Hash tables (dictionaries) in PowerShell - Mini Project: Build & Apply

Working with Hash Tables (Dictionaries) in PowerShell

📖 Scenario: You are managing a small store's inventory. You want to keep track of product names and their quantities using a hash table in PowerShell.

🎯 Goal: Build a PowerShell script that creates a hash table for products and quantities, sets a minimum stock threshold, filters products below that threshold, and displays those products.

📋 What You'll Learn

Create a hash table named inventory with exact product names and quantities

Create a variable named minStock with a numeric value

Use a foreach loop to find products with quantity less than minStock

Store filtered products in a new hash table named lowStock

Print the lowStock hash table at the end

💡 Why This Matters

🌍 Real World

Managing inventory levels in small businesses or stores to know when to reorder products.

💼 Career

PowerShell scripting for system administrators or IT professionals automating inventory or configuration tasks.

1

Create the inventory hash table

Create a hash table called inventory with these exact entries: 'Apples' = 30, 'Bananas' = 12, 'Oranges' = 20, 'Grapes' = 5

PowerShell

# Create the inventory hash table
# Your code here

Hint

Use @{} to create a hash table in PowerShell. Separate entries with semicolons.

2

Set the minimum stock threshold

Create a variable called minStock and set it to 15

PowerShell

$inventory = @{ 'Apples' = 30; 'Bananas' = 12; 'Oranges' = 20; 'Grapes' = 5 }
# Set the minimum stock threshold
# Your code here

Hint

Just assign the number 15 to the variable minStock.

3

Find products with low stock

Create an empty hash table called lowStock. Use a foreach loop with variables product and quantity to iterate over $inventory.GetEnumerator(). Inside the loop, if quantity is less than minStock, add the product and quantity to lowStock.

PowerShell

$inventory = @{ 'Apples' = 30; 'Bananas' = 12; 'Oranges' = 20; 'Grapes' = 5 }
$minStock = 15
# Create lowStock hash table and fill it with products below minStock
# Your code here

Hint

Use $inventory.GetEnumerator() to loop through the hash table. Use $lowStock[$product] = $quantity to add entries.

4

Display the low stock products

Print the lowStock hash table using Write-Output

PowerShell

$inventory = @{ 'Apples' = 30; 'Bananas' = 12; 'Oranges' = 20; 'Grapes' = 5 }
$minStock = 15
$lowStock = @{}
foreach ($item in $inventory.GetEnumerator()) {
    $product = $item.Key
    $quantity = $item.Value
    if ($quantity -lt $minStock) {
        $lowStock[$product] = $quantity
    }
}
# Print the lowStock hash table
# Your code here

Hint

Use Write-Output $lowStock to display the hash table.

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