Loops are control flow statements that allow you to repeatedly execute a block of code. This is incredibly useful for iterating over collections of items or performing actions a fixed number of times. Python's primary loop for iteration is the for loop.

The for Loop

The for loop is used to iterate over a sequence (like a string, list, tuple, or dictionary) or other iterable objects. It executes a block of code once for each item in the sequence.

Syntax:

python
for variable_name in sequence:
    # Code to execute for each item
    statement_1
    statement_2

Example 1: Iterating over a string

python
word = "Python"
for char in word:
    print(char)

# Output:
# P
# y
# t
# h
# o
# n

Example 2: Iterating over a list (will cover lists in more detail later)

python
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(f"I like {fruit}.")

# Output:
# I like apple.
# I like banana.
# I like cherry.

The range() Function

The range() function is often used with for loops to generate a sequence of numbers. It's particularly useful when you want to loop a specific number of times.

Syntax:

• range(stop): Generates numbers from 0 up to (but not including) stop.
• range(start, stop): Generates numbers from start up to (but not including) stop.
• range(start, stop, step): Generates numbers from start up to (but not including) stop, incrementing by step.

Example 1: Loop a fixed number of times (0 to 4)

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

# Output:
# 0
# 1
# 2
# 3
# 4

Example 2: Loop from a specific start number (2 to 6)

python
for j in range(2, 7):
    print(j)

# Output:
# 2
# 3
# 4
# 5
# 6

Example 3: Loop with a step (0, 2, 4, 6, 8)

python
for k in range(0, 10, 2):
    print(k)

# Output:
# 0
# 2
# 4
# 6
# 8

Example 4: Looping backwards (countdown)

python
for countdown in range(5, 0, -1):
    print(countdown)
print("Blast off!")

# Output:
# 5
# 4
# 3
# 2
# 1
# Blast off!

Combining for with if

You can combine for loops with if statements to perform conditional actions within a loop.

python
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
for num in numbers:
    if num % 2 == 0: # Check if number is even
        print(f"{num} is even.")
    else:
        print(f"{num} is odd.")

# Output:
# 1 is odd.
# 2 is even.
# ...

The for loop and range() function are indispensable tools for automating repetitive tasks and processing collections of data. They form a core part of Python's control flow mechanisms.

Key Takeaways:

• for loops iterate over sequences (strings, lists, etc.) or other iterable objects.
• The code block inside the loop executes once for each item in the sequence.
• range() generates a sequence of numbers, commonly used to control the number of loop iterations.
• range() can take one (stop), two (start, stop), or three (start, stop, step) arguments.
• Loops can be combined with conditional statements for more complex logic.