader.ReadLine() }

   $writer.WriteLine($line + $secondLine)
}
$reader.Close()
$writer.Flush()
$writer.Close()

Answer 2

PowerShell-idiomatic solutions:

Use Get-Content with -ReadCount 2 in order to read the lines from your file in pairs, which allows you to process each pair in a ForEach-Object call, where the constituent lines can be joined to form a single output line.

Get-Content -ReadCount 2 yourFile.txt | 
  ForEach-Object { $_[0] + ' ' +  $_[1].TrimStart() }

The above directly outputs the resulting lines (as the for command in your question does), causing them to print to the display by default.

Pipe to Set-Content to save the output to a file:

Get-Content -ReadCount 2 yourFile.txt | 
  ForEach-Object { $_[0] + ' ' +  $_[1].TrimStart() } |
  Set-Content yourOutputFile.txt

Performance notes:

• Unfortunately (as of PowerShell 7.3.2), Get-Content is quite slow by default - see GitHub issue #7537, and the performance of ForEach-Object and Where-Object could be improved too - see GitHub issue #10982.

• At the expense of collecting all inputs and outputs in memory first, you can noticeably improve the performance with the following variation, which avoids the ForEach-Object cmdlet in favor of the intrinsic .ForEach() method, and, instead of piping to Set-Content, passes all output lines via the -Value parameter:

  Set-Content $tempOutFile -Value (
    (Get-Content -ReadCount 2 $tempInFile).ForEach({ $_[0] + ' ' + $_[1].TrimStart() })
  )
  

• Read on for even faster alternatives, but remember that optimizations are only worth undertaking if actually needed - if the first PowerShell-idiomatic solution above is fast enough in practice, it is worth using for its conceptual elegance and concision.

  • See this Gist for benchmarks that compare the relative performance of the solutions in this answer as well as that of the solution from jdweng's .NET API-based answer.

---

An better-performing alternative is to use a switch statement with the -File parameter to process files line by line:

$i = 1
switch -File yourFile.txt {
  default {
    if ($i++ % 2) { $firstLineInPair = $_ }
    else          { $firstLineInPair + ' ' + $_.TrimStart() } 
  }
} 

^{Helper index variable $i and the modulo operation (%) are simply used to identify which line is the start of a (new) pair, and which one is its second half.}

• The switch statement is itself streaming, but it cannot be used as-is as pipeline input. By enclosing it in & { ... }, it can, but that forfeits some of the performance benefits, making it only marginally faster than the optimized Get-Content -ReadCount 2 solution:

  & {
    $i = 1
    switch -File yourFile.txt {
      default {
        if ($i++ % 2) { $firstLineInPair = $_ }
        else          { $firstLineInPair + ' ' + $_.TrimStart() } 
      }
    } 
  } | Set-Content yourOutputFile.txt
  

• For the best performance when writing to a file, use Set-Content $outFile -Value $(...), albeit at the expense of collecting all output lines in memory first:

  Set-Content yourOutputFile.txt -Value $(
    $i = 1
    switch -File yourFile.txt {
      default {
        if ($i++ % 2) { $firstLineInPair = $_ }
        else          { $firstLineInPair + ' ' + $_.TrimStart() } 
      }
    } 
  )
  

---

The fastest and most concise solution is to use a regex-based approach, which reads the entire file up front:

(Get-Content -Raw yourFile.txt) -replace '(.+)\r?\n(?: *)(.+\r?\n)', '$1 $2'

Note:

• The assumption is that all lines are paired, and that the last line has a trailing newline.

• The -replace operation matches two consecutive lines, and joins them together with a space, ignoring leading spaces on the second line. For a detailed explanation of the regex and the ability to interact with it, see this regex101.com page.

• To save the output to a file, you can pipe directly to Set-Content:

  (Get-Content -Raw yourFile.txt) -replace '(.+)\r?\n(?: *)(.+\r?\n)', '$1 $2' |
    Set-Content yourOutputFile.txt
  

  • In this case, because the pipeline input to Set-Content is provided by an expression that doesn't involve for-every-input-line calls to script blocks ({ ... }) (as the switch solution requires), there is virtually no slowdown resulting from use of the pipeline (whose use is generally preferable for conceptual elegance and concision).

---

As for what you tried:

The $splitLines-based solution in your question is predicated on having assigned all lines of the input file to this self-chosen variable as an array, which your code does not do.

While you could fill variable $splitLines with an array of lines from your input file with $splitLines = Get-Content yourFile.txt, given that Get-Content reads text files line by line by default, the switch-based line-by-line solution is more efficient and streams its results (which - if saved to a file - keeps memory usage constant, which matters with large input sets (though rarely with text files)).

A performance tip when reading all lines at once into an array with Get-Content: use -ReadCount 0, which greatly speeds up the operation:
$splitLines = Get-Content -ReadCount 0 yourFile.txt