Is there a PowerShell equivalent to the 'zip' from functional programming?
What is zip (functional programming?)
I want to take two input sequences and return a sequence of tuples containing the paired elements.
Is there a built-in method or something built-in to the language that makes this easy?
BTW, I want the solution to be very 'natural' for PowerShell and integrate with the pipeline.
This request has come up from time to time. The idiomatic PowerShell solution would be a Join-Object command. The PowerShell team wrote this script along with some examples of using it on their blog - http://blogs.msdn.com/b/powershell/archive/2012/07/13/join-object.aspx
A bit cumbersome, but it works:
Script file test.ps1:
param( $fnop, $first, $second)
$retval = #()
$len = if($first.length -lt $second.length) { $first.length } else { $second.length }
for( $i = 0; $i -lt $len; $i++) {
$oprslt = $fnop.invoke( $first[$i], $second[$i])
$retval = $retval + $oprslt
}
$retval
Example:
# add and sort
PS> .\test.ps1 {param($a,$b) return $a+$b} (3,2,1) (8,-4,5) | sort
11
-2
6
# add only
PS> .\test.ps1 {param($a,$b) return $a+$b} (3,2,1) (8,-4,5)
-2
6
11
# multiply and sort
PS> .\test.ps1 {param($a,$b) return $a*$b} (3,2,1) (8,-4,5) | sort
-8
5
24
The function block {param($a,$b) return $a+$b} is your zip function.
I just want to point out that you can use tuples natively in Powershell:
$q = [System.Tuple]::Create('One','Two')
$q.Item1
$q.Item2
This doesn't necessarily help you or even make sense for this situation, but I thought it might interest you.
Related
I have trouble of getting index of the current element for multiple elements that are exactly the same object:
$b = "A","D","B","D","C","E","D","F"
$b | ? { $_ -contains "D" }
Alternative version:
$b = "A","D","B","D","C","E","D","F"
[Array]::FindAll($b, [Predicate[String]]{ $args[0] -contains "D" })
This will return:
D
D
D
But this code:
$b | % { $b.IndexOf("D") }
Alternative version:
[Array]::FindAll($b, [Predicate[String]]{ $args[0] -contains "D" }) | % { $b.IndexOf($_) }
Returns:
1
1
1
so it's pointing at the index of the first element. How to get indexes of the other elements?
You can do this:
$b = "A","D","B","D","C","E","D","F"
(0..($b.Count-1)) | where {$b[$_] -eq 'D'}
1
3
6
mjolinor's answer is conceptually elegant, but slow with large arrays, presumably due to having to build a parallel array of indices first (which is also memory-inefficient).
It is conceptually similar to the following LINQ-based solution (PSv3+), which is more memory-efficient and about twice as fast, but still slow:
$arr = 'A','D','B','D','C','E','D','F'
[Linq.Enumerable]::Where(
[Linq.Enumerable]::Range(0, $arr.Length),
[Func[int, bool]] { param($i) $arr[$i] -eq 'D' }
)
While any PowerShell looping solution is ultimately slow compared to a compiled language, the following alternative, while more verbose, is still much faster with large arrays:
PS C:\> & { param($arr, $val)
$i = 0
foreach ($el in $arr) { if ($el -eq $val) { $i } ++$i }
} ('A','D','B','D','C','E','D','F') 'D'
1
3
6
Note:
Perhaps surprisingly, this solution is even faster than Matt's solution, which calls [array]::IndexOf() in a loop instead of enumerating all elements.
Use of a script block (invoked with call operator & and arguments), while not strictly necessary, is used to prevent polluting the enclosing scope with helper variable $i.
The foreach statement is faster than the Foreach-Object cmdlet (whose built-in aliases are % and, confusingly, also foreach).
Simply (implicitly) outputting $i for each match makes PowerShell collect multiple results in an array.
If only one index is found, you'll get a scalar [int] instance instead; wrap the whole command in #(...) to ensure that you always get an array.
While $i by itself outputs the value of $i, ++$i by design does NOT (though you could use (++$i) to achieve that, if needed).
Unlike Array.IndexOf(), PowerShell's -eq operator is case-insensitive by default; for case-sensitivity, use -ceq instead.
It's easy to turn the above into a (simple) function (note that the parameters are purposely untyped, for flexibility):
function get-IndicesOf($Array, $Value) {
$i = 0
foreach ($el in $Array) {
if ($el -eq $Value) { $i }
++$i
}
}
# Sample call
PS C:\> get-IndicesOf ('A','D','B','D','C','E','D','F') 'D'
1
3
6
You would still need to loop with the static methods from [array] but if you are still curious something like this would work.
$b = "A","D","B","D","C","E","D","F"
$results = #()
$singleIndex = -1
Do{
$singleIndex = [array]::IndexOf($b,"D",$singleIndex + 1)
If($singleIndex -ge 0){$results += $singleIndex}
}While($singleIndex -ge 0)
$results
1
3
6
Loop until a match is not found. Assume the match at first by assigning the $singleIndex to -1 ( Which is what a non match would return). When a match is found add the index to a results array.
I'm trying to get something that looks like UNIX ls output in PowerShell. This is getting there:
Get-ChildItem | Format-Wide -AutoSize -Property Name
but it's still outputting the items in row-major instead of column-major order:
PS C:\Users\Mark Reed> Get-ChildItem | Format-Wide -AutoSize -Property Name
Contacts Desktop Documents Downloads Favorites
Links Music Pictures Saved Games
Searches Videos
Desired output:
PS C:\Users\Mark Reed> My-List-Files
Contacts Downloads Music Searches
Desktop Favorites Pictures Videos
Documents Links Saved Games
The difference is in the sorting: 1 2 3 4 5/6 7 8 9 reading across the lines, vs 1/2/3 4/5/6 7/8/9 reading down the columns.
I already have a script that will take an array and print it out in column-major order using Write-Host, though I found a lot of PowerShellish idiomatic improvements to it by reading Keith's and Roman's takes. But my impression from reading around is that's the wrong way to go about this. Instead of calling Write-Host, a script should output objects, and let the formatters and outputters take care of getting the right stuff written to the user's console.
When a script uses Write-Host, its output is not capturable; if I assign the result to a variable, I get a null variable and the output is written to the screen anyway. It's like a command in the middle of a UNIX pipeline writing directly to /dev/tty instead of standard output or even standard error.
Admittedly, I may not be able to do much with the array of Microsoft.PowerShell.Commands.Internal.Format.* objects I get back from e.g. Format-Wide, but at least it contains the output, which doesn't show up on my screen in rogue fashion, and which I can recreate at any time by passing the array to another formatter or outputter.
This is a simple-ish function that formats column major. You can do this all in PowerShell Script:
function Format-WideColMajor {
[CmdletBinding()]
param(
[Parameter(ValueFromPipeline)]
[AllowNull()]
[AllowEmptyString()]
[PSObject]
$InputObject,
[Parameter()]
$Property
)
begin {
$list = new-object System.Collections.Generic.List[PSObject]
}
process {
$list.Add($InputObject)
}
end {
if ($Property) {
$output = $list | Foreach {"$($_.$Property)"}
}
else {
$output = $list | Foreach {"$_"}
}
$conWidth = $Host.UI.RawUI.BufferSize.Width - 1
$maxLen = ($output | Measure-Object -Property Length -Maximum).Maximum
$colWidth = $maxLen + 1
$numCols = [Math]::Floor($conWidth / $colWidth)
$numRows = [Math]::Ceiling($output.Count / $numCols)
for ($i=0; $i -lt $numRows; $i++) {
$line = ""
for ($j = 0; $j -lt $numCols; $j++) {
$item = $output[$i + ($j * $numRows)]
$line += "$item$(' ' * ($colWidth - $item.Length))"
}
$line
}
}
}
I was wondering is it possible to have variables after a conditions, e.g. when normally a while sentence would be like that:
$i = 5
while ($i -le 5) {Write-Host $i; $i++}
But because I have so many variables in my script I thought may be I could write the variables at the end of the code so I could read the script's logic first not the variables, something like that:
while ($i -le 5) {Write-Host $i; $i++}
$i = 10
$Variable2 = 5
...
$Variable100 = 25
Powershell executes the lines in your script in order from top to bottom except where a loop, condition, or a function call make the execution jump somewhere else.
So, no, you cannot initialise a variable after you used it, that just wouldn't make sense.
Your solution here would be to move all the logic inside one or more functions, and then at the end of your script you can initialise variables and call the function(s). That does roughly what you are asking for.
You don't need to expressly declare a variable in PowerShell, if that's what you're asking. Variables that weren't defined before are automatically defined and initialized with default value the first time they're used.
However, depending on how the variable is used, you may inadvertently get different variables with the same name in different scopes:
PS C:\> function Foo { $i; $i = 23; $i }
PS C:\> Foo
23
PS C:\> $i = 42
PS C:\> $i
42
PS C:\> Foo
42
23
PS C:\> $i
42
PS C:\> function Bar { $i; $global:i = 23; $i }
PS C:\> Bar
42
23
PS C:\> $i
23
Also, if you want a variable to start with a specific value, you need to initialize it before you use it. If you have numerous initializations, you could put them into a separate PowerShell script and dot-source that second script in your "worker" script:
. 'C:\path\to\config.ps1'
is it possible to do something like below in powershell?
$condition = "-eq 10"
$a=11
if ($a &$condition) {"yes"} else {"no"}
basically to create part of the statement outside the statement and then use it dynamically?
I am aware I can create the whole statement in variable and then run it using Invoke-Expression or as &$cmd, but I have a very long and complex if/else statement and trying to build part of it on the fly.
Hope it makes sense :)
Yes, if you're OK with swapping order e.g.:
C:\PS> $conditionEq10 = {param($a) $a -eq 10}
C:\PS> $a = 11
C:\PS> if (&$conditionEq10 $a) {'yes'} else {'no'}
no
C:\PS> $a = 10
C:\PS> if (&$conditionEq10 $a) {'yes'} else {'no'}
yes
You can do prefix style notation using scriptblocks {code} with PowerShell but not infix nor postfix. That's because PowerShell wants the arguments to follow the scriptblock (not before and not between).
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A while back I was reading about multi-variable assignments in PowerShell. This lets you do things like this
64 > $a,$b,$c,$d = "A four word string".split()
65 > $a
A
66 > $b
four
Or you can swap variables in a single statement
$a,$b = $b,$a
What little known nuggets of PowerShell have you come across that you think may not be as well known as they should be?
The $$ command. I often have to do repeated operations on the same file path. For instance check out a file and then open it up in VIM. The $$ feature makes this trivial
PS> tf edit some\really\long\file\path.cpp
PS> gvim $$
It's short and simple but it saves a lot of time.
By far the most powerful feature of PowerShell is its ScriptBlock support. The fact that you can so concisely pass around what are effectively anonymous methods without any type constraints are about as powerful as C++ function pointers and as easy as C# or F# lambdas.
I mean how cool is it that using ScriptBlocks you can implement a "using" statement (which PowerShell doesn't have inherently). Or, pre-v2 you could even implement try-catch-finally.
function Using([Object]$Resource,[ScriptBlock]$Script) {
try {
&$Script
}
finally {
if ($Resource -is [IDisposable]) { $Resource.Dispose() }
}
}
Using ($File = [IO.File]::CreateText("$PWD\blah.txt")) {
$File.WriteLine(...)
}
How cool is that!
A feature that I find is often overlooked is the ability to pass a file to a switch statement.
Switch will iterate through the lines and match against strings (or regular expressions with the -regex parameter), content of variables, numbers, or the line can be passed into an expression to be evaluated as $true or $false
switch -file 'C:\test.txt'
{
'sometext' {Do-Something}
$pwd {Do-SomethingElse}
42 {Write-Host "That's the answer."}
{Test-Path $_} {Do-AThirdThing}
default {'Nothing else matched'}
}
$OFS - output field separator. A handy way to specify how array elements are separated when rendered to a string:
PS> $OFS = ', '
PS> "$(1..5)"
1, 2, 3, 4, 5
PS> $OFS = ';'
PS> "$(1..5)"
1;2;3;4;5
PS> $OFS = $null # set back to default
PS> "$(1..5)"
1 2 3 4 5
Always guaranteeing you get an array result. Consider this code:
PS> $files = dir *.iMayNotExist
PS> $files.length
$files in this case may be $null, a scalar value or an array of values. $files.length isn't going to give you the number of files found for $null or for a single file. In the single file case, you will get the file's size!! Whenever I'm not sure how much data I'll get back I always enclose the command in an array subexpression like so:
PS> $files = #(dir *.iMayNotExist)
PS> $files.length # always returns number of files in array
Then $files will always be an array. It may be empty or have only a single element in it but it will be an array. This makes reasoning with the result much simpler.
Array covariance support:
PS> $arr = '127.0.0.1','192.168.1.100','192.168.1.101'
PS> $ips = [system.net.ipaddress[]]$arr
PS> $ips | ft IPAddressToString, AddressFamily -auto
IPAddressToString AddressFamily
----------------- -------------
127.0.0.1 InterNetwork
192.168.1.100 InterNetwork
192.168.1.101 InterNetwork
Comparing arrays using Compare-Object:
PS> $preamble = [System.Text.Encoding]::UTF8.GetPreamble()
PS> $preamble | foreach {"0x{0:X2}" -f $_}
0xEF
0xBB
0xBF
PS> $fileHeader = Get-Content Utf8File.txt -Enc byte -Total 3
PS> $fileheader | foreach {"0x{0:X2}" -f $_}
0xEF
0xBB
0xBF
PS> #(Compare-Object $preamble $fileHeader -sync 0).Length -eq 0
True
Fore more stuff like this, check out my free eBook - Effective PowerShell.
Along the lines of multi-variable assignments.
$list = 1,2,3,4
While($list) {
$head, $list = $list
$head
}
1
2
3
4
I've been using this:
if (!$?) { # if previous command was not successful
Do some stuff
}
and I also use $_ (current pipeline object) quite a bit, but these might be more known than other stuff.
The fact that many operators work on arrays as well and return the elements where a comparison is true or operate on each element of the array independently:
1..1000 -lt 800 -gt 400 -like "?[5-9]0" -replace 0 -as "int[]" -as "char[]" -notmatch "\d"
This is faster than Where-Object.
Not a language feature but super helpful
f8 -- Takes the text you have put in already and searches for a command that starts with that text.
Tab-search through your history with #
Example:
PS> Get-Process explorer
PS> "Ford Explorer"
PS> "Magellan" | Add-Content "great explorers.txt"
PS> type "great explorers.txt"
PS> #expl <-- Hit the <tab> key to cycle through history entries that have the term "expl"
Love this thread. I could list a ton of things after reading Windows Powershell in Action. There's a disconnect between that book and the documentation. I actually tried to list them all somewhere else here, but got put on hold for "not being a question".
I'll start with foreach with three script blocks (begin/process/end):
Get-ChildItem | ForEach-Object {$sum=0} {$sum++} {$sum}
Speaking of swapping two variables, here's swapping two files:
${c:file1.txt},${c:file2.txt} = ${c:file2.txt},${c:file1.txt}
Search and replace a file:
${c:file.txt} = ${c:file.txt} -replace 'oldstring','newstring'
Using assembly and using namespace statements:
using assembly System.Windows.Forms
using namespace System.Windows.Forms
[messagebox]::show('hello world')
A shorter version of foreach, with properties and methods
ps | foreach name
'hi.there' | Foreach Split .
Use $() operator outside of strings to combine two statements:
$( echo hi; echo there ) | measure
Get-content/Set-content with variables:
$a = ''
get-content variable:a | set-content -value there
Anonymous functions:
1..5 | & {process{$_ * 2}}
Give the anonymous function a name:
$function:timestwo = {process{$_ * 2}}
Anonymous function with parameters:
& {param($x,$y) $x+$y} 2 5
You can stream from foreach () with these, where normally you can't:
& { foreach ($i in 1..10) {$i; sleep 1} } | out-gridview
Run processes in background like unix '&', and then wait for them:
$a = start-process -NoNewWindow powershell {timeout 10; 'done a'} -PassThru
$b = start-process -NoNewWindow powershell {timeout 10; 'done b'} -PassThru
$c = start-process -NoNewWindow powershell {timeout 10; 'done c'} -PassThru
$a,$b,$c | wait-process
Or foreach -parallel in workflows:
workflow work {
foreach -parallel ($i in 1..3) {
sleep 5
"$i done"
}
}
work
Or a workflow parallel block where you can run different things:
function sleepfor($time) { sleep $time; "sleepfor $time done"}
workflow work {
parallel {
sleepfor 3
sleepfor 2
sleepfor 1
}
'hi'
}
work
Three parallel commands in three more runspaces with the api:
$a = [PowerShell]::Create().AddScript{sleep 5;'a done'}
$b = [PowerShell]::Create().AddScript{sleep 5;'b done'}
$c = [PowerShell]::Create().AddScript{sleep 5;'c done'}
$r1,$r2,$r3 = ($a,$b,$c).begininvoke()
$a.EndInvoke($r1); $b.EndInvoke($r2); $c.EndInvoke($r3) # wait
($a,$b,$c).Streams.Error # check for errors
($a,$b,$c).dispose() # cleanup
Parallel processes with invoke-command, but you have to be at an elevated prompt with remote powershell working:
invoke-command localhost,localhost,localhost { sleep 5; 'hi' }
An assignment is an expression:
if ($a = 1) { $a }
$a = $b = 2
Get last array element with -1:
(1,2,3)[-1]
Discard output with [void]:
[void] (echo discard me)
Switch on arrays and $_ on either side:
switch(1,2,3,4,5,6) {
{$_ % 2} {"Odd $_"; continue}
4 {'FOUR'}
default {"Even $_"}
}
Get and set variables in a module:
'$script:count = 0
$script:increment = 1
function Get-Count { return $script:count += $increment }' > counter.psm1 # creating file
import-module .\counter.psm1
$m = get-module counter
& $m Get-Variable count
& $m Set-Variable count 33
See module function definition:
& $m Get-Item function:Get-Count | foreach definition
Run a command with a commandinfo object and the call operator:
$d = get-command get-date
& $d
Dynamic modules:
$m = New-Module {
function foo {"In foo x is $x"}
$x=2
Export-ModuleMember -func foo -var x
}
flags enum:
[flags()] enum bits {one = 1; two = 2; three = 4; four = 8; five = 16}
[bits]31
Little known codes for the -replace operator:
$number Substitutes the last submatch matched by group number.
${name} Substitutes the last submatch matched by a named capture of the form (?).
$$ Substitutes a single "$" literal.
$& Substitutes a copy of the entire match itself.
$` Substitutes all the text from the argument string before the matching portion.
$' Substitutes all the text of the argument string after the matching portion.
$+ Substitutes the last submatch captured.
$_ Substitutes the entire argument string.
Demo of workflows surviving interruptions using checkpoints. Kill the window or reboot. Then start PS again. Use get-job and resume-job to resume the job.
workflow test1 {
foreach ($b in 1..1000) {
$b
Checkpoint-Workflow
}
}
test1 -AsJob -JobName bootjob
Emacs edit mode. Pressing tab completion lists all the options at once. Very useful.
Set-PSReadLineOption -EditMode Emacs
Any command that begins with "get-", you can leave off the "get-":
date
help
End parsing --% and end of parameters -- operators.
write-output --% -inputobject
write-output -- -inputobject
Tab completion on wildcards:
cd \pro*iles # press tab
Compile and import a C# module with a cmdlet inside, even in Osx:
Add-Type -Path ExampleModule.cs -OutputAssembly ExampleModule.dll
Import-Module ./ExampleModule.dll
Iterate backwards over a sequence just use the len of the sequence with a 1 on the other side of the range:
foreach( x in seq.length..1) { Do-Something seq[x] }