Loops (for & while)

Objective

Iterate with for and while loops, use built-in iteration helpers, and control loop flow with break and continue.

Tools & Technologies

for
while
range()
enumerate()
zip()
break
continue

Key Commands

for i in range(10):

for i, v in enumerate(lst):

while condition:

break

continue

list(zip(a, b))

Architecture Overview

flowchart TD ITER[Iterator/Sequence] --> LOOP[for item in sequence] LOOP -->|has item| BODY[Loop body] BODY --> BREAK{break?} BREAK -->|yes| EXIT[Exit loop] BREAK -->|no| CONT{continue?} CONT -->|yes| LOOP CONT -->|no| LOOP LOOP -->|exhausted| ELSE[else block\nif no break] EXIT --> AFTER[Code after loop] style BREAK fill:#1a1a2e,stroke:#ff4444,color:#ff4444 style ELSE fill:#1a1a2e,stroke:#ffd700,color:#ffd700

Step-by-Step Process

for Loop

Iterate over any iterable.

# Over range
for i in range(5):         # 0 1 2 3 4
    print(i)

# With start, stop, step
for i in range(0, 10, 2):  # 0 2 4 6 8
    print(i)

# Over list
fruits = ['apple', 'banana', 'cherry']
for fruit in fruits:
    print(fruit)

# enumerate: index + value
for i, fruit in enumerate(fruits):
    print(f'{i}: {fruit}')

while Loop & break/continue

Control flow within loops.

# Count down
n = 5
while n > 0:
    print(n)
    n -= 1

# break: exit loop early
for item in data:
    if item == target:
        print('Found!')
        break
else:
    print('Not found')  # runs if no break

# continue: skip to next iteration
for n in range(10):
    if n % 2 == 0:
        continue  # skip even
    print(n)       # only odd

zip and List Comprehensions

Combine and transform iterables.

# zip
names = ['Alice', 'Bob']
scores = [95, 87]
for name, score in zip(names, scores):
    print(f'{name}: {score}')

# List comprehension
squares = [x**2 for x in range(10)]
even = [x for x in range(20) if x % 2 == 0]

# Dict comprehension
name_len = {name: len(name) for name in names}

Challenges & Solutions

Modifying a list while iterating over it causes skipped items — iterate a copy
range() is lazy (doesn't create the list) — list(range(10)) creates it

Key Takeaways

The for...else clause is unique to Python — runs if loop wasn't terminated by break
enumerate(lst, start=1) starts index at 1