Hash tables (dictionaries) in PowerShell - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a hash table in PowerShell?

A hash table is a collection of key-value pairs. It lets you store and quickly find data by using a unique key, like a labeled box for your information.

beginner

How do you create an empty hash table in PowerShell?

You create an empty hash table using @{} like this:
$myHash = @{}

beginner

How do you add a key-value pair to a hash table?

Use the syntax:
$hashTable["key"] = "value"
This adds or updates the value for the given key.

beginner

How do you access a value from a hash table by its key?

Use the key inside square brackets:
$value = $hashTable["key"]
This gets the value stored for that key.

beginner

What happens if you try to access a key that does not exist in a hash table?

PowerShell returns nothing (null). It does not cause an error, but you get no value back.

Which symbol is used to create a hash table in PowerShell?

A ()

B []

C {}

D @{}

The @{} syntax creates a hash table in PowerShell.

How do you add a new key 'Name' with value 'Alice' to a hash table $h?

A $h.Add('Name', 'Alice')

B $h['Name'] = 'Alice'

C $h.Name = 'Alice'

D $h->Name = 'Alice'

In PowerShell, you add or update a key-value pair using $h['Name'] = 'Alice'.

What does $h['Age'] return if 'Age' is not a key in $h?

A Null (no value)

B An error

C 0

D The string 'Age'

Accessing a missing key returns null, not an error.

How do you create a hash table with keys 'City' and 'Country' and values 'Paris' and 'France'?

A $h = @{City='Paris'; Country='France'}

B $h = ['City'='Paris', 'Country'='France']

C $h = {'City':'Paris', 'Country':'France'}

D $h = (City='Paris', Country='France')

The correct syntax uses @{} with semicolons separating key-value pairs.

Which of these is NOT a valid way to access a hash table value?

A $h['Key']

B $h.Key

C $h->Key

D $h.Get_Item('Key')

PowerShell does not use '->' to access hash table items.

Explain how to create, add to, and retrieve data from a hash table in PowerShell.

Think of a hash table as a labeled box where you put and get items by their labels.

What happens when you try to get a value from a hash table using a key that does not exist? How can this behavior be useful?

Consider how you might safely check for data without breaking your script.

Practice

1. What is the correct way to create an empty hash table in PowerShell?

easy

A. ()

B. []

C. @{}

D. {}

Solution

1. Step 1: Understand hash table syntax in PowerShell

   PowerShell uses @{} to define hash tables, which store key-value pairs.

2. Step 2: Identify the empty hash table syntax

   An empty hash table has no key-value pairs inside, so it is @{}.

3. Final Answer:

   @{} -> Option C

4. Quick Check:

   Empty hash table = @{ } [OK]

Hint: Use @{} to create hash tables in PowerShell [OK]

Common Mistakes:

• Using {} which is a script block, not a hash table
• Using [] which is an array, not a hash table
• Using () which is for grouping or method calls

2. Which of the following is the correct syntax to add a key-value pair 'Name' = 'Alice' to an existing hash table $person?

easy

A. $person[Name] = 'Alice'

B. $person.Name = Alice

C. $person->Add('Name', 'Alice')

D. $person.Add('Name', 'Alice')

Solution

1. Step 1: Recall how to add key-value pairs to hash tables

   PowerShell hash tables have an Add() method to add new keys and values.

2. Step 2: Identify correct method call syntax

   The correct syntax is $person.Add('Name', 'Alice'). Note that the Add() method specifically adds new entries.

3. Final Answer:

   $person.Add('Name', 'Alice') -> Option D

4. Quick Check:

   Add key-value with Add() method [OK]

Hint: Use Add() method to add new keys to hash tables [OK]

Common Mistakes:

• Using -> which is not PowerShell syntax
• Using dot notation with unquoted value
• Using unquoted key in square brackets

3. What will be the output of this PowerShell code?

$hash = @{ 'a' = 1; 'b' = 2 }
$hash['a'] = 3
$hash['c'] = 4
$hash['b']

medium

A. 2

B. 1

C. 3

D. 4

Solution

1. Step 1: Understand the initial hash table and updates

   The hash table starts with keys 'a' = 1 and 'b' = 2. Then 'a' is updated to 3, and a new key 'c' = 4 is added.

2. Step 2: Identify the value accessed

   The code outputs $hash['b'], which was never changed and remains 2.

3. Final Answer:

   2 -> Option A

4. Quick Check:

   Access unchanged key 'b' = 2 [OK]

Hint: Check which key is accessed after updates [OK]

Common Mistakes:

• Confusing updated key 'a' with accessed key 'b'
• Assuming last added key 'c' is accessed
• Mixing values of different keys

4. Identify the error in this PowerShell code snippet:

$ht = @{ 'x' = 10 }
$ht.Add('x', 20)

medium

A. Runtime error: key 'x' already exists

B. Syntax error in hash table creation

C. No error, code runs fine

D. Add method does not exist for hash tables

Solution

1. Step 1: Understand hash table initialization

   The hash table $ht is created with key 'x' = 10.

2. Step 2: Analyze the Add() method call

   Calling $ht.Add('x', 20) tries to add a duplicate key 'x', which causes a runtime error.

3. Final Answer:

   Runtime error: key 'x' already exists -> Option A

4. Quick Check:

   Adding duplicate key causes runtime error [OK]

Hint: Add() fails if key already exists in hash table [OK]

Common Mistakes:

• Assuming Add() updates existing keys
• Thinking Add() method doesn't exist
• Confusing syntax error with runtime error

5. You have a list of users and their ages:

$users = @('Alice', 'Bob', 'Charlie')
$ages = @(25, 30, 35)

Which PowerShell code correctly creates a hash table mapping each user to their age?

hard

A. $userAges = @{}; foreach ($user in $users) { $userAges.Add($user, $ages) }

B. $userAges = @{}; for ($i=0; $i -lt $users.Count; $i++) { $userAges[$users[$i]] = $ages[$i] }

C. $userAges = @{}; $users | ForEach-Object { $userAges.Add($_, $ages[$_]) }

D. $userAges = @{}; for ($i=0; $i -le $users.Count; $i++) { $userAges[$i] = $ages[$i] }

Solution

1. Step 1: Understand the goal

   Create a hash table where each user name is a key and their corresponding age is the value.

2. Step 2: Analyze each option

   $userAges = @{}; for ($i=0; $i -lt $users.Count; $i++) { $userAges[$users[$i]] = $ages[$i] } uses a for loop with index to assign each user key to the correct age value. $userAges = @{}; foreach ($user in $users) { $userAges.Add($user, $ages) } tries to add all ages at once, which is incorrect. $userAges = @{}; $users | ForEach-Object { $userAges.Add($_, $ages[$_]) } uses $_ as index but $_ is the user string, not an index. $userAges = @{}; for ($i=0; $i -le $users.Count; $i++) { $userAges[$i] = $ages[$i] } loops one too many times and uses numeric keys instead of user names.

3. Final Answer:

   $userAges = @{}; for ($i=0; $i -lt $users.Count; $i++) { $userAges[$users[$i]] = $ages[$i] } -> Option B

4. Quick Check:

   Use indexed loop to map keys to values [OK]

Hint: Use index loop to map two lists into hash table [OK]

Common Mistakes:

• Using foreach without index to access parallel array
• Using wrong loop bounds causing errors
• Assigning numeric keys instead of user names