end of day 4 week 1

week 1/day 4/exo1.ps1

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+# exo 1
+$number = [int](Read-Host('please enter a number'));
+
+if(!$number || $number -isnot [int])
+{
+    Write-Host('you should enter a number !')
+    exit
+}
+
+if ($number % 2)
+{
+    Write-Host('Odd');
+}else
+{
+    Write-Host('Even');
+}

week 1/day 4/exo2.ps1

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+# exo 2
+# return the number of character of a string only if they're letter
+
+$text = Read-Host('please enter a phrase')
+$counter = 0
+
+foreach ($char in $text.ToCharArray()) {
+    if($char  -match '[A-Za-z]'){
+        $counter++
+    }
+}
+
+Write-Host("your string contain ${counter} character")

week 1/day 4/exo3.ps1

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+# accept the username only if
+# it contain letters, the username is inferior at 20
+# start with a letter, no digit, no space
+
+$username = Read-Host("Please enter your username");
+$badChar = $false;
+
+if($username -eq ''){
+    Write-Host("The username can't be empty");
+    exit;
+}
+
+foreach($char in $username.ToCharArray()){
+    if($char -notmatch '[A-Za-z]' -or $char -eq ' '){
+        $badChar = $true;
+        break
+    }
+}
+
+$firstLetter = $username[0]
+$validFirstLetter = $firstLetter -match '[A-Za-z]'
+
+if(-not $badChar  -and $username.Length -le 20  -and $validFirstLetter ){
+    Write-Host("Accepted");
+}else{
+    Write-Host("Invalid");
+
+}

week 1/day 4/exo4.ps1

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+$users = @("admin", "root1", "John_Doe", "Alice", "Bob42")
+
+foreach($username in $users){
+
+    $badChar = $false
+
+    foreach($char in $username.ToCharArray()){
+        if($char -notmatch '[A-Za-z]' -or $char -eq ' '){
+            $badChar = $true
+            break
+        }
+
+    }
+
+    $firstLetter = $username[0]
+    $validFirstLetter = $firstLetter -match '[A-Za-z]'
+
+    if(-not $badChar  -and $username.Length -le 20  -and $validFirstLetter ){
+        Write-Host("$username is valid");
+    }else{
+        Write-Host("$username is Invalid");
+
+    }
+}

week 1/day 4/exo5.md

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+**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`
+
+**Answer**
+
+if the number of user double in size 
+
+$$
+T(2n, m) = n /times 2 /times m
+$$
+
+so the number of execution will multiply by 2
+
+the big O notation will be O(n⋅m)

week 1/day 4/readme.md

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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

week 1/day 4/test.ps1

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+# a little script to test if powershell work with my machine
+
+Write-Host("hello from powershell");
+
+# ! it work :)