# Day 2

## Exercises (Submit for Grading)

### Exercise 1 — Even / Odd

Ask for a number and print:

"Even" or "Odd"

use: 
```python
n % 2
```

## Exercise 2 — Count Letters

Given a string:

count only alphabetic characters

ignore digits and symbols

## Exercise 3 — Rebuild Your C Validator (Python)

Write your own version (no copy-paste):

≤ 20 characters

starts with letter

no digits

no spaces

## Exercise 4 — Real-World Scenario

Given:

users = ["admin", "root1", "John_Doe", "Alice", "Bob42"]

Print only valid usernames.

Exercise 5 — Algorithm Explanation (Text)

Answer:
1. What happens to runtime if the list doubles in size?

2. Write the formula.

Expected:

T(n) = c × n

Explain in words.