Lesson 20: For Loops - Python Course

For Loops

For loops are one of the most powerful and commonly used features in Python. They allow you to iterate over sequences like lists, strings, ranges, and other iterable objects.

For loops are preferred when you know how many times you want to iterate or when you're iterating over a collection of items. They're more Pythonic and often more readable than while loops for these cases.

For Loop with range()

The range() function generates a sequence of numbers, perfect for creating loops that run a specific number of times:

for.py
# Basic range: range(stop) - starts at 0, stops before the number
for i in range(5):
    print(i)  # Output: 0, 1, 2, 3, 4

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

# Range with step: range(start, stop, step)
for i in range(0, 10, 2):
    print(i)  # Output: 0, 2, 4, 6, 8 (even numbers)

# Countdown with negative step
for i in range(5, 0, -1):
    print(i)  # Output: 5, 4, 3, 2, 1

Iterating Over Sequences

For loops can iterate over any sequence: strings, lists, tuples, and more:

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

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

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

Using enumerate() for Index and Value

When you need both the index and value, use enumerate():

enumerate.py
fruits = ["apple", "banana", "cherry"]

# Get both index and value
for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")

# Output:
# 0: apple
# 1: banana
# 2: cherry

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

Loop Control: break and continue

You can control for loops with break (exit) and continue (skip to next iteration):

control.py
numbers = [1, 2, 3, 4, 5]

# Break: exit loop early
for num in numbers:
    if num == 4:
        break
    print(num)  # Output: 1, 2, 3

# Continue: skip current iteration
for num in numbers:
    if num % 2 == 0:
        continue  # Skip even numbers
    print(num)  # Output: 1, 3, 5

Nested For Loops

You can nest for loops to work with multi-dimensional data or create patterns:

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

# Pattern printing
for i in range(1, 5):
    for j in range(i):
        print("*", end="")
    print()

The else Clause in For Loops

For loops can have an else clause that executes after the loop completes normally (not via break):

else_clause.py
numbers = [2, 4, 6, 8]

for num in numbers:
    if num == 5:
        print("Found 5!")
        break
else:
    print("5 was not found in the list")

# The else runs because break was never executed

Practical Examples

Here are some common real-world uses of for loops:

practical.py
# Sum of numbers
numbers = [1, 2, 3, 4, 5]
total = 0
for num in numbers:
    total += num
print(f"Sum: {total}")

# Find maximum value
max_value = numbers[0]
for num in numbers:
    if num > max_value:
        max_value = num
print(f"Maximum: {max_value}")

# Process each character in a string
text = "Hello"
for char in text:
    print(char.upper())

Range Function Details

💡 Range Function Forms

• range(stop) - from 0 to stop-1

• range(start, stop) - from start to stop-1

• range(start, stop, step) - with step size

💡 Important Notes

• Range is exclusive on the stop value (doesn't include it)

• Range doesn't create a list - it's an iterator (memory efficient)

• To see the values, convert to list: list(range(5))

For Loops

For Loops

For Loop with range()

Iterating Over Sequences

Using enumerate() for Index and Value

Loop Control: break and continue

Nested For Loops

The else Clause in For Loops

Practical Examples

Range Function Details

💡 Range Function Forms

💡 Important Notes

Lesson Complete!