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Module 015: Loops: for

Phase: 2. Control Flow & Data | Estimated time: 2 hours | Milestone Project: No

Prerequisites

Learning Objectives

  • Write for loops to iterate over sequences
  • Use range() with start, stop, and step parameters
  • Iterate over strings character by character
  • Understand the for-else clause
  • Choose between for and while loops appropriately
  • Use indexing patterns with range(len())

Why This Matters

The for loop is the most commonly used loop in Python. It's cleaner and safer than while for iterating over sequences because you don't have to manage a loop variable manually. Whether you're processing each character in a string, generating a sequence of numbers with range(), or working with collections (coming in Module 018), for is your go-to tool.

Concept Explanation

The for Loop

A for loop iterates over each item in a sequence:

for item in sequence:
    # do something with item

The loop variable (item) takes each value from the sequence, one at a time:

for name in ["Alice", "Bob", "Charlie"]:
    print(f"Hello, {name}!")
# Output:
# Hello, Alice!
# Hello, Bob!
# Hello, Charlie!

Iterating Over Strings

Strings are sequences of characters:

word = "Python"
for letter in word:
    print(letter, end=" ")
# Output: P y t h o n

The range() Function

range() generates a sequence of integers. It's commonly used with for loops.

range(stop) — Numbers from 0 up to (but not including) stop:

for i in range(5):
    print(i, end=" ")
# Output: 0 1 2 3 4

range(start, stop) — Numbers from start to stop - 1:

for i in range(2, 7):
    print(i, end=" ")
# Output: 2 3 4 5 6

range(start, stop, step) — Numbers from start, incrementing by step:

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

for i in range(10, 0, -2):
    print(i, end=" ")
# Output: 10 8 6 4 2

Key facts about range():

  • range is lazy — it doesn't create a list of all numbers in memory
  • The stop value is exclusive (not included)
  • Default start is 0
  • Step can be negative to count downward

for-else

The else clause after a for loop runs only if the loop completed without hitting break:

numbers = [1, 3, 5, 7, 9]
for n in numbers:
    if n % 2 == 0:
        print(f"Found an even number: {n}")
        break
else:
    print("No even numbers found.")
# Output: No even numbers found.

(Detailed use of break comes in Module 016.)

for vs while

Use for when... Use while when...
You know the sequence to iterate over The number of iterations is unknown
You need to process items one by one You're waiting for a condition to change
You want to avoid manual index management You need a sentinel-controlled loop
You're using range() You're doing input validation 
# for loop: clean iteration over a range
for i in range(10):
    print(i)

# while loop: equivalent but more verbose
i = 0
while i < 10:
    print(i)
    i = i + 1

Iterating with Index Patterns

Sometimes you need both the index and the value:

word = "hello"
for i in range(len(word)):
    print(f"Index {i}: {word[i]}")
# Output:
# Index 0: h
# Index 1: e
# Index 2: l
# Index 3: l
# Index 4: o

Common Pitfalls

  1. Modifying the sequence while iterating — Don't add or remove items from a list while looping over it (leads to skipped items). We'll cover safe patterns later.

  2. Forgetting that range(stop) excludes stop — range(10) gives 0-9, not 0-10. If you need to include 10, use range(11).

  3. Using for when input validation needs while — If you're waiting for a user to enter valid input, you don't know how many tries it will take. Use while.

  4. Loop variable leaking after the loop — In Python 3, the loop variable keeps its last value after the loop ends. Be aware of this.

  5. Unnecessary index tracking — If you don't need the index, just iterate directly: for item in sequence: not for i in range(len(sequence)):.

Hands-On Walkthrough

Let's count vowels in a string using a for loop:

text = input("Enter some text: ")  # Assume "Hello World"
vowels = "aeiou"
count = 0

for char in text.lower():
    if char in vowels:
        count = count + 1

print(f"Number of vowels: {count}")  # Number of vowels: 3

Now let's use range() to print a multiplication table:

n = int(input("Which multiplication table? "))  # Assume 7

for i in range(1, 11):
    print(f"{n} x {i} = {n * i}")
# Output:
# 7 x 1 = 7
# 7 x 2 = 14
# 7 x 3 = 21
# ...
# 7 x 10 = 70

Key Takeaways

  • for item in sequence: iterates over each element in a sequence
  • range(start, stop, step) generates integer sequences without creating a list
  • range(stop) goes from 0 to stop-1; range(start, stop) goes from start to stop-1
  • Strings are iterable — for char in "hello": works
  • for-else runs the else block only if no break occurred
  • Use for when iterating over sequences; use while when waiting for a condition
  • for i in range(len(seq)): gives you indexes; prefer direct iteration when you don't need them

Further Reading

Next Module

Module 016: Loop Control (break, continue, loop else) — Take control of your loops with break, continue, and the loop else clause.