Find next number in a sequence

Question

I'm working to find the next available number in a set of numbers and have trouble.

My dataset is an array of strings that is then converted to an array of integers. The original dataset may have duplicate numbers.

When I try and find the next available number, the duplicates seem to cause issues yielding bad results

$dataset = "0001","0002","0004","0006","2","5"

#Convert our strings to integers
[array]$Used = foreach($number in $dataset) {
   try {
         [int]::parse($number)
   } catch {
         Invoke-Expression -Command $number;
   }
}

[array]::sort($Used)

$range = 1..10
$Available = compare $range $Used -PassThru

My results are:

$Dataset = 
0001
0002
0004
0006
2
5

$Used = 
1
2
2
4
5
6

Noticed that $Used (which is sorted) shows that 2 is a duplicate:

$Available = 
2
3
7
8
9
10

Finally $Available lists 2 as an available number which is wrong. 2 is actually used twice and the correct answer should be 3.

Any ideas?

`$Available = compare $range $used -PassThru | where SideIndicator -eq '<='` — JosefZ, Jun 19 '19 at 21:24
Or `$Available = Compare-object $range ($used|Sort-Object -Unique) -PassThru` — , Jun 19 '19 at 21:40

BentChainRing · Accepted Answer · 2019-06-19T22:15:00.387

1

This question has been answered before: Removing duplicate values from a PowerShell array

Lots of great answer options, but basically:

$Used = $Used | select -uniq or another option $Used | sort -uniq

edited Jun 19 '19 at 22:15

answered Jun 19 '19 at 21:43

BentChainRing

382
5
14

mklement0 · Answer 2 · 2019-06-20T01:22:13.667

Converting your strings to numbers can be done with a simple cast:

$dataset = "0001","0002","0004","0006","2","5"

$numbers = [int[]] $dataset

To sort these numbers and eliminate duplicates:

$uniqueSortedNumbers = $numbers | Sort-Object -Unique

To find the first available number inside the resulting list, or possibly the next higher number:

# Find the first gap in the list of sorted numbers, if any.
$nextAvailableNumber = 
  Compare-Object -PassThru $uniqueSortedNumbers (1..($uniqueSortedNumbers[-1])) |
    Select-Object -First 1 

# If there was no gap, use the highest number + 1
if ($null -eq $nextAvailableNumber) { 
  $nextAvailableNumber = $uniqueSortedNumbers[-1] + 1
}

As for what you tried:

Compare-Object lists objects that are unique to either collection.

By having duplicates in $used, the duplicates are also reported as unique, because $range has no duplicate counterparts.

With -PassThru, the unique values are passed through without (obvious) distinction as to which input collection they're unique to, so you end up with a mix of numbers unique to either collection, comprising both the numbers unique to $range and the duplicates, which explains your symptom.

By eliminating duplicates from $used beforehand, the problem is avoided, because all unique values are then guaranteed to be from $range only.

score 0 · Answer 3 · answered Jun 19 '19 at 22:40

the Compare-Object cmdlet always confuses me [blush], so i avoided it entirely. [grin] this works by ...

converting the data set [string] items into [int] items
removing the dupes
filtering for target range numbers that are NOT in the used numbers list

here's the code ...

$DataSet = '0001','0002','0004','0006','2','5'
$UsedNumbers = $DataSet.
    ForEach({[int]$_}) |
    Sort-Object -Unique

$TargetRange = 1..10

$AvailableNumbers = $TargetRange.
    Where({
        $_ -notin $UsedNumbers
        })

$AvailableNumbers

output ...

Find next number in a sequence

3 Answers3