Behaviour of PowerShell's -eq vs compare-object

Question

Why does Compare-Object work as expected where -EQ fails to accurately compare arrays of strings?

I had a PowerShell script which was populating an array of strings and was using the -EQ operator to test against the expected values - this was always failing - I think the following code illustrates the issues

    # Setting up 4 Lists - $Lists1 2 and 3 should be identical and $List4 differs
    [string[]]$List1  = "AA","BBB"
              $List2  = $List1

    [string[]]$List3  = "AA"
              $List3 += "BBB"

    [string[]]$List4  = $List3
              $List4 += "CCCC"

    "--------"
    "Checking for Equality of the Lists using the -EQ comparison operator (why do all fail--- when only List4 should fail)"
    "--------"
    if ($List1 -eq $List1) {"List 1 and 1 are equal"} else {"List 1 and 1 are NOT equal"}
    if ($List1 -eq $List2) {"List 1 and 2 are equal"} else {"List 1 and 2 are NOT equal"}
    if ($List1 -eq $List3) {"List 1 and 3 are equal"} else {"List 1 and 3 are NOT equal"}
    if ($List1 -eq $List4) {"List 1 and 4 are equal"} else {"List 1 and 4 are NOT equal"}
    ""
    ""
    "--------"
    "Checking using Compare-object (operates as expected - only List4 Differs)"
    "--------"
    if ((compare-object $List1 $List1) -eq $null) {"List 1 and 1 are equal"} else {"List 1 and 1 are NOT equal"}
    if ((compare-object $List1 $List2) -eq $null) {"List 1 and 2 are equal"} else {"List 1 and 2 are NOT equal"}
    if ((compare-object $List1 $List3) -eq $null) {"List 1 and 3 are equal"} else {"List 1 and 3 are NOT equal"}
    if ((compare-object $List1 $List4) -eq $null) {"List 1 and 4 are equal"} else {"List 1 and 4 are NOT equal"}

`$List1 -eq $List1` doesn't return anything, I assume `-eq` doesn't work for arrays. — 4c74356b41, Dec 08 '16 at 15:49
You should read how comparison operators (including `-eq`) work when left hand side operand is collection. Also, `Compare-Object` does not compare lists, it compare content of lists. — user4003407, Dec 08 '16 at 15:51
Is there another operator that can be used to compare the Array - or do I have to use the compare-object cmdlet ? are there inefficiencies in using the cmdlet? — Gene Lowy, Dec 08 '16 at 15:53

score 0 · Answer 1 · edited Jun 20 '19 at 02:12

0

Let me explain it to you with an example:

$a1 = @(1, 2, 3, 4, 5)
$b1 = @(1, 2, 3, 4, 5, 6)
$c = Compare-Object -ReferenceObject (1..5) -DifferenceObject (1..6) -PassThru

$c will be 6.

That's what compare object does

Where -EQ only checks like if the left-hand side is equal to the right or not. It results in a boolean value.

Example

$DNS = (Test-Connection www.google.com -quiet)
If($DNS -eq "True") {Write-Host "The Internet is available"}
ElseIf($DNS -ne "True") {Restart-Service dnscache}

edited Jun 20 '19 at 02:12

Peter Mortensen

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answered Dec 08 '16 at 15:57

Ranadip Dutta

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Thanks Ranadip - My problem - and the reason for this query is that I thought EQ worked like you stated and expected that EQ would check if the left-hand equals the right hand operand - but that does not seem to work as expected for arrays. for example ... try this [string[]]$List1="A","B" [string[]]$List2="A","B" if ($List1 -eq $List1) {"List1 ---$List1--- Equals itself"} else {"List1 ---$List1--- DOES NOT Equal itself"} if ($List1 -eq $List2) {"List1 ---$List1--- and 2 ---$List2--- are equal"} else {"List1 ---$List1--- and 2 ---$List2--- are NOT equal"}` – Gene Lowy Dec 09 '16 at 13:29
Yes.. I second you.. eq has some limitations also. – Ranadip Dutta Dec 09 '16 at 13:39
But if you do remove one element from the list. Then it will take. The reason being you are converting an array to string as a Type Cast and EQ cannot handle that. Try this. It will work . `[string[]]$List1="A" [string[]]$List2="A" if ($List1 -eq $List1) {"List1 ---$List1--- Equals itself"} else {"List1 ---$List1--- DOES NOT Equal itself"} if ($List1 -eq $List2) {"List1 ---$List1--- and 2 ---$List2--- are equal"} else {"List1 ---$List1--- and 2 ---$List2--- are NOT equal"}` – Ranadip Dutta Dec 09 '16 at 14:06

score 0 · Accepted Answer · edited May 23 '17 at 12:13

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Taken from this answer - which is certainly worth reading as it very elequently explains your question:

When the -eq operator is used between two array variables, things are a bit different. PowerShell will, in fact, enumerate only the array on the left side and compare each item to the array on the right side as a whole. The result will be an array of matching items or nothing at all when there are no matches.

Compare-Object will return an array of differences between the two arrays or $null when the arrays are equal. More precisely, the resulting array will contain an object for each item that exists only in one array and not the other.

edited May 23 '17 at 12:13

Community

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answered Dec 08 '16 at 16:00

henrycarteruk

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I think I have it now - thanks. In summary THE -EQ operator only returns a status of TRUE when There is only one element in the 2nd operand - and .... - and - ANY element in the first operand has to match THE SINGLE ELEMENT in the 2nd operand. So if List1= A, B and List2=A then List1 -eq List2 returns TRUE. if List1= A and List2=A, B then List1 -eq List2 returns FALSE – Gene Lowy Dec 09 '16 at 13:19

Behaviour of PowerShell's -eq vs compare-object

2 Answers2