Python — For Loops

Basic For Loop

for item in [1, 2, 3]:
    print(item)
# Output:
# 1
# 2
# 3

Looping Over Different Types

Strings

for char in "Hello":
    print(char)
# H, e, l, l, o

Lists

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

Tuples

colors = ("red", "green", "blue")
for color in colors:
    print(color)

Dictionaries

person = {"name": "Alice", "age": 25}
for key in person:
    print(f"{key}: {person[key]}")

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

Sets

unique = {1, 2, 3, 4, 5}
for num in unique:
    print(num)

range() Function

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

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

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

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

enumerate()

Get index and value together:

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}")
# 1. apple
# 2. banana
# 3. cherry

zip()

Loop over multiple lists simultaneously:

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

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

Nested Loops

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

# Loop through 2D list
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
for row in matrix:
    for cell in row:
        print(cell, end=" ")
    print()

break, continue, else

# break — exit loop early
for num in range(10):
    if num == 5:
        break
    print(num)  # 0, 1, 2, 3, 4

# continue — skip to next iteration
for num in range(10):
    if num % 2 == 0:
        continue
    print(num)  # 1, 3, 5, 7, 9

# else — runs if loop completes without break
for num in range(10):
    if num == 15:
        break
else:
    print("15 not found")  # This runs

List Comprehensions (Concise Loops)

# Instead of:
squares = []
for x in range(10):
    squares.append(x**2)

# Write:
squares = [x**2 for x in range(10)]

# With condition
evens = [x for x in range(20) if x % 2 == 0]

Practical Examples

# Sum of numbers
total = 0
for num in [1, 2, 3, 4, 5]:
    total += num
print(total)  # 15

# Find maximum
numbers = [23, 45, 12, 67, 34]
max_val = numbers[0]
for num in numbers:
    if num > max_val:
        max_val = num
print(max_val)  # 67

# Count occurrences
text = "hello world"
count = 0
for char in text:
    if char == 'l':
        count += 1
print(count)  # 3

Best Practices

• Use enumerate() instead of manual index tracking
• Use zip() for parallel iteration
• Keep loops simple — extract complex logic into functions
• Use list comprehensions for simple transformations
• Avoid modifying a list while iterating over it

Mini Practice

1. Print numbers 1-20 using range()
2. Loop through a dictionary and print key-value pairs
3. Use enumerate() to number a list of items
4. Use zip() to combine two lists
5. Write a nested loop to print a multiplication table

Up Next

Next: While Loops →