🔁 Topic 05 · Iteration
Loops & Iteration
Repeat actions efficiently using for and while loops, and control iteration with break, continue, and handy built-ins like enumerate and zip.
⏱ ~40 min
🟢 Beginner
🔄 Core skill
for Loop
The for loop iterates over any sequence: lists, strings, ranges, tuples, and more.
for_loop.py
# Iterate over a list
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
print(fruit)
# Iterate with range
for i in range(5): # 0,1,2,3,4
print(i)
# range(start, stop, step)
for i in range(0, 10, 2): # 0,2,4,6,8
print(i)
while Loop
Runs as long as a condition stays true. Always ensure the condition eventually becomes false to avoid an infinite loop.
while_loop.py
count = 0
while count < 5:
print(f"count = {count}")
count += 1
# Prints count = 0 through count = 4
⚠️ Infinite loop risk If the condition never becomes False the program hangs. Press Ctrl+C to interrupt. Always verify the loop variable changes each iteration.
break and continue
break exits the loop immediately. continue skips the rest of the current iteration and moves to the next.
break_continue.py
# break — stop at first positive even number
for n in range(10):
if n % 2 == 0 and n > 0:
print(f"Found: {n}")
break # prints 2, then exits
# continue — skip odd numbers
for n in range(6):
if n % 2 != 0:
continue
print(n) # 0, 2, 4
enumerate & zip
enumerate gives index + value in each iteration. zip pairs two sequences together.
enum_zip.py
items = ["a", "b", "c"]
for idx, val in enumerate(items):
print(idx, val) # 0 a · 1 b · 2 c
names = ["Alice", "Bob"]
scores = [90, 85]
for name, score in zip(names, scores):
print(f"{name}: {score}")
List Comprehensions
A concise, readable way to build a new list by transforming or filtering an existing sequence.
list_comp.py
# Standard loop
squares = []
for x in range(5):
squares.append(x ** 2)
# Same thing as comprehension
squares = [x ** 2 for x in range(5)]
print(squares) # [0, 1, 4, 9, 16]
# With filter
evens = [x for x in range(10) if x % 2 == 0]
