For Loops

Beginner ~25 min read

The for loop is Python's workhorse for iteration. Unlike traditional counter-based loops in C or Java, Python's for loop iterates directly over items in a sequence - making it elegant, readable, and powerful!

Iterating Over Sequences

Python's for loop works with any iterable: lists, strings, tuples, sets, dictionaries, and more. The loop variable takes each value in the sequence, one at a time.

# Basic For Loop - Iterating Over Sequences

# Loop through a list
print("=== Looping Through a List ===")
fruits = ["apple", "banana", "cherry", "date"]
for fruit in fruits:
    print(f"I like {fruit}")

print()

# Loop through a string (character by character)
print("=== Looping Through a String ===")
word = "Python"
for char in word:
    print(char, end=" ")
print()  # New line

print()

# Loop through a tuple
print("=== Looping Through a Tuple ===")
colors = ("red", "green", "blue")
for color in colors:
    print(f"Color: {color}")

print()

# Loop through a set (order not guaranteed)
print("=== Looping Through a Set ===")
unique_numbers = {3, 1, 4, 1, 5, 9}
for num in unique_numbers:
    print(num, end=" ")
print()

print()

# Loop through dictionary keys (default)
print("=== Looping Through Dictionary ===")
person = {"name": "Alice", "age": 30, "city": "NYC"}

print("Keys:")
for key in person:
    print(f"  {key}")

print("Values:")
for value in person.values():
    print(f"  {value}")

print("Key-Value pairs:")
for key, value in person.items():
    print(f"  {key}: {value}")

Output

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Python vs Other Languages: In C-style languages, you write for (int i = 0; i < n; i++). In Python, you simply write for item in collection:. This is called "iterating over" a sequence rather than "counting up to" a number.

The `range()` Function

When you need to repeat something a specific number of times or need numeric sequences, use range(). It generates numbers on-demand without creating a full list in memory.

Syntax	Description	Example Output
`range(5)`	0 to 4 (stop-1)	0, 1, 2, 3, 4
`range(2, 6)`	2 to 5 (start to stop-1)	2, 3, 4, 5
`range(0, 10, 2)`	0 to 8, step by 2	0, 2, 4, 6, 8
`range(5, 0, -1)`	5 to 1, counting down	5, 4, 3, 2, 1

# The range() Function

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

print()

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

print()

# range(start, stop, step)
print("=== range(0, 10, 2) - Even numbers ===")
for i in range(0, 10, 2):
    print(i, end=" ")
print()  # 0 2 4 6 8

print()

# Counting backwards with negative step
print("=== range(10, 0, -1) - Countdown ===")
for i in range(10, 0, -1):
    print(i, end=" ")
print()  # 10 9 8 7 6 5 4 3 2 1

print()

# Using range with len() for index-based iteration
print("=== Using range(len()) ===")
fruits = ["apple", "banana", "cherry"]
for i in range(len(fruits)):
    print(f"fruits[{i}] = {fruits[i]}")

print()

# Creating a list from range
print("=== Converting range to list ===")
numbers = list(range(1, 6))
print(f"list(range(1, 6)) = {numbers}")

squares = [x**2 for x in range(1, 6)]
print(f"Squares: {squares}")

Output

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Memory Efficiency: range() doesn't create a list in memory - it generates numbers one at a time. This means range(1000000) uses minimal memory, unlike list(range(1000000)) which would create a million-item list!

Using `enumerate()`

When you need both the index and the value while iterating, enumerate() is your friend. It's cleaner than maintaining a manual counter!

# enumerate() - Get Index and Value Together

fruits = ["apple", "banana", "cherry", "date"]

# Without enumerate (manual index tracking)
print("=== Without enumerate (manual) ===")
index = 0
for fruit in fruits:
    print(f"{index}: {fruit}")
    index += 1

print()

# With enumerate (much cleaner!)
print("=== With enumerate ===")
for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")

print()

# Starting index from a different number
print("=== enumerate with start=1 ===")
for num, fruit in enumerate(fruits, start=1):
    print(f"{num}. {fruit}")

print()

# Practical example: Finding item position
print("=== Finding Item Position ===")
names = ["Alice", "Bob", "Charlie", "Diana"]
search_name = "Charlie"

for index, name in enumerate(names):
    if name == search_name:
        print(f"Found '{search_name}' at position {index}")
        break

print()

# Creating a numbered menu
print("=== Numbered Menu ===")
options = ["New Game", "Load Game", "Settings", "Exit"]
for i, option in enumerate(options, start=1):
    print(f"[{i}] {option}")

Output

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Pro Tip: Use enumerate(items, start=1) when you want human-friendly numbering starting from 1 instead of 0. Perfect for menus and lists!

Practical Examples

For loops are essential for processing data, building new collections, and combining information from multiple sources.

# Practical For Loop Examples

# 1. Sum of numbers
print("=== Sum of Numbers ===")
numbers = [10, 20, 30, 40, 50]
total = 0
for num in numbers:
    total += num
print(f"Sum of {numbers}: {total}")

print()

# 2. Find maximum value
print("=== Finding Maximum ===")
values = [45, 22, 89, 34, 67]
max_value = values[0]
for value in values:
    if value > max_value:
        max_value = value
print(f"Maximum in {values}: {max_value}")

print()

# 3. Count occurrences
print("=== Counting Occurrences ===")
text = "hello world"
target = "l"
count = 0
for char in text:
    if char == target:
        count += 1
print(f"'{target}' appears {count} times in '{text}'")

print()

# 4. Building a new list
print("=== Building a New List ===")
original = [1, 2, 3, 4, 5]
squared = []
for num in original:
    squared.append(num ** 2)
print(f"Original: {original}")
print(f"Squared:  {squared}")

print()

# 5. Filtering items
print("=== Filtering Items ===")
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
evens = []
for num in numbers:
    if num % 2 == 0:
        evens.append(num)
print(f"All numbers: {numbers}")
print(f"Even numbers: {evens}")

print()

# 6. zip() - Iterate over multiple sequences
print("=== Using zip() ===")
names = ["Alice", "Bob", "Charlie"]
scores = [85, 92, 78]

for name, score in zip(names, scores):
    print(f"{name}: {score} points")

Output

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Don't Modify While Iterating: Never add or remove items from a list while looping over it! This can cause unexpected behavior or errors. Instead, create a new list or iterate over a copy: for item in list.copy():

Common Mistakes

1. Off-by-one errors with range()

# Wrong - this prints 0-9, not 1-10!
for i in range(10):
    print(i)

# Correct - for 1-10
for i in range(1, 11):  # stop is exclusive!
    print(i)

2. Using range(len()) when not needed

# Unnecessary - don't do this!
for i in range(len(fruits)):
    print(fruits[i])

# Better - iterate directly
for fruit in fruits:
    print(fruit)

# If you need the index too, use enumerate
for i, fruit in enumerate(fruits):
    print(i, fruit)

3. Modifying list while iterating

# Wrong - unpredictable behavior!
for item in items:
    if some_condition:
        items.remove(item)  # Don't do this!

# Correct - iterate over a copy
for item in items.copy():
    if some_condition:
        items.remove(item)

4. Forgetting the colon

# Wrong - SyntaxError!
for item in items
    print(item)

# Correct
for item in items:
    print(item)

Exercise: Sum Calculator

Task: Calculate sums using for loops.

Requirements:

Calculate the sum of the numbers list
Calculate the sum of numbers from 1 to 100 using range()

Output

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Show Solution

numbers = [10, 20, 30, 40, 50]
total_sum = 0

# 1. Sum of list
for num in numbers:
    total_sum += num
print(f"Sum of list: {total_sum}")  # 150

# 2. Sum of 1-100
range_sum = 0
for i in range(1, 101):  # 101 because stop is exclusive
    range_sum += i
print(f"Sum of 1-100: {range_sum}")  # 5050

Summary

for item in sequence: Iterates over each item directly
range(stop): Generates 0 to stop-1
range(start, stop, step): Full control over the sequence
enumerate(): Get both index and value
zip(): Iterate over multiple sequences in parallel
dict.items(): Iterate over dictionary key-value pairs
Don't modify lists while iterating over them

What's Next?

Now that you can iterate with for loops, let's learn about while loops - loops that continue as long as a condition is true. While loops are perfect when you don't know in advance how many iterations you need!

Previous Ternary Operator

Next While Loops

Iterating Over Sequences

The range() Function

Using enumerate()

Practical Examples

Common Mistakes

1. Off-by-one errors with range()

2. Using range(len()) when not needed

3. Modifying list while iterating

4. Forgetting the colon

Exercise: Sum Calculator

Summary

What's Next?

Enjoying these tutorials?

The `range()` Function

Using `enumerate()`