# 📘 Week 1 · Day 4 — PowerShell Exercises

This document describes **all exercises for Week 1 Day 4**, focused on **PowerShell conditionals, loops, and sysadmin-oriented input validation**.

The goal is to translate your existing algorithmic knowledge (from C, Python, SQL) into **PowerShell syntax and pipelines**, preparing you for Windows administration and automation tasks.

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## 🎯 Learning Objectives

By completing today’s exercises, you will:

* Use `if / elseif / else` for conditional logic
* Loop over strings and collections with `foreach`
* Apply input validation logic in a Windows scripting context
* Understand early exit using `break`
* Reason about algorithm runtime (`T(n)`) in PowerShell
* Practice defensive programming for security-sensitive scripts

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## 🛠 Exercises

### Exercise 1 — Even / Odd Number Checker

**Task:**

* Prompt the user for a number
* Output `Even` or `Odd`

**Concepts practiced:**

* `Read-Host`
* Integer conversion
* Conditional branching (`if / else`)

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### Exercise 2 — Count Letters in a String

**Task:**

* Prompt the user for a string
* Count only letters (`A-Z`, `a-z`)
* Output the number of letters

**Concepts practiced:**

* `foreach` loop over string characters
* Regex matching with `-match`
* Counter variable logic

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### Exercise 3 — Username Validator

**Task:**

* Prompt the user for a username
* Validate using these rules:

  * Length ≤ 20
  * Starts with a letter
  * No digits
  * No spaces
* Output `Accepted` or `Refused`

**Concepts practiced:**

* Conditional logic
* Early exit using `break`
* Character validation via regex
* Defensive programming

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### Exercise 4 — Validate Multiple Usernames

**Task:**

* Given an array of usernames:

```powershell
$users = @("admin", "root1", "John_Doe", "Alice", "Bob42")
```

* Print only valid usernames (using the same rules as Exercise 3)

**Concepts practiced:**

* Iterating over collections with `foreach`
* Reusing validation logic
* Output formatting

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### Exercise 5 — Algorithm Explanation (Written)

**Task:**

* Explain what happens if the number of users doubles
* Write the runtime formula using:

  * `n` = number of users
  * `m` = average username length

**Expected reasoning:**

* Doubling the list doubles the execution time
* Linear runtime: `T(n, m) = n × m`

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## 🔐 Security Perspective

PowerShell scripts are heavily used in **Windows administration and security tasks**. Today’s exercises reinforce:

* Input validation to prevent bad data
* Predictable scaling of loops
* Defensive coding practices

These habits are essential for:

* Active Directory scripting
* Log parsing and auditing
* Automating security tasks
* Incident response and analysis

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## ✅ Completion Criteria

Day 4 is complete when:

* Exercises 1–4 run correctly and produce expected outputs
* Exercise 5 demonstrates correct understanding of scaling and runtime
* Scripts are secure, readable, and reusable