Python Loops & Conditions · Control Your Program's Flow

• 1. Conditional statements (if, elif, else)
• 2. For loop
• 3. While loop
• 4. For vs While - When to use which?
• 5. Nested loops
• 6. Break, continue, and pass
• 7. Quick reference
• 8. Quick Challenge: Spot the Flow Control
• 9. Exercises
• 10. Common Errors & Solutions
• 11. Check Your Understanding
• 12. Your Progress Tracker

Conditionals allow your program to make decisions based on whether a condition is True or False.

# Basic if statement
age = 18
if age >= 18:
    print("You are an adult")

# if-else statement
score = 75
if score >= 60:
    print("You passed!")
else:
    print("You failed.")

# if-elif-else (multiple conditions)
grade = 85
if grade >= 90:
    print("A")
elif grade >= 80:
    print("B")
elif grade >= 70:
    print("C")
elif grade >= 60:
    print("D")
else:
    print("F")

# Comparison operators
# ==  equal to
# !=  not equal to
# >   greater than
# <   less than
# >=  greater than or equal to
# <=  less than or equal to

# Logical operators
# and - both conditions must be True
# or  - at least one condition must be True
# not - reverses the condition

age = 20
has_license = True
if age >= 18 and has_license:
    print("You can drive")

if age < 12 or age > 65:
    print("You get a discount")

# Nested conditions (if inside if)
num = 10
if num > 0:
    if num % 2 == 0:
        print("Positive even number")
    else:
        print("Positive odd number")

A for loop is used to iterate over a sequence (list, tuple, string, range, etc.). Use it when you know how many times to repeat.

# Looping through a list
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

# Looping through a string
for char in "Python":
    print(char)

# Using range() function
for i in range(5):      # 0, 1, 2, 3, 4
    print(i)

for i in range(2, 8):   # 2, 3, 4, 5, 6, 7
    print(i)

for i in range(1, 10, 2):  # 1, 3, 5, 7, 9 (step = 2)
    print(i)

# Looping through dictionary
student = {"name": "Alice", "age": 25, "grade": "A"}
for key in student:
    print(f"{key}: {student[key]}")

for key, value in student.items():
    print(f"{key}: {value}")

# Using enumerate() to get index and value
colors = ["red", "green", "blue"]
for index, color in enumerate(colors):
    print(f"{index}: {color}")

# Using zip() to loop through multiple lists together
names = ["Alice", "Bob", "Charlie"]
scores = [85, 92, 78]
for name, score in zip(names, scores):
    print(f"{name} scored {score}")

A while loop repeats as long as a condition is True. Use it when you DON'T know how many times to repeat.

# Basic while loop
count = 0
while count < 5:
    print(count)
    count += 1

# While loop with user input
user_input = ""
while user_input != "quit":
    user_input = input("Enter 'quit' to exit: ")
    print(f"You entered: {user_input}")

# While loop with break (exit loop early)
num = 0
while True:
    if num >= 10:
        break
    print(num)
    num += 1

# While loop with continue (skip current iteration)
num = 0
while num < 10:
    num += 1
    if num % 2 == 0:
        continue    # skip even numbers
    print(num)      # prints only odd numbers

# While loop with else (runs after loop ends normally, not if break)
num = 0
while num < 3:
    print(num)
    num += 1
else:
    print("Loop completed without break")

# For loop example - known number of iterations
for i in range(10):
    print(i)

# While loop example - unknown number of iterations (game loop!)
game_active = True
while game_active:
    # game keeps running until player loses
    if player_hits_cactus:
        game_active = False

Nested loops are loops inside other loops. The inner loop completes all its iterations for each iteration of the outer loop.

# Nested for loops
for i in range(3):
    for j in range(3):
        print(f"({i}, {j})", end=" ")
    print()   # new line after each outer loop

# Output:
# (0, 0) (0, 1) (0, 2)
# (1, 0) (1, 1) (1, 2)
# (2, 0) (2, 1) (2, 2)

# Multiplication table
for i in range(1, 6):
    for j in range(1, 6):
        print(f"{i * j:4}", end="")
    print()

# break - exits the loop entirely
for i in range(10):
    if i == 5:
        break
    print(i)    # prints 0,1,2,3,4

# continue - skips to the next iteration
for i in range(10):
    if i % 2 == 0:
        continue
    print(i)    # prints 1,3,5,7,9

# pass - does nothing, used as a placeholder
for i in range(5):
    if i == 2:
        pass     # TODO: add code later
    print(i)

# Example: Find first even number (using break)
numbers = [1, 3, 5, 8, 10, 13]
for num in numbers:
    if num % 2 == 0:
        print(f"First even number is {num}")
        break   # stop after finding it

# CONDITIONALS
if condition:
    # code
elif condition2:
    # code
else:
    # code

# Comparison operators: ==, !=, <, >, <=, >=
# Logical operators: and, or, not

# FOR LOOP
for item in sequence:
    # code

for i in range(stop):
for i in range(start, stop):
for i in range(start, stop, step):

for index, item in enumerate(sequence):
for item1, item2 in zip(seq1, seq2):
for key, value in dict.items():

# WHILE LOOP
while condition:
    # code

# LOOP CONTROL
break   # exit loop completely
continue # skip to next iteration
pass    # do nothing (placeholder)

1. What is the difference between if and elif?
2. What does range(5) produce?
3. When would you use a while loop instead of a for loop?
4. What is the difference between break and continue?
5. What happens if you write while True: without a break?
6. What is a nested loop?