Chapter 2 — Control Flow: Conditions & Loops

1) Truthiness & Comparisons

if "BeSmart":
    print("truthy")
if []:
    print("won't run")
# comparisons: ==, !=, <, >, <=, >= ; chained:
x = 3
print(1 < x < 5)  # True

2) if / elif / else (+ nested)

marks = float(input("Percent: "))
if marks >= 90:
    grade = "A+"
elif marks >= 75:
    grade = "A"
elif marks >= 60:
    grade = "B"
else:
    grade = "C"
print("Grade:", grade)

Nested example

age = int(input("Age: "))
if age >= 18:
    id_proof = input("Do you have ID? (y/n): ").lower()
    if id_proof == "y":
        print("Allowed")
    else:
        print("Bring ID")
else:
    print("Not allowed")

3) Conditional Expression (Ternary)

n = int(input("Number: "))
parity = "even" if n % 2 == 0 else "odd"
print(parity)

4) match–case (Python 3.10+)

day = input("Day (mon/tue/...): ").lower()
match day:
    case "mon" | "tue" | "wed" | "thu" | "fri":
        print("Weekday")
    case "sat" | "sun":
        print("Weekend")
    case _:
        print("Unknown")

5) Loops Overview

6) while Loop

# Sum of positive numbers until user enters 0
total = 0
n = int(input("Enter number (0 to stop): "))
while n != 0:
    if n > 0:
        total += n
    n = int(input("Enter number (0 to stop): "))
print("Total:", total)

7) for Loop & range()

# Multiplication table
n = int(input("Table of: "))
for i in range(1, 11):
    print(f"{n} x {i} = {n*i}")

# Reverse loop
for i in range(5, 0, -1):
    print(i, end=" ")  # 5 4 3 2 1

8) Loop else

# Prime check using for-else
n = int(input("Enter number: "))
if n < 2:
    print("Not prime")
else:
    for d in range(2, int(n**0.5)+1):
        if n % d == 0:
            print("Not prime"); break
    else:
        print("Prime")

9) break, continue, pass

# find first multiple of 7 between 1..100
for i in range(1, 101):
    if i % 7 == 0:
        print("Found:", i)
        break

# print 1..10 except multiples of 3
for i in range(1, 11):
    if i % 3 == 0:
        continue
    print(i, end=" ")

# TODO placeholder
def feature():
    pass

10) enumerate() & zip()

names = ["Riya", "Aarav", "Kabir"]
scores = [92, 88, 76]
for idx, (n, s) in enumerate(zip(names, scores), start=1):
    print(idx, n, s)

11) Nested Loops & Pattern Programs

rows = int(input("Rows: "))
for i in range(1, rows+1):
    print("*" * i)

n = int(input("Rows: "))
for i in range(1, n+1):
    spaces = " " * (n - i)
    stars = "*" * (2*i - 1)
    print(spaces + stars)

n = int(input("Rows: "))
for i in range(1, n+1):
    line = ""
    for j in range(1, i+1):
        line += str(j) + " "
    print(line)

n = int(input("Rows: "))
for i in range(n, 0, -1):
    print("*" * i)

12) Input Validation Patterns

while True:
    try:
        x = int(input("Enter positive int: "))
        if x > 0:
            break
        print("Must be > 0")
    except ValueError:
        print("Enter a valid integer")
print("OK:", x)

13) Mini Programs (5)

A) Simple Menu (while + match/if)

balance = 1000.0
while True:
    print("\\n1) Check  2) Deposit  3) Withdraw  4) Exit")
    ch = input("Choice: ")
    if ch == "1":
        print("Balance:", balance)
    elif ch == "2":
        amt = float(input("Amount: "))
        if amt > 0: balance += amt
        else: print("Invalid")
    elif ch == "3":
        amt = float(input("Amount: "))
        if 0 <= amt <= balance: balance -= amt
        else: print("Insufficient/Invalid")
    elif ch == "4":
        print("Bye"); break
    else:
        print("Invalid choice")

B) Guess the Number (1–100)

import random
target = random.randint(1, 100)
tries = 0
while True:
    n = int(input("Guess (1-100): "))
    tries += 1
    if n == target:
        print("Correct in", tries, "tries"); break
    elif n < target:
        print("Too low")
    else:
        print("Too high")

C) Factorial (for)

n = int(input("n: "))
fact = 1
for i in range(2, n+1):
    fact *= i
print("Factorial:", fact)

D) GCD (Euclid, while)

a = int(input("a: ")); b = int(input("b: "))
while b != 0:
    a, b = b, a % b
print("GCD:", a)

E) Armstrong Number (3-digit)

n = int(input("n (100..999): "))
s = sum(int(d)**3 for d in str(n))
print("Armstrong" if s == n else "Not Armstrong")

14) Practice Questions (Easy → Hard)

1. Concept: Truthy vs Falsy values list karo; examples do.
2. Code: Percent → Grade (A+/A/B/C) if/elif/else se likho (cutoffs aap choose karo).
3. Ternary: Input number ki parity (even/odd) 1-line me print karo.
4. match–case: Month number (1–12) → season print karo (3.10+).
5. Loop: 1..N ka sum (while & for dono versions).
6. Loop-else: List me target mila ya nahi — message print karo.
7. break/continue: 1..50 me sirf multiples of 5 print karo; 25 ke baad stop.
8. enumerate/zip: Do lists (subjects, marks) ko side-by-side index ke saath print karo.
9. Patterns: User rows ke hisab se hollow square print karo.
10. Challenge: Prime numbers between A..B print karo (efficient √n check), count bhi batao.

15) Quick Cheatsheet