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Loops in Python

Loops in Python allow you to execute a block of code repeatedly. They are essential for automating repetitive tasks and processing collections of data efficiently.

The for Loop​

The for loop is used to iterate over a sequence (like a list, tuple, string, or range).

Syntax:

for variable in sequence:
    # block of code

Example:

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

Output:

apple
banana
cherry

The range() Function​

The range() function generates a sequence of numbers, commonly used with for loops.

# Print numbers 0 to 4
for i in range(5):
    print(i)

# Print numbers 2 to 6
for i in range(2, 7):
    print(i)

# Print even numbers from 0 to 8
for i in range(0, 10, 2):
    print(i)

Output:

0
1
2
3
4

The while Loop​

The while loop continues executing as long as a condition is True.

Syntax:

while condition:
    # block of code

Example:

count = 1
while count <= 5:
    print(f"Count: {count}")
    count += 1

Output:

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

Loop Control Statements​

The break Statement​

break exits the loop immediately.

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

Output:

0
1
2
3
4

The continue Statement​

continue skips the current iteration and moves to the next one.

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

Output:

0
1
3
4

The pass Statement​

pass is a placeholder that does nothing. Useful when you need syntactically correct code but no action.

for i in range(3):
    if i == 1:
        pass  # Do nothing for i == 1
    else:
        print(i)

Nested Loops​

You can place one loop inside another loop.

# Multiplication table
for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i} x {j} = {i * j}")
    print()  # Empty line after each row

Output:

1 x 1 = 1
1 x 2 = 2
1 x 3 = 3

2 x 1 = 2
2 x 2 = 4
2 x 3 = 6

3 x 1 = 3
3 x 2 = 6
3 x 3 = 9

Looping Through Different Data Types​

Strings​

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

Dictionaries​

student = {"name": "Alice", "age": 20, "grade": "A"}

# Loop through keys
for key in student:
    print(key)

# Loop through values
for value in student.values():
    print(value)

# Loop through key-value pairs
for key, value in student.items():
    print(f"{key}: {value}")

Lists with Index​

colors = ["red", "green", "blue"]

# Using enumerate() to get index and value
for index, color in enumerate(colors):
    print(f"{index}: {color}")

Output:

0: red
1: green
2: blue

The else Clause in Loops​

Loops can have an else clause that executes when the loop completes normally (not broken by break).

# With for loop
for i in range(3):
    print(i)
else:
    print("Loop completed!")

# With while loop
count = 0
while count < 3:
    print(count)
    count += 1
else:
    print("While loop finished!")

Common Loop Patterns​

Counting Pattern​

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

Accumulation Pattern​

# Sum of numbers
numbers = [1, 2, 3, 4, 5]
total = 0
for num in numbers:
    total += num
print(f"Sum: {total}")

Finding Pattern​

# Find first even number
numbers = [1, 3, 5, 8, 9, 10]
for num in numbers:
    if num % 2 == 0:
        print(f"First even number: {num}")
        break

Best Practices​

  1. Use for loops when you know the number of iterations
  2. Use while loops when the condition determines when to stop
  3. Avoid infinite loops by ensuring the condition eventually becomes False
  4. Use meaningful variable names in loops
  5. Consider list comprehensions for simple transformations

Summary Table​

Loop TypeUse Case
for loopIterating over sequences (lists, strings)
while loopRepeating until a condition is met
breakExit loop immediately
continueSkip current iteration
passPlaceholder for empty loop body
Nested loopsWorking with multi-dimensional data
List comprehensionsCreating lists with concise syntax

Practice Questions:​

1. Print Even Numbers​

Write a Python program to print all even numbers from 1 to 20 using a for loop.

Expected Output:

2 4 6 8 10 12 14 16 18 20

2. Sum of Natural Numbers​

Write a program to calculate the sum of the first 10 natural numbers using a loop.

Expected Output:

The sum is: 55

3. Loop Through a String​

Given the string:

word = "Python"

Write a loop to print each character on a new line.

4. Reverse a List Using Loop​

Without using the built-in reverse() or slicing, write a program to print a list in reverse order using a loop.

numbers = [1, 2, 3, 4, 5]

Expected Output:

5
4
3
2
1

5. Multiplication Table​

Write a Python program to print the multiplication table of 7 using a loop.

Expected Output:

7 x 1 = 7  
7 x 2 = 14  
...  
7 x 10 = 70

6. Using break Statement​

Write a loop that prints numbers from 1 to 10.

  • If the number is 5, use break to stop the loop.

Expected Output:

1  
2  
3  
4

7. Using continue Statement​

Write a loop that prints numbers from 1 to 10.

  • Skip printing the number 5 using continue.

Expected Output:

1  
2  
3  
4  
6  
7  
8  
9  
10

8. Nested Loop – Pattern Printing​

Write a program using nested loops to print the following pattern:

*
* *
* * *
* * * *
* * * * *

Conclusion​

Loops are fundamental to programming in Python. Whether you're processing data, automating tasks, or building complex algorithms, mastering for and while loops will make your code more efficient and powerful. Practice with different data types and loop patterns to become proficient in using loops effectively.