7-Modern-Languages-in-7-Weeks---Systems--Security---Algorithms-Track/week 1/day 4/readme.md

📘 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:

$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