Get Random with Excluded Numbers

An interesting question came up about using the Get-Random cmdlet to return a random number but with a predefined list of numbers to exclude.

Even though Get-Random will return a random number, you can also specify a list or range of numbers for it to run against.

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PS C:\> Get-Random 1,2,3,4,5

…will return a random number between 1 and 5.

What if you want to exclude one or more specific numbers? Get-Random doesn’t have this capability built in but we can still accomplish this with a little extra work.

Let’s say I want to exclude numbers 1 and 3.

Here’s how we’ll do it:

1. Assign the entire range (1 to 5) to an ArrayList
2. Assign the numbers we want to exclude (1 and 3) to another ArrayList
3. Loop through the (second) ArrayList that contains the excluded numbers, and any that match what’s in the first ArrayList, remove it from that ArrayList.
   This is accomplished by passing the excluded numbers to the IndexOf() method of the first ArrayList object, and invoking it within its RemoveAt() method. The example will shed more light.
4. With the first ArrayList now containing all numbers except for the exclusions, use the result with Get-Random.

Here’s the code (see below for explanation):

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# Create ArrayList for entire range
[System.Collections.ArrayList]$NumRange = @(1..5)

# Create ArrayList for excluded numbers
[System.Collections.ArrayList]$Exclusions = @(1,3)

# Loop through excluded numbers
foreach ($num in $Exclusions) {

    # If excluded number exists in entire range,
    if ($NumRange.Contains($num)) {
        # remove it from entire range
        $NumRange.RemoveAt($NumRange.IndexOf($num))
    }
}

Here’s the output:

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PS C:\> $NumRange
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4
5

Now, pipe the results to Get-Random:

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PS C:\> $NumRange | Get-Random
4

There you have it, using Get-Random and a range of numbers with exclusions.