For Loops and Iteration in Python

One of the foundational skills in any programming language is the ability to repeat actions efficiently. In Python, iteration is handled primarily through for loops, a simple yet powerful tool that lets you loop over sequences such as lists, strings, or ranges.

Whether you’re processing data, automating tasks, or just learning to program, understanding for loops is crucial. In this article, we’ll explore everything you need to know about for loops and iteration in Python—from basics to best practices—with plenty of examples along the way.

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Why Learn About For Loops and Iteration?

Repetition is common in programming. We often need to:

• Process items in a list one by one
• Repeat a task a certain number of times
• Check each character in a string
• Apply logic to data structures like dictionaries

Instead of writing the same code multiple times, loops automate the repetition. The for loop is Python’s primary tool for iterating over items in a sequence.

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Prerequisites

Before you dive into for loops, make sure you understand:

• Basic Python syntax
• Variables and data types
• Data structures like lists, tuples, dictionaries, and strings
• The concept of sequences and indexing

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1. What is a for Loop?

A for loop in Python is used to iterate over a sequence of elements. Unlike some languages that use a counter to loop, Python’s for loop reads each element from the sequence directly.

Example:

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

Output:

apple
banana
cherry

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2. Syntax of for Loops

Here’s the basic syntax:

for variable in sequence:
    # Code block to execute

• variable is a name for each element in the sequence.
• The indented block runs once for each item.

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3. Iterating Over Different Data Types

🔹 Lists:

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

🔹 Strings:

word = "hello"
for letter in word:
    print(letter)

🔹 Tuples:

coordinates = (10, 20, 30)
for value in coordinates:
    print(value)

🔹 Dictionaries:

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

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4. Using range() in Loops

range() is often used to generate a sequence of numbers.

for i in range(5):
    print(i)

Output:

0
1
2
3
4

You can customize range(start, stop, step):

for i in range(1, 10, 2):
    print(i)

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5. Nested for Loops

A for loop inside another is called a nested loop.

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

This is useful in matrix operations or dealing with multi-dimensional data.

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6. Using break and continue

🔸 break — exits the loop prematurely:

for num in range(10):
    if num == 5:
        break
    print(num)

🔸 continue — skips the current iteration:

for num in range(10):
    if num % 2 == 0:
        continue
    print(num)

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7. Looping with else

Python allows an else clause in loops.

for num in range(3):
    print(num)
else:
    print("Loop completed")

If the loop completes without hitting a break, the else runs.

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8. Real-World Examples

🔹 1. Summing Numbers in a List:

numbers = [1, 2, 3, 4]
total = 0
for num in numbers:
    total += num
print("Total:", total)

🔹 2. Counting Vowels in a String:

text = "education"
vowels = "aeiou"
count = 0
for char in text:
    if char in vowels:
        count += 1
print("Vowels:", count)

🔹 3. Generating a Multiplication Table:

for i in range(1, 6):
    for j in range(1, 6):
        print(i * j, end="\t")
    print()

🔹 4. Filtering Even Numbers:

numbers = [1, 2, 3, 4, 5, 6]
for n in numbers:
    if n % 2 == 0:
        print(n)

🔹 5. Reading User Input:

names = []
for i in range(3):
    name = input("Enter a name: ")
    names.append(name)
print("Names:", names)

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9. Common Mistakes to Avoid

• Forgetting indentation
• Modifying a list while iterating over it
• Using incorrect range values
• Assuming for loops behave like C-style loops (they don’t)
• Over-nesting loops unnecessarily

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10. Best Practices

• Use descriptive loop variables (item, char, number)
• Avoid deeply nested loops—consider functions or list comprehensions
• Prefer for over while for finite sequences
• Use enumerate() when you need index and value:

for i, value in enumerate(["a", "b", "c"]):
    print(i, value)

• Leverage list comprehensions for concise looping:

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

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The for loop is one of the most important tools in Python, allowing you to iterate over sequences efficiently. Whether you’re dealing with a list of numbers, a string of characters, or a dictionary of key-value pairs, the for loop is your go-to solution for repetition and automation.

Once you’ve mastered the basics, you’ll be able to build powerful logic, manipulate data, and write more dynamic and responsive programs.