Koans/Constructs/AboutLists.Koans.ps1
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#Requires -Module PSKoans [Koan(301)] param() <# Lists - Flexible, Generic Collections Arrays have one critical weakness: they are immutable collections. While the members within them can change, the number of items they hold cannot. Consider the folllowing code: $Array = 0, 1 $Array = $Array + 2 $Array += 3 # So, if the number of items in an array can't change, what happens here? # You might think it should fail with an error, but it won't! Actually, in PowerShell, the addition operator for arrays is defined something like: "Take the elements in this array, and the thing(s) we're adding in, and build a new array with them." This is very convenient, but it can create a problem - this gets significantly more expensive if you need to do it a lot. For large collections, adding items to them becomes very slow to do. Introducing: Lists! Lists are generic collections, which means that they have a defined type when created, and will only hold objects of that type. Other types will either be converted, or cause an error to be thrown if there is no available conversion. However, all objects in PowerShell inherit from [PSObject] or [object], so a List with that typing will hold any mix of data types. There are several ways to create a List object, but we'll just cover the main methods. For more details, check out the available overloads, which you can see by executing the following line as-is in the PowerShell console: [System.Collections.Generic.List[psobject]]::new #> Describe 'Lists' { Context 'Strict Typing' { It 'can enforce strict typing on its contents' { # New integer-only List $IntList = New-Object System.Collections.Generic.List[int] $IntList.Add(7) $IntList.Add(5.34) # Items are accessed by index just like arrays. $IntList[0] | Should -Be __ $IntList[1] | Should -Be __ } It 'will convert items to its data type when possible' { $StringList = [System.Collections.Generic.List[string]]::new() $StringList.Add("FILL_ME_IN") $StringList.Add(12) # What happens if we add a number to a string-typed list? $StringList[0] | Should -Be 'hello!' $StringList[1] | Should -Be __ } } Context 'Loose Typing' { It 'can accept loose typing' { # We can convert from an array. [object] or [PSObject] typed lists hold any type(s) of items. $List = [System.Collections.Generic.List[Object]]@(1, 2) # Items must be added to Lists using their .Add() or .AddRange() method. # .AddRange() takes an array or other collection object. $List.AddRange(@(12, 1, 2, 3)) $List.Add(12.5) $List[0] | Should -Be __ $List[6] -eq 12.5 | Should -Be $true } } Context 'Removing List Entries' { It 'allows you to remove entries' { # Lists have a few methods for removing entries: # .Remove($value), .RemoveAt($Index) and .RemoveRange($Index,$Count) (among others) $List = [System.Collections.Generic.List[string]]@(12, 15, 15, 18, 'hello', 19, 4, 12, 5, 10) $List[3] | Should -Be '18' $List.RemoveAt(3) $List[3] | Should -Be '__' $List[1] | Should -Be '15' # The .Remove() method returns $true if the item was removed, and $false if it couldn't be # found or removed. $List.Remove('15') | Should -BeTrue $List[1] | Should -Be __ # Now see if you can reduce the list down to only two values using the # .RemoveRange($Index, $Count) method # Feel free to add an additional ' | Should -Be ' test if you need it to guide your way! $List | Should -Be @('12', '10') } It 'allows you to remove multiple entries based on conditions' { $List = [System.Collections.Generic.List[string]]@(1..11) <# This method takes a lambda expression in C#, which translates to a script block for PowerShell. It also outputs a $true or $false depending on whether elements were removed, so we'll check that #> $List.RemoveAll({ # The input variable representing each entry must be named, or use $args[0] param($_) # Remove everything that contains the number 9, essentially $_ -match '9' # The output must boil down to a $true/$false, or will be coerced to it }) | Should -BeTrue $RemainingEntries = @('1', '2', '__', '__', '5', '__', '7', '__', '__', '__', '11') $List | Should -Be $RemainingEntries $List.RemoveAll( { param($_) # Change this expression to make the assertions below true }) | Should -BeTrue $List | Should -Be @(1, 3, 5, 7, 11) } } } |