$prompt = ($defaultValue,$prompt)[[bool]$prompt] - emulating a ternary conditional in PowerShell - powershell

Question

I'm learning to write scripts with PowerShell, and I found this code that will help me with a project The example comes from Is there a one-liner for using default values with Read-Host?.
$defaultValue = 'default'
$prompt = Read-Host "Press enter to accept the default [$($defaultValue)]"
$prompt = ($defaultValue,$prompt)[[bool]$prompt]
I think I understand that $prompt = ($defaultValue,$prompt) is creating a two-element array and that the [bool] part is forcing the $prompt data type to Boolean, but I don’t understand what this third line of code does as a whole.

Answer 1

This is a common programming pattern:
if (user entered a price)
{
price = user entered value
}
else
{
price = default value
}
and because that is quite common, and also long winded, some languages have a special ternary operator to write all that code much more concisely and assign a variable to "this value or that value" in one move. e.g. in C# you can write:
price = (user entered a price) ? (user entered value) : (default value)
# var = IF [boolean test] ? THEN (x) ELSE (y)
and the ? assigns (x) if the test is true, and (y) if the test is false.
In Python, it's written:
price = (user entered value) if (user entered a price) else (default value)
And in PowerShell, it's written:
# you can't have a ternary operator in PowerShell, because reasons.
Yeah. No nice short code pattern allowed.
But what you can do, is abuse array-indexing (#('x', 'y')[0] is 'x' and #('x', 'y')[1] is 'y' and ) and write that ugly and confusing code-golf line:
$price = ($defaultValue,$userValue)[[bool]$UserEnteredPrice]
# var (x,y) is an array $array[ ] is array indexing
(0,1) are the array indexes of the two values
[bool]$UserEnteredPrice casts the 'test' part to a True/False value
[True/False] used as indexes into an array indexing makes no sense
so they implicitly cast to integers, and become 0/1
# so if the test is true, the $UserValue is assigned to $price, and if the test fails, the $DefaultValue is assigned to price.
And it behaves like a ternary operator, except it's confusing and ugly and in some situations it will trip you up if you're not careful by evaluating both array expressions regardless of which one is chosen (unlike real ? operators).
Edit: What I should really add is a PowerShell form I prefer - you can assign the result of an if test directly in PowerShell and do:
$price = if ($userValue) { $userValue } else { $DefaultValue }
# ->
$prompt = if ($prompt) { $prompt } else { $DefaultValue }

Answer 2

Casting $prompt to [bool] produces a value of $true or $false depending on whether the variable is empty ($null or empty string both become $false) or not (non-emtpy strings become $true).
[bool]'' → $false
[bool]'something' → $true
Using that boolean value in the index operator then implicitly casts the value to an integer where $false becomes 0 and $true becomes 1, hence selecting the first or second element of the array.
[int]$false → 0
[int]$true → 1
($defaultValue,$prompt)[0] → $defaultValue
($defaultValue,$prompt)[1] → $prompt

Answer 3

To complement the great answers given by Ansgar Wiechers and by TessellatingHeckler:
It would be great if PowerShell had operators for ternary conditionals and null-coalescing, such as follows (applied to the example in the question):
Update: PowerShell (Core) 7+ now does have these operators.
# Ternary conditional
# Note: does NOT work in *Windows PowerShell*,
# only in PowerShell (Core) v7+
$prompt = $prompt ? $prompt : $defaultValue
# Or, more succinctly, with null coalescence:
# Note: does NOT work in *Windows PowerShell*,
# only in PowerShell (Core) v7+
# (Note: This example assumes that $prompt will be $null in the default
# case, whereas the code in the question actually assigns the
# empty string to $prompt if the user just presses Enter.)
$prompt = $prompt ?? $defaultValue
Unfortunately, these expressive constructs are not part of Windows PowerShell, the legacy edition of PowerShell, which will see no new functionality.
Below are adapted versions of the functions from a 2006 PowerShell Team blog post with associated alias definitions, whose use then allows the following solution:
# Ternary conditional - note how the alias must come *first*
# Note: Requires the function and alias defined below.
$prompt = ?: $prompt $prompt $defaultValue
# Or, more succinctly, with null coalescence - note how the alias must come *first*
# Note: Requires the function and alias defined below.
$prompt = ?? $prompt $defaultValue
Source code:
Note that the actual functions are quite short; it is the comment-based help that makes this listing lengthy.
Set-Alias ?: Invoke-Ternary -Option AllScope
<#
.SYNOPSIS
Emulation of a ternary conditional operator.
.DESCRIPTION
An emulation of the still-missing-from-the-PS-language ternary conditional,
such as the C-style <predicate> ? <if-true> : <if-false>
Because a function is used for emulation, however, the function name must
come first in the invocation.
If you define a succinct alias, e.g., set-alias ?: Invoke-Ternary,
concise in-line conditionals become possible.
To specify something other than a literal or a variable reference, pass a
script block for any of the tree operands.
A predicate script block is of necessity always evaluated, but a script block
passed to the true or false branch is only evaluated on demand.
.EXAMPLE
> Invoke-Ternary { 2 -lt 3 } 'yes' 'no'
Evaluates the predicate script block, which outputs $true, and therefore
selects and outputs the true-case expression, string 'yes'.
.EXAMPLE
> Invoke-Ternary $false { $global:foo = 'bar' } { Get-Date }
Outputs the result of executing Get-Date.
Note that the true-case script block is NOT evaluated in this case.
.NOTES
Gratefully adapted from http://blogs.msdn.com/powershell/archive/2006/12/29/dyi-ternary-operator.aspx
#>
function Invoke-Ternary
{
[CmdletBinding()]
param($Predicate, $Then, $Otherwise = $null)
if ($(if ($Predicate -is [scriptblock]) { & $Predicate } else { $Predicate })) {
if ($Then -is [ScriptBlock]) { & $Then } else { $Then }
} else {
if ($Otherwise -is [ScriptBlock]) { & $Otherwise } else { $Otherwise }
}
}
Set-Alias ?? Invoke-NullCoalescence -Option AllScope
<#
.SYNOPSIS
Emulation of a null-coalescence operator.
.DESCRIPTION
An emulation of a null-coalescence operator such as the following:
<expr> ?? <alternative-expr-if-expr-is-null>
Because a function is used for emulation, however, the function name must
come first in the invocation.
If you define a succinct alias, e.g., set-alias ?? Invoke-NullCoalescence,
concise in-line null-coalescing becomes possible.
To specify something other than a literal or a variable reference, pass a
script block for any of the two operands.
A first-operand script block is of necessity always evaluated, but a
second-operand script block is only evaluated on demand.
Note that only a true $null value in the first operand causes the second
operand to be returned.
.EXAMPLE
> Invoke-NullCoalescence $null '(empty)'
Since the first operand is $null, the second operand, string '(empty)', is
output.
.EXAMPLE
> Invoke-NullCoalescence '' { $global:foo = 'bar' }
Outputs the first operand, the empty string, because it is not $null.
Note that the second-operand script block is NOT evaluated in this case.
.NOTES
Gratefully adapted from http://blogs.msdn.com/powershell/archive/2006/12/29/dyi-ternary-operator.aspx
#>
function Invoke-NullCoalescence
{
[CmdletBinding()]
param($Value, $Alternative)
if ($Value -is [scriptblock]) { $Value = & $Value }
if ($null -ne $Value) {
$Value
} else {
if ($Alternative -is [ScriptBlock]) { & $Alternative } else { $Alternative }
}
}