Python in 7 Days – Day 2: Control Flow, Loops & Building Your First Game

1. VOTING AGE CHECKER USING IF-ELSE STATEMENT

The Code:

age = int(input("Enter your age: "))
if age >= 18:
    print("You are eligible to vote.")
else:
    print("You are not eligible to vote.")

Line-by-Line Explanation:

Line 1: age = int(input("Enter your age: "))

• input("Enter your age: ") - This displays the prompt "Enter your age: " and waits for the user to type something

• int() - This converts whatever the user types (which comes as text/string) into a whole number (integer)

• age = - This stores that number in a variable called age

• Example: If user types 25, then age becomes 25

Line 2: if age >= 18:

• if - This starts a conditional statement (a decision)

• age >= 18 - This checks if the value in age is greater than or equal to 18

• >= is the "greater than or equal to" operator

• The colon : tells Python the next indented line belongs to this condition

Line 3: print("You are eligible to vote.")

• This line ONLY runs if the condition on line 2 is TRUE (age is 18 or more)

• The indentation (4 spaces) is important - it shows this line belongs to the if

Line 4: else:

• else means "otherwise" - this runs when the if condition is FALSE

• The colon : again indicates the next indented block belongs to else

Line 5: print("You are not eligible to vote.")

• This line ONLY runs if age is LESS than 18

• The indentation shows it belongs to the else

Flow Chart of How It Works:

Start → Ask for age → User enters 20 → Is 20 >= 18? → YES → Print "eligible"
                                    ↓
                        User enters 16 → Is 16 >= 18? → NO → Print "not eligible"

Example Scenarios:

2. PRINTING EVEN NUMBERS FROM 1-20 USING FOR LOOP

The Code:

for i in range(1, 21):
    if i % 2 == 0:
        print(i)

Line-by-Line Explanation:

Line 1: for i in range(1, 21):

• for - This starts a loop that will repeat multiple times

• i - This is a loop variable (can be any name, but i is common for "index")

• in - Specifies what we're looping through

• range(1, 21) - This creates a sequence of numbers starting at 1, up to but NOT including 21

  • So it generates: 1, 2, 3, 4, ..., 20

• The colon : indicates the loop body starts next

Understanding range():

range(1, 21)  # Gives: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20
range(5)      # Gives: 0, 1, 2, 3, 4 (starts at 0 by default)
range(2, 10, 2) # Gives: 2, 4, 6, 8 (start, stop, step)

Line 2: if i % 2 == 0:

• i % 2 - The % is the modulo operator (gives the REMAINDER after division)

  • If i = 5, then 5 % 2 = 1 (5 divided by 2 is 2 with remainder 1)

  • If i = 4, then 4 % 2 = 0 (4 divided by 2 is 2 with remainder 0)

• == 0 - Checks if the remainder equals zero

• So this condition is TRUE for even numbers (remainder 0)

Line 3: print(i)

• This runs only when the number is even

• Prints the current value of i

What Happens During Loop Execution:

Visual Representation:

Start
  ↓
i = 1 → Is 1 even? → No → Next i
  ↓
i = 2 → Is 2 even? → Yes → Print 2 → Next i
  ↓
i = 3 → Is 3 even? → No → Next i
  ↓
i = 4 → Is 4 even? → Yes → Print 4 → Next i
  ↓
... continues until i = 20 ...
  ↓
End (when i reaches 21, loop stops)

Alternative Ways to Write This:

# Method 2: Using step parameter (more efficient)
for i in range(2, 21, 2):
    print(i)

# Method 3: Using list comprehension
even_numbers = [i for i in range(1, 21) if i % 2 == 0]
print(even_numbers)

3. NUMBER GUESSING GAME USING WHILE LOOP

The Code:

import random
number_to_guess = random.randint(1, 100)
guess = 0
while guess != number_to_guess:
    guess = int(input("Guess a number between 1 and 100: "))
    if guess < number_to_guess:
        print("Too low! Try again.")
    elif guess > number_to_guess:
        print("Too high! Try again.")
    else:
        print("Congratulations! You guessed the number.")

Line-by-Line Explanation:

Line 1: import random

• import - This brings in Python's random module

• random - A built-in library that provides functions for generating random numbers

• Without this line, you can't use random.randint()

Line 2: number_to_guess = random.randint(1, 100)

• random.randint(1, 100) - Generates a random integer between 1 and 100 (inclusive)

  • randint = Random Integer

  • Could return any number like 42, 17, 99, etc.

• number_to_guess = - Stores that random number in this variable

• Example result: If random gives 73, then number_to_guess = 73

Line 3: guess = 0

• Creates a variable called guess and sets it to 0

• Set to 0 initially so it's different from any valid guess (1-100)

• This ensures the while loop runs at least once

Line 4: while guess != number_to_guess:

• while - Creates a loop that continues as long as the condition is TRUE

• guess != number_to_guess - Checks if guess is NOT equal to the target number

• != means "not equal to"

• The loop continues UNTIL the user guesses correctly

Line 5: guess = int(input("Guess a number between 1 and 100: "))

• input("Guess a number between 1 and 100: ") - Prompts user for input

• int() - Converts the string input to an integer

• guess = - Updates the guess variable with the user's input

Line 6: if guess < number_to_guess:

• Checks if the user's guess is less than the secret number

Line 7: print("Too low! Try again.")

• Tells the user they need to guess higher

Line 8: elif guess > number_to_guess:

• elif = "else if" - Checks if guess is greater than secret number

• Only runs if the first condition (guess <) was false

Line 9: print("Too high! Try again.")

• Tells the user they need to guess lower

Line 10: else:

• Runs if neither condition was true

• This means guess is NOT less than AND NOT greater than secret number

• Therefore, guess MUST equal the secret number

Line 11: print("Congratulations! You guessed the number.")

• Congratulates the user for guessing correctly

• After this, the loop condition will be checked again

• Since guess now equals number_to_guess, the loop ends

Detailed Walkthrough Example:

Let's trace through the game assuming the secret number is 42:

Step 1: import random (load the random module)
Step 2: number_to_guess = random.randint(1, 100) → gets 42 (hypothetically)
Step 3: guess = 0

→ Check while condition: Is 0 != 42? YES → Enter loop

Step 4: User enters 50 → guess = 50
Step 5: Is 50 < 42? NO
Step 6: Is 50 > 42? YES → Print "Too high! Try again."

→ Loop continues (guess is 50, secret is 42, 50 != 42 is TRUE)

Step 7: User enters 30 → guess = 30
Step 8: Is 30 < 42? YES → Print "Too low! Try again."

→ Loop continues (30 != 42 is TRUE)

Step 9: User enters 42 → guess = 42
Step 10: Is 42 < 42? NO
Step 11: Is 42 > 42? NO
Step 12: else → Print "Congratulations! You guessed the number."

→ Check while condition: Is 42 != 42? NO → Exit loop

Program ends

Visual Flow Chart:

Start
  ↓
Generate secret number (e.g., 42)
  ↓
Initialize guess = 0
  ↓
┌─────────────────────────────────┐
│  WHILE guess != secret?         │
│       ↓ YES                     │
│  Ask user for guess             │
│       ↓                         │
│  Is guess < secret?  → YES → Print "Too low" → Back to loop start
│       ↓ NO                      │
│  Is guess > secret?  → YES → Print "Too high" → Back to loop start
│       ↓ NO                      │
│  (must be equal)                │
│  Print "Congratulations"        │
│       ↓                         │
│  Exit loop                      │
└─────────────────────────────────┘
  ↓
End

Common Issues and Solutions:

# ISSUE 1: User enters a letter instead of number
# This would crash the program. FIX with try-except:

try:
    guess = int(input("Guess a number: "))
except ValueError:
    print("Please enter a valid number!")
    continue  # Ask again

# ISSUE 2: User guesses outside 1-100 range
# FIX with validation:

if guess < 1 or guess > 100:
    print("Please guess between 1 and 100!")
    continue  # Don't count this guess, ask again

# ISSUE 3: Want to limit number of attempts
# FIX with attempt counter:

attempts = 0
max_attempts = 10
while guess != number_to_guess and attempts < max_attempts:
    attempts += 1
    # rest of game logic

Key Concepts Demonstrated:

SUMMARY TABLE OF KEY CONCEPTS

PRACTICE EXERCISES

Try modifying these on your own:

1. Voting Checker Challenge: Add different voting ages for different countries

2. Even Numbers Challenge: Print odd numbers instead of even ones

3. Guessing Game Challenge: Add a hint system, difficulty levels, or score tracking

TIPS FOR BEGINNERS

1. Indentation is CRITICAL in Python - always use consistent spaces (4 spaces is standard)

2. Read error messages - they tell you exactly what went wrong

3. Test with different inputs - try edge cases (like age = 18 exactly)

4. Add print statements to see what your variables contain while debugging

5. Comment your code - explain what each part does for future reference