Powershell: Delete Every n Files - loops

Question

I just imported a bunch of pictures, and realized that there's 3 copies of each pictures, but they're named sequentially.
Basically these three files are the same:
P5240901.dng
P5240902.dng
P5240903.dng
And that, for about 1600 pictures.
I was looking into writing a simple PowerShell script (I use Windows) that would look into the directory of these files, and keep 1 file out of three, just looping through a range of files.
I didn't find something that would deal with the 'P' character before my file, and I'm not familiar with PowerShell language.
Any ideas?
Thank you!

Answer 1

Assuming everything in the dir follows the naming convention & is in a set of 3 something like this should work:
$mydir = 'C:\path\to\files'
[int]$idx = 1
get-childitem $mydir|sort-object {$_.Name} |foreach-object{
if ($idx % 3 -ne 1){ #get the modulus
$_ |remove-item
}
$idx++
}

Answer 2

Try the following, which will keep only the 1st file in each group of files whose names are the same except for the last character before the filename extension, assuming that character is a digit (syntax assumes PSv3+):
'P5240901.dng', 'P5240902.dng', 'P5240903.dng', 'A1.dng', 'A2.dng', 'singleton.dng' |
Group-Object { $_ -replace '^(.+)\d\.', '$1' } |
? Count -gt 1 |
% { $_.Group[1..$($_.Group.Count)] }
yields:
P5240902.dng
P5240903.dng
A2.dng
Replace the sample input array with a call to Get-ChildItem -File, and prepend Remove-Item to $_.Group[1..$($_.Group.Count)] to perform actual deletion.
The above command uses a string array with input filenames, but the [System.IO.FileInfo] instances output by Get-ChildItem will effectively act the same in a string context: they will expand to their respective filenames.
The advantage of this solution is that it doesn't rely on input files appearing strictly in groups of 3:
Any group of input files sharing the same name except for a digit before the filename extension that has at least 2 members (and any number beyond that) will have every member but the 1st deleted.
Any other files are left untouched.
Explanation:
Group-Object { $_ -replace '^(.+)\d\.', '$1' }
effectively groups the input files by the portion of the filename they share (but only if they share everything but the last char. before the filename extension, and if that char. is a digit).
? Count -gt 1
only passes on those resulting groups that have at least 2 members.
% { $_.Group[1..$($_.Group.Count)] }
processes each group's files, except the 1st.
Update: Here's a variation prompted by the OP's later comments:
The following, given input filenames such as P5240901.dng, P5240902.dng, ..., P5240910.dng, P5240911.dng, ..., P5240990.dng, P5240991.dng, ..., P5240999.dng, will consider each group of 10 files a group (based on the tens place), and within each group only retain the 1st file:
1..99 | % { "P52409$('{0:00}' -f $_).dng" } |
Group-Object { $_ -replace '^(.+\d)\d\.', '$1' } |
? Count -gt 1 |
% { $_.Group[1..$($_.Group.Count)]}
yields:
# tens place of 0; skips ...01.dng
P5240902.dng
P5240903.dng
... # up to ...09.dng
# tens place of 1; skips ...10.dng
P5240911.dng
P5240912.dng
... # skips ...20.dng, ...30.dng, ...
# tens place of 9; skips ...90.dng
P5240991.dng
P5240992.dng
...
P5240999.dng
In order to only pass the files of interest to the command, replace the sample input array with
Get-ChildItem P52515[0-9][0-9].dng.