For loops are fundamental to Python programming. Here's everything you need to know to use them effectively:

1. Basic For Loop Syntax

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

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

# Iterate over a tuple
coordinates = (10, 20, 30)
for coord in coordinates:
    print(coord)

2. Using range() Function

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

# range(start, stop) - from start to stop-1
for i in range(2, 8):
    print(i)  # 2, 3, 4, 5, 6, 7

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

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

3. enumerate() - Get Index and Value

# Get both index and value ✅
fruits = ['apple', 'banana', 'cherry']

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

# Start index at 1 instead of 0
for index, fruit in enumerate(fruits, start=1):
    print(f"{index}. {fruit}")
# 1. apple
# 2. banana
# 3. cherry

4. Iterate Over Dictionaries

person = {'name': 'Alice', 'age': 30, 'city': 'NYC'}

# Iterate over keys (default)
for key in person:
    print(key)  # name, age, city

# Iterate over values
for value in person.values():
    print(value)  # Alice, 30, NYC

# Iterate over key-value pairs ✅ (most common)
for key, value in person.items():
    print(f"{key}: {value}")
# name: Alice
# age: 30
# city: NYC

5. Nested For Loops

# Create a multiplication table
for i in range(1, 6):
    for j in range(1, 6):
        print(f"{i} x {j} = {i*j}")

# Iterate over 2D list (matrix)
matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

for row in matrix:
    for num in row:
        print(num, end=' ')
    print()  # New line after each row

# Flatten 2D list
flat = [num for row in matrix for num in row]
# [1, 2, 3, 4, 5, 6, 7, 8, 9]

6. Loop Control: break and continue

# break - exit loop early
for i in range(10):
    if i == 5:
        break  # Stop loop when i is 5
    print(i)  # 0, 1, 2, 3, 4

# continue - skip current iteration
for i in range(10):
    if i % 2 == 0:
        continue  # Skip even numbers
    print(i)  # 1, 3, 5, 7, 9

# Find first match
numbers = [1, 3, 5, 8, 9, 11]
for num in numbers:
    if num % 2 == 0:
        print(f"First even number: {num}")
        break

7. Loop with else Clause

# else runs if loop completes without break
numbers = [1, 3, 5, 7, 9]

for num in numbers:
    if num % 2 == 0:
        print("Found even number")
        break
else:
    print("No even numbers found")  # This runs

# Practical example: Search for item
items = ['apple', 'banana', 'cherry']
search = 'grape'

for item in items:
    if item == search:
        print(f"Found {search}")
        break
else:
    print(f"{search} not found")

8. zip() - Iterate Multiple Lists

# Iterate two lists together
names = ['Alice', 'Bob', 'Charlie']
ages = [25, 30, 35]

for name, age in zip(names, ages):
    print(f"{name} is {age} years old")

# Zip three lists
cities = ['NYC', 'LA', 'Chicago']
for name, age, city in zip(names, ages, cities):
    print(f"{name}, {age}, lives in {city}")

# Create dictionary from two lists
person_dict = dict(zip(names, ages))
# {'Alice': 25, 'Bob': 30, 'Charlie': 35}

9. Practical Examples

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

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

# Filter list
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
evens = []
for num in numbers:
    if num % 2 == 0:
        evens.append(num)

# Better: Use list comprehension ✅
evens = [num for num in numbers if num % 2 == 0]

# Process files
import os
for filename in os.listdir('.'):
    if filename.endswith('.txt'):
        print(f"Processing {filename}")

Best Practices

• ✅ Use enumerate() when you need both index and value
• ✅ Use zip() to iterate multiple lists together
• ✅ Use range() only when you need numeric indices
• ✅ Use list comprehensions for simple transformations (faster)
• ✅ Use break to exit early and save computation
• ❌ Don't modify list while iterating over it (make a copy first)
• ❌ Don't use for i in range(len(list)) - iterate directly
• ⚠️ Avoid deeply nested loops (more than 3 levels) - refactor into functions