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+# 📘 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.
+
+---
+
+## 🎯 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
+
+---
+
+## 🛠 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`)
+
+---
+
+### 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
+
+---
+
+### 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
+
+---
+
+### 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
+
+---
+
+### 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`
+
+---
+
+## 🔐 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
+
+---
+
+## ✅ 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