Loops

Loops are an essential part of programming that allow us to execute a block of code multiple times without writing it repeatedly. Instead of manually repeating tasks, loops help automate repetitive actions efficiently.

Python provides two main types of loops:

1. for loop - Used for iterating over a sequence (such as a list, tuple, dictionary, string, or range).
2. while loop - Executes a block of code as long as a condition is true.

In this section, we will focus on the for loop, as it is one of the most commonly used loops in Python.

Understanding for Loops

A for loop in Python is used to iterate over a sequence (like a list or string). Unlike traditional C-style loops (which rely on an index variable), Python's for loop directly iterates over elements in a sequence.

Syntax of a for Loop

for item in sequence:
    # Code block to execute

• item takes the value of each element in the sequence, one at a time.
• The loop continues until all elements in the sequence are processed.
• The colon (:) is required after the for statement.
• The indented block under for is executed for each item.

Example: Looping Through a List

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

Output:

apple
banana
cherry

Looping Through a String

A string is a sequence of characters, so we can iterate through it using a for loop.

for letter in "Python":
    print(letter)

Output:

P
y
t
h
o
n

Using for Loop with range()

The range() function generates a sequence of numbers, which is useful when we need to loop a specific number of times.

for i in range(5):
    print("Iteration", i)

Output:

Iteration 0
Iteration 1
Iteration 2
Iteration 3
Iteration 4

We'll explore range() in more detail in the next lesson.

Nested for Loops

We can use a loop inside another loop to iterate over multiple sequences.

for i in range(3):
    for j in range(2):
        print(f"i: {i}, j: {j}")

Output:

i: 0, j: 0
i: 0, j: 1
i: 1, j: 0
i: 1, j: 1
i: 2, j: 0
i: 2, j: 1

break and continue

• break: Stops the loop entirely.
• continue: Skips the current iteration and moves to the next one.

Example: Using break

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

Output:

0
1
2
3
4

Example: Using continue

for num in range(5):
    if num == 2:
        continue
    print(num)

Output:

0
1
3
4

Exercise: Try It Yourself!

1. Write a for loop that prints the numbers from 1 to 10.
2. Modify the loop to skip number 5 using continue.
3. Create a for loop that iterates over the word "Python" and prints each letter in uppercase.
4. Use break to stop the loop when it encounters the letter "t" in "Python".