PowerShell -contains logic question

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PowerShell -contains logic question

techresearch7777777 1,981

Hello, sorry if novice question I realize the following returns True:

"abc", "def" -contains "def"                  # Output: True

But when when adding more items why does the following return False?

"abc", "def", "ghi" -contains "abc", "def"    # Output: False

The first one seems to make sense since "def" does contain within the left side set...but the second example both "abc" & "def" looks like also exists within the left side set so I would think should return True but it doesn't?

Guess I'm missing something or not clear on this logic.

Thanks in advance.

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Answer 1

Here's another mind-bender you may run into (and it's a VERY COMMON coding error!):

$a = 1,2,$null,4
"There's a `$null in `$a, but `$a is an array"

if ($a -eq $null){
    "`$a is `$null"
}
else{
    "`$a is not `$null"
}
"You'd expect to get either `$True or `$false for that! But . . . uh-oh!

"==="   # just a separator
"`$a isn't `$null"
$null -eq $a

To avoid this kind of conundrum, ALWAYS ALWAYS ALWAYS place the $null on the LEFT SIDE of the comparison.

If you need help understanding why that happens, and why there's a missing $true/$false, leave a comment. :-)

Answer 2

Anonymous

Hi,

When the right-hand side operand is a collection, these operators convert the value to its string representation before comparing it to the left-hand side collection.

https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_comparison_operators?view=powershell-7.3#containment-operators

Best Regards,

Ian Xue

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techresearch7777777 1,981 Reputation points

2023-11-30T19:47:44.66+00:00

Thanks for your reply Ian_Xue, appreciate it.

Oh...so does that mean the right hand side is actually 1 entire full string only variable and not 2 separate items?

Something like $RightHandSideFullStringOneVariableOnly = ""abc", "def"" (two double quotes encapsulating it)

If yes, then I would see that this then would not match any of the 3 left side items.

Is this correct?

(Yes I did check out that link and is where I got those examples but still bit unclear)
Rich Matheisen 48,026 Reputation points

2023-11-30T20:14:45.8666667+00:00

The documentation @Anonymous pointed out says that the right-hand side of the -contains and -notcontains operators are scalars (e.g., a string, single integer, single floating point number, etc.). To convert that list (array) "abc", "def" to a scalar, the entire right-hand side is cast to a string (in this case) object (e.g., [string]("abc", "def")). The result is then used in the comparison.

As you note, the list on the left-hand side in your example does not contain the string "abc def".
Rich Matheisen 48,026 Reputation points

2023-11-30T20:22:10.5933333+00:00

FYI, the quoting in your example is incorrect. Either of these would work to produce an array of two quoted strings:

$RightHandSideFullStringOneVariableOnly = """abc"", ""def"""

$RightHandSideFullStringOneVariableOnly = '"abc"', '"def"'
techresearch7777777 1,981 Reputation points

2023-11-30T21:49:28.44+00:00

(Re-entered here as duplicate followup question.)
techresearch7777777 1,981 Reputation points

2023-11-30T21:52:43.59+00:00
Thanks Rich for your reply in trying to help, much appreciated.

When using the second example I listed earlier where I'm scratching my head in leaving the right hand side as is but can change the left hand side...

"What item or value can be listed here for this to return True?" -contains "abc", "def"
Rich Matheisen 48,026 Reputation points

2023-11-30T22:34:57.8866667+00:00

This:

"abc", "def", "ghi", "abc def" -contains "abc", "def"

Why? Because [string]("abc", "def") returns the string "abc def" (a string is a scalar object -- it has only one value.

Why doesn't this work: "abc", "def", "ghi", @("abc","def") -contains "abc", "def"?

Because "abc", "def" is not a scalar object -- it's an array, and, in this case, it contains two values.

Conversely, this does work: "abc", "def", "ghi", @("abc","def") -contains @("abc")

Why? Because the array (on the right-hand side), while it has the potential to hold more than one value, contains only a single value that, when you cast the array to a string, it returns just that one value (i.e., a scalar). That scalar value matches the 1st element in the collection on the left-hand side of the -contains operator.

Does all that make sense?
techresearch7777777 1,981 Reputation points

2023-11-30T22:41:32.6133333+00:00

Wow, yes. Thanks much for your great examples and clarifying.

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PowerShell -contains logic question

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