Vista - Ternary Operator in Powershell

One this that has bothered me for some time, is the lack of a ternary
operator in powershell (disclaimer, maybe there is one, and i just
haven't found it. This seems unintuitive expecially for an expression
based sort of shell language like powershell.. Well the great thing
with powershell, in the way that functions/cmdlets etc are evaluated
before keywords , and also particularly scriptblocks, how scriptblocks
are first class citizens. So you can easily make a function to do this,
so i did. In a couple of lines, and a couple of minutes

function Invoke-Ternary ([bool]
$decider,[scriptblock]$iftrue,[scriptblock]$iffalse)
{
if ($decider) { &$iftrue} else { &$iffalse }
}
set-Alias ?: Invoke-Ternary


So you can easily call it like this

$a = 10
?: ($a -gt 5) {"its greater than five"} {"its not greater than 5"}

i love powershell. its so easy to adapt.

Karl Prosser : http://www.karlprosser.com/coder
PowerShell Analyzer : http://www.powershellanalyzer.com

maybe this technique is even better

function Invoke-Ternary ([scriptblock]
$decider,[scriptblock]$iftrue,[scriptblock]$iffalse)
{
begin {}
process {
if (&$decider) { &$iftrue} else { &$iffalse }
}
end {}
}

because it works with the pipeline, and alows the condition to be
calculated each time.. i.e in the following examples

$a = 10
?: {$a -gt 5} {"its greater than five"} {"its not greater than 5"}

1..10 | ?: {$_ -gt 5} {"its greater than five";$_} {"its not greater
than 5";$_}

doh, even more simpler

filter Invoke-Ternary ([scriptblock]
$decider,[scriptblock]$iftrue,[scriptblock]$iffalse)
{
if (&$decider) { &$iftrue} else { &$iffalse }
}

I like it, definitely a keeper.

For others following along, the first version showed this usage example:
?: ($a -gt 5) {"its greater than five"} {"its not greater than 5"}

but that won't work with v3, replace () with {}:
?: {$a -gt 5} {"its greater than five"} {"its not greater than 5"}


<klumsy@gmail.com> wrote in message
news:1163630863.578013.281630@f16g2000cwb.googlegroups.com...
> doh, even more simpler
>
> filter Invoke-Ternary ([scriptblock]
> $decider,[scriptblock]$iftrue,[scriptblock]$iffalse)
> {
> if (&$decider) { &$iftrue} else { &$iffalse }
> }
>

<klumsy@gmail.com> wrote in message
news:1163630863.578013.281630@f16g2000cwb.googlegroups.com...
> doh, even more simpler
>
> filter Invoke-Ternary ([scriptblock]
> $decider,[scriptblock]$iftrue,[scriptblock]$iffalse)
> {
> if (&$decider) { &$iftrue} else { &$iffalse }
> }
>

BTW, if you would like to see this implemented directly in the language vote
on it:

https://connect.microsoft.com/feedba...3059&SiteID=99

This one is variant of null coalescing operator in C# for working with null
values (and nullables):

https://connect.microsoft.com/feedba...3061&SiteID=99

--
Keith

"Keith Hill [MVP]" <r_keith_hill@mailhot.moc.nospam> wrote in message news:OkiRlsSCHHA.4892@TK2MSFTNGP04.phx.gbl...
> This one is variant of null coalescing operator in C# for working with null
> values (and nullables):
>
> https://connect.microsoft.com/feedba...3061&SiteID=99
>

Regarding the ?? operator request, the following comes close but doesn't work in a pipeline:

# Close but not quite working filter
set-alias ?? Invoke-NullAlt
filter Invoke-NullAlt ([scriptblock]$expression, [scriptblock]$executeIfNull, [bool]$debug = $false) {
if ($debug) {"`$expression is `"$expression`""}
if ($expression -ne $null) {
$result = &$expression
if ($debug) {"`$exprResult is `"$result`""}
if ($result -ne $null) {
$result
}
else {
&$executeIfNull
}
}
else {
&$executeIfNull
}
}

16# ?? {$logdir} {"${env:systemroot}\System32\LogFiles"}
C:\WINDOWS\System32\LogFiles

In this case $logdir isn't defined so it evaluates the second scriptblock as the result of the filter.

17# ?? {$env:temp} {"${env:systemroot}\System32\LogFiles"}
C:\DOCUME~1\Keith\LOCALS~1\Temp

In the case above the first scriptblock evaluates to non-null and that result is used.

Here's where I run into problems though:

18# gps | %{?? {$_.Description} {"UNKNOWN"} $true}
$expression is "$_.Description"
$exprResult is ""
UNKNOWN
<snip>

Is there no way to evaluate the $_ in the $expression scriptblock?

--
Keith

You declared it as a filter, so you can use it directly without having to parse the pipeline input with foreach:

PS> gps | ?? {$_.description} {"UNKNOWN"}
UNKNOWN
UNKNOWN
UNKNOWN
UNKNOWN
UNKNOWN
Desktop Window Manager
Windows Explorer
UNKNOWN
Microsoft Firewall Client Management
....

Jacques

"Keith Hill [MVP]" <r_keith_hill@mailhot.moc.nospam> wrote in message news:O004A4TCHHA.4024@TK2MSFTNGP04.phx.gbl...
"Keith Hill [MVP]" <r_keith_hill@mailhot.moc.nospam> wrote in message news:OkiRlsSCHHA.4892@TK2MSFTNGP04.phx.gbl...
> This one is variant of null coalescing operator in C# for working with null
> values (and nullables):
>
> https://connect.microsoft.com/feedba...3061&SiteID=99
>

Regarding the ?? operator request, the following comes close but doesn't work in a pipeline:

# Close but not quite working filter
set-alias ?? Invoke-NullAlt
filter Invoke-NullAlt ([scriptblock]$expression, [scriptblock]$executeIfNull, [bool]$debug = $false) {
if ($debug) {"`$expression is `"$expression`""}
if ($expression -ne $null) {
$result = &$expression
if ($debug) {"`$exprResult is `"$result`""}
if ($result -ne $null) {
$result
}
else {
&$executeIfNull
}
}
else {
&$executeIfNull
}
}

16# ?? {$logdir} {"${env:systemroot}\System32\LogFiles"}
C:\WINDOWS\System32\LogFiles

In this case $logdir isn't defined so it evaluates the second scriptblock as the result of the filter.

17# ?? {$env:temp} {"${env:systemroot}\System32\LogFiles"}
C:\DOCUME~1\Keith\LOCALS~1\Temp

In the case above the first scriptblock evaluates to non-null and that result is used.

Here's where I run into problems though:

18# gps | %{?? {$_.Description} {"UNKNOWN"} $true}
$expression is "$_.Description"
$exprResult is ""
UNKNOWN
<snip>

Is there no way to evaluate the $_ in the $expression scriptblock?

--
Keith

sure there is , you don't need the foreach as this is a filter, and you
are processing it via the pipeline

so instead of

gps | %{?? {$_.Description} {"UNKNOWN"} $true}

try

gps | ?? {$_.Description} {"UNKNOWN"} $true

<klumsy@gmail.com> wrote in message
news:1163698564.620489.240720@m7g2000cwm.googlegroups.com...
> sure there is , you don't need the foreach as this is a filter, and you
> are processing it via the pipeline
>
> so instead of
>
> gps | %{?? {$_.Description} {"UNKNOWN"} $true}
>
> try
>
> gps | ?? {$_.Description} {"UNKNOWN"} $true

Doh! :-) Thanks for pointing that out.

--
Keith

Hi klumsy,

> So you can easily call it like this
>
> $a = 10
> ?: ($a -gt 5) {"its greater than five"} {"its not greater than 5"}
>
> i love powershell. its so easy to adapt.

IMHO it would be much nicer to have a built-in ternary operator so one
could write this:

$a=$true
$b=$a?"True":"False"

or

$a = 10
$b=$a -gt 5?"its greater than five":"its not greater than 5"

or

$b=$a -gt 5?5:$a

cu

Max