Select-String sometimes results in "System.Object[]" - powershell

Question

I'm working on a script that combines parts of two text files. These files are not too large (about 2000 lines each).
I'm seeing strange output from select-string that I don't think should be there.
Here's samples of my two files:
CC.csv - 2026 lines
LS126L47L6/1L2#519,07448,1,B
LS126L47L6/1R1-1#503,07449,1,B
LS126L47L6/1L3#536,07450,1,B
LS126L47L6/2R1#515,07451,1,B
LS126L47L6/10#525,07452,1,B
LS126L47L6/1L4#538,07453,1,B
GI.txt - 1995 lines
07445,B,SH,1
07446,B,SH,1
07448,B,SH,1
07449,B,SH,1
07450,B,SH,1
07451,B,SH,1
07452,B,SH,1
07453,B,SH,1
07454,B,SH,1
And here's a sample of the output file:
output in myfile.csv
LS126L47L6/3R1#516,07446,1,B
LS126L47L6/1L2#519,07448,1,B
LS126L47L6/1R1-1#503,07449,1,B
System.Object[],B
LS126L47L6/2R1#515,07451,1,B
This is the script I'm using:
sc ./myfile.csv "col1,col2,col3,col4"
$mn = gc cc.csv | select -skip 1 | % {$_.tostring().split(",")[1]}
$mn | % {
$a = (gc cc.csv | sls $_ ).tostring() -replace ",[a-z]$", ""
if (gc GI.txt | sls $_ | select -first 1)
{$b = (gc GI.txt | sls $_ | select -first 1).tostring().split(",")[1]}
else {$b = "NULL"
write-host "$_ is not present in GI file"}
$c = $a + ',' + $b
ac ./myfile.csv -value $c
}
The $a variable is where I am sometimes seeing the returned string as System.Object[]
Any ideas why? Also, this script takes quite some time to finish. Any tips for a newb on how to speed it up?
Edit: I should add that I've taken one line from the cc.csv file, saved in a new text file, and run through the script in console up through assigning $a. I can't get it to return "system.object[]".
Edit 2: After follow the advice below and trying a couple of things I've noticed that if I run
$mn | %{(gc cc.csv | sls $_).tostring()}
I get System.Object[].
But if I run
$mn | %{(gc cc.csv | sls $_)} | %{$_.tostring()}
It comes out fine. Go figure.

Answer 1

The problem is caused by a change in multiplicity of matches. If there are multiple matching elements an Object[] array (of MatchInfo elements) is returned; a single matching element results in a single MatchInfo object (not in an array); and when there are no matches, null is returned.
Consider these results, when executed against the "cc.csv" test-data supplied:
# matches many
(gc cc.csv | Select-String "LS" ).GetType().Name # => Object[]
# matches one
(gc cc.csv | Select-String "538").GetType().Name # => MatchInfo
# matches none
(gc cc.csv | Select-String "FAIL") # => null
The result of calling ToString on Object[] is "System.Object[]" while the result is a more useful concatenation of the matched values when invoked directly upon a MatchInfo object.
The immediate problem can be fixed with selected | Select -First 1, which will result in a MatchInfo being returned for the first two cases. Select-String will still search the entire input - extra results are simply discarded.
However, it seems like the look-back into "cc.csv" (with the Select-String) could be eliminated entirely as that is where $_ originally comes from. Here is a minor [untested] adaptation, of what it may look like:
gc cc.csv | Select -Skip 1 | %{
$num = $_.Split(",")[1]
$a = $_ -Replace ",[a-z]$", ""
# This is still O(m*n) and could be improved with a hash/set probe.
$gc_match = Select-String $num -Path gi.csv -SimpleMatch | Select -First 1
if ($gc_match) {
# Use of "Select -First 1" avoids the initial problem; but
# it /may/ be more appropriate for an error to indicate data problems.
# (Likewise, an error in the original may need further investigation.)
$b = $gc_match.ToString().Split(",")[1]
} else {
$b = "NULL"
Write-Host "$_ is not present in GI file"
}
$c = $a + ',' + $b
ac ./myfile.csv -Value $c
}