Loops: for and while - Python Mastery

for Loops

Python's for loop iterates over any iterable — lists, strings, ranges, dictionaries, files, and more. Unlike C-style for loops, Python's version is always "for each item in collection".

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

# Iterate over a string (character by character)
for char in "Python":
    print(char, end="-")
# P-y-t-h-o-n-

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

for i in range(1, 6):        # 1, 2, 3, 4, 5
    print(i)

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

for i in range(10, 0, -1):  # 10, 9, 8, ... 1  (countdown)
    print(i)

enumerate() — Index and Value Together

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

# BAD — old-school, don't do this
for i in range(len(fruits)):
    print(i, fruits[i])

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

# Start counting from 1
for i, fruit in enumerate(fruits, start=1):
    print(f"{i}. {fruit}")
# 1. apple
# 2. banana
# 3. cherry

zip() — Iterate Multiple Lists in Parallel

names = ["Alice", "Bob", "Carol"]
scores = [95, 87, 92]
grades = ["A", "B", "A"]

for name, score, grade in zip(names, scores, grades):
    print(f"{name}: {score} ({grade})")

# zip stops at the shortest iterable
# Use itertools.zip_longest to continue to the longest

while Loops

Repeat a block while a condition is True. Always ensure the condition eventually becomes False.

count = 0
while count < 5:
    print(count)
    count += 1  # Must modify condition variable!

# Classic input validation pattern
while True:                              # Infinite loop until break
    user_input = input("Enter a positive number: ")
    if user_input.isdigit() and int(user_input) > 0:
        number = int(user_input)
        break
    print("Invalid! Please enter a positive integer.")
print(f"You entered: {number}")

# Countdown
n = 10
while n > 0:
    print(n, end=" ")
    n -= 1
print("Go!")

Loop Control: break, continue, else

# break — exit the loop immediately
for i in range(10):
    if i == 5:
        break
    print(i)
# Prints: 0 1 2 3 4

# continue — skip to next iteration
for i in range(10):
    if i % 2 == 0:
        continue   # Skip even numbers
    print(i)
# Prints: 1 3 5 7 9

# Loop else — runs ONLY if loop completed without break
# (Useful for search patterns)
target = 7
for i in range(10):
    if i == target:
        print(f"Found {target}!")
        break
else:
    print(f"{target} not found")

# Practical: check if all items pass a condition
items = [2, 4, 6, 8, 10]
for item in items:
    if item % 2 != 0:
        print("Not all even!")
        break
else:
    print("All items are even!")  # Runs — no break occurred

Useful Loop Patterns

# Accumulate results
total = 0
for n in range(1, 101):
    total += n
print(total)  # 5050

# Build a list (better as comprehension, but useful to understand)
squares = []
for x in range(1, 6):
    squares.append(x ** 2)
# [1, 4, 9, 16, 25]

# Iterate over dictionary
person = {"name": "Alice", "age": 30, "city": "NYC"}
for key, value in person.items():
    print(f"{key}: {value}")

# Nested loops — multiplication table
for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i*j:3}", end="")
    print()
#   1  2  3
#   2  4  6
#   3  6  9

# Flatten nested structure
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
flat = []
for row in matrix:
    for num in row:
        flat.append(num)
# Better as: flat = [n for row in matrix for n in row]

# Fibonacci sequence
a, b = 0, 1
for _ in range(10):       # _ means "don't care about the variable"
    print(a, end=" ")
    a, b = b, a + b
# 0 1 1 2 3 5 8 13 21 34

Key Takeaways

for loops iterate over iterables: lists, strings, range(), dicts, files...
Use enumerate() not range(len()): cleaner and more Pythonic
while for condition-based repetition: ensure the condition changes or use break
break exits, continue skips: loop else runs only if no break occurred
_ as throwaway variable: for _ in range(n) when you don't need the counter

Practice Exercises

Write a loop that prints the first 15 Fibonacci numbers.
Find all prime numbers between 1 and 100 using a nested loop with break.
Write a number guessing game: pick random.randint(1, 100), let the user guess with "too high"/"too low" hints, count and display attempts.
Given two lists of equal length, use zip to create a dictionary pairing them, then print sorted by value descending.

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