$UserChoice = Read-Host "Enter # of tool you want to run"
switch -exact ($UserChoice) {
    1 {Write-Host 'You selected 1'}
    1a {Write-Host 'You selected 1a'}
    1b {Write-Host 'You selected 1b'}
    1c {Write-Host 'You selected 1c'}
    1d {Write-Host 'You selected 1d'}
}

Preface: I know the fix is to put the comparison values in quotes, ex: '1d'. I want to know WHY PowerShell is acting the way it is for learning sake.

1. If I enter 1b at prompt, it returns 1b whereas if I enter 1d it returns nothing, why?
2. If I enter 1b at prompt, it returns 1b whereas if I enter 1 it returns 1 and 1d, why?
3. It seems PowerShell is interpreting d in a switch statement comparison value different for some reason, why?

--EDIT: I just noticed VISUALLY the comparison values are different colors (1 & 1d are darker) so I guess PowerShell ISE syntax highlighting is telling us they're being treated differently. 4. What do the colors mean (maroon/dark-RedOrPurple VS purple?

CodePudding user response：

Before explaining why the 1d label is "special", I should note that the -exact mode (which is the default mode of comparison for a switch statement) is probably a bit misleading.

It simply means "use the -eq operator to compare input values to case labels".

The reason 1d behaves differently is that PowerShell doesn't recognize the expression 1d as a string. Instead, it interprets d is a numerical suffix signifying the [decimal] type, and the case label value is thus the same as if you'd written 1.0 or $([decimal]1).

The result is that comparison to the input string "1" comes out the same for both - "1" -eq 1 and "1" -eq 1d are both true, thanks to PowerShell's overloaded operators.

If you ever expand your options further, you'll encounter the same problem with 1l (l = [long]), and, if using PowerShell 7, eventually 1n, 1s, 1u, and 1y.

Quote the switch labels to avoid PowerShell parsing them as a numerical expressions:

$UserChoice = Read-Host "Enter # of tool you want to run"
switch -exact ($UserChoice) {
    '1'  {Write-Host 'You selected 1'}
    '1a' {Write-Host 'You selected 1a'}
    '1b' {Write-Host 'You selected 1b'}
    '1c' {Write-Host 'You selected 1c'}
    '1d' {Write-Host 'You selected 1d'}
}

See the about_Numeric_Literals help topic for a more comprehensive overview of suffixes interpreted as numerical modfiers by PowerShell.

CodePudding user response：

To add to Mathias' excellent answer:

• The switch statement's branch conditionals should never have allowed the use of barewords (unquoted tokens serving as strings).

  • It amounts to an inappropriate blending of PowerShell's two fundamental parsing modes, argument mode and expression mode.

    • In expression mode - which is the appropriate one to apply - strings must be quoted; e.g., to match a string with verbatim value 1d, it must be represented as '1d', which disambiguates it from the number literal 1d, a [decimal].

  • See GitHub issue #3668 for a discussion.

• The -exact switch opts into the default behavior and, given the non-exact nature of this behavior, is confusingly named.

  • GitHub issue #8599 therefore suggests (soft-)deprecating it.