02.2 - Loops: for, while, break, continue, else

Theory 25 min Beginner

The `for` Loop

The for loop iterates over any iterable (list, string, range, dict, file…).

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

# Iterate over a string
for char in "Python":
    print(char)

# Iterate over a range
for i in range(5):
    print(i)   # 0, 1, 2, 3, 4

`range()` Function

range() generates a sequence of integers without creating a list in memory.

range(stop)           # 0 to stop-1
range(start, stop)    # start to stop-1
range(start, stop, step)  # with step

range(5)          # 0, 1, 2, 3, 4
range(1, 6)       # 1, 2, 3, 4, 5
range(0, 10, 2)   # 0, 2, 4, 6, 8
range(10, 0, -1)  # 10, 9, 8, 7, 6, 5, 4, 3, 2, 1

# Convert to list
list(range(5))    # [0, 1, 2, 3, 4]

# Sum of 1 to 100
total = sum(range(1, 101))   # 5050

Useful `for` Loop Patterns

`enumerate()` — Index + Value

fruits = ["apple", "banana", "cherry"]

for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")
# 0: apple
# 1: banana
# 2: cherry

# Start from 1
for i, fruit in enumerate(fruits, start=1):
    print(f"{i}. {fruit}")

`zip()` — Multiple Iterables in Parallel

names = ["Alice", "Bob", "Charlie"]
scores = [92, 87, 95]

for name, score in zip(names, scores):
    print(f"{name}: {score}")
# Alice: 92
# Bob: 87
# Charlie: 95

`reversed()` and `sorted()`

numbers = [3, 1, 4, 1, 5, 9, 2, 6]

for n in reversed(numbers):
    print(n)   # 6, 2, 9, 5, 1, 4, 1, 3

for n in sorted(numbers):
    print(n)   # 1, 1, 2, 3, 4, 5, 6, 9

for n in sorted(numbers, reverse=True):
    print(n)   # 9, 6, 5, 4, 3, 2, 1, 1

Iterating Over Dictionaries

person = {"name": "Alice", "age": 30, "city": "Montreal"}

# Keys only (default)
for key in person:
    print(key)

# Values only
for value in person.values():
    print(value)

# Keys and values
for key, value in person.items():
    print(f"{key}: {value}")

The `while` Loop

while repeats as long as a condition is True.

count = 0
while count < 5:
    print(count)
    count += 1
# 0, 1, 2, 3, 4

⚠️ Always ensure the condition eventually becomes False — or use break — to avoid infinite loops.

# Infinite loop pattern (controlled with break)
while True:
    user_input = input("Command (quit to exit): ").strip()
    if user_input == "quit":
        break
    print(f"You entered: {user_input}")

Loop Control: `break`, `continue`, `pass`

Statement	Effect
`break`	Exit the loop immediately
`continue`	Skip current iteration, continue with next
`pass`	Do nothing — placeholder

# break — stop when found
numbers = [1, 3, 5, 8, 11, 13]
for n in numbers:
    if n % 2 == 0:
        print(f"First even: {n}")
        break

# continue — skip odd numbers
for i in range(10):
    if i % 2 != 0:
        continue
    print(i)   # 0, 2, 4, 6, 8

# pass — placeholder for empty blocks
for item in range(5):
    pass   # TODO: implement later

Loop `else` Clause

Python's for and while loops have an optional else clause that runs only if the loop completed without break:

# Search for a prime
numbers = [4, 6, 8, 9, 11, 15]

for n in numbers:
    for i in range(2, n):
        if n % i == 0:
            break
    else:
        print(f"{n} is prime!")
# 11 is prime!

# While with else
attempts = 0
while attempts < 3:
    password = input("Password: ")
    if password == "secret":
        print("Access granted!")
        break
    attempts += 1
else:
    print("Too many failed attempts — locked!")

Nested Loops

# Multiplication table
for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i} × {j} = {i*j}", end="   ")
    print()  # newline after each row

# 1 × 1 = 1   1 × 2 = 2   1 × 3 = 3
# 2 × 1 = 2   2 × 2 = 4   2 × 3 = 6
# 3 × 1 = 3   3 × 2 = 6   3 × 3 = 9

`for` vs `while` — When to Use

Situation	Use
Iterating over a collection	`for`
Counting a fixed number of times	`for + range()`
Unknown number of iterations	`while`
User input validation	`while True` + `break`
Search (stop when found)	`for` + `break`

Pythonic Loops

# ❌ Non-Pythonic: using index manually
for i in range(len(fruits)):
    print(fruits[i])

# ✅ Pythonic: iterate directly
for fruit in fruits:
    print(fruit)

# ❌ Non-Pythonic: manually tracking index
i = 0
for fruit in fruits:
    print(f"{i}: {fruit}")
    i += 1

# ✅ Pythonic: enumerate()
for i, fruit in enumerate(fruits):
    print(f"{i}: {fruit}")

Key Vocabulary

Term	Definition
Iterable	Any object that can be looped over (`list`, `str`, `range`, `dict`…)
`range()`	Generates an integer sequence without storing it in memory
`enumerate()`	Returns `(index, value)` pairs when iterating
`zip()`	Pairs elements from multiple iterables
`break`	Immediately exits the innermost loop
`continue`	Skips the rest of the current iteration
`else` on loop	Executes only if the loop completed without `break`
Infinite loop	A `while True` loop that only exits via `break`

Summary

for loops iterate over any iterable — lists, strings, ranges, dicts
range(start, stop, step) generates integer sequences efficiently
enumerate() gives both index and value; zip() pairs multiple iterables
while loops run as long as a condition is True — always provide an exit
break exits a loop; continue skips to the next iteration
The else clause on a loop executes only when no break occurred

📄️ 02.1 - Conditional Statements

Control program execution with if/elif/else, match/case, ternary expressions, and guard clauses