LOOP IN PYTHON

Introduction

In programming, we often need to repeat certain actions multiple times — for example, printing 10 similar lines. To handle these repetitive tasks efficiently, we use loops. Python supports two main types of loops:

• for loop – used to iterate over a sequence (like a list, range, tuple, etc.)
• while loop – repeats as long as a condition is true

FOR Loop

The for loop is used to iterate over each element in a list, string, tuple, dictionary, or any iterable object.

Breakdown of components:

• Keyword: Marks the start of the for loop. It tells Python you want to iterate over something.
• Colon: Indicates the end of the loop header and the beginning of the loop body.
• Indentation: Python uses indentation (usually 4 spaces or 1 tab) to define the block of code inside the loop
• element: A variable that represents each element from the iterable. On each iteration, it takes the next value.
• iterable : Any iterable object (like a list, string, tuple, or range) that provides the elements for the loop.

The break statement is used to terminate the loop prematurely when a specific condition is met. The continue statement is used to skip the rest of the code inside the loop for the current iteration and jump to the next iteration.

Examples

# Example 1: Loop through a string
word = "Python"
for i in word:
    print(i)
    
# Output:
# P
# y
# t
# h
# o
# n
 
 
# Example 2: Loop through a list
fruits = ["Apple", "Orange", "Banana"]
for fruit in fruits:
    print(fruit)
    
# Output:
# Apple
# Orange
# Banana
 
 
# Example 3: Loop through a sequence of numbers using range(begin, end, step)
# Explanation:
# - begin: starting number (inclusive)
# - end: ending number (exclusive)
# - step: step size between numbers (default is 1)
 
for i in range(2, 10, 2):
    print(i)
# Output:
# 2
# 4
# 6
# 8
 

• When you call range(stop), passing only one argument:
  • stop is the ending value (exclusive).
  • The default begin is 0.
  • The default step is 1.
• When you call range(start, stop), passing two arguments:
  • start is the starting value (inclusive).
  • stop is the ending value (exclusive).
  • The default step is still 1.
• When you call range(start, stop, step), passing three arguments:
  • start is the starting value.
  • stop is the ending value.
  • step is the step size (can be positive or negative).

# Only one argument: range(stop)
print("range(5):")
for i in range(5):
    print(i)
 
# Output:
# 0
# 1
# 2
# 3
# 4
 
 
# Two arguments: range(start, stop)
print("\nrange(2, 6):")
for i in range(2, 6):
    print(i)
 
# Output:
# 2
# 3
# 4
# 5
 
 
# Three arguments: range(start, stop, step)
print("\nrange(1, 10, 3):")
for i in range(1, 10, 3):
    print(i)
 
# Output:
# 1
# 4
# 7
 
 
# Negative step example: counting backwards
print("\nrange(10, 0, -2):")
for i in range(10, 0, -2):
    print(i)
 
# Output:
# 10
# 8
# 6
# 4
# 2

Using break and continue in the loop

print("Example with break:")
for i in range(1, 10):
    if i == 5:
        break
    print(i)
 
# Output:
# Example with break:
# 1
# 2
# 3
# 4
 
print("\nExample with continue:")
for i in range(1, 6):
    if i == 3:
        continue
    print(i)
 
# Output:
# Example with continue:
# 1
# 2
# 4
# 5

WHILE Loop

Breakdown of components:

• Keyword (while):

  Marks the beginning of the loop. It tells Python to repeatedly execute the loop body as long as the condition is True.

• Condition:

  A boolean expression that is evaluated before each iteration.

  If the condition is True, the loop continues; if False, the loop stops.

• Colon (:):

  Indicates the end of the loop header and the start of the loop body.

  It’s required to define the structure of the loop.

• Indentation:

  Defines the block of code that belongs to the loop body.

  Python uses indentation (usually 4 spaces) to determine which statements are inside the loop.

The break statement is used to terminate the loop prematurely when a specific condition is met. The continue statement is used to skip the rest of the code inside the loop for the current iteration and jump to the next iteration.

Examples

i = 0
while i < 5:  # Loop continues while i < 5
    print("i =", i)
    i += 1    # When i becomes 5, the condition is False and the loop stops
 
# Output:
# i = 0
# i = 1
# i = 2
# i = 3
# i = 4
 
#Using break to exit the loop
i = 0
while True:  # Infinite loop
    print("i =", i)
    if i == 3:
        break  # Breaks the loop when i == 3
    i += 1
 
# Output:
# i = 0
# i = 1
# i = 2
# i = 3
#Using continue to skip an iteration
i = 0
while i < 5:
    i += 1
    if i == 3:
        continue  # Skips the rest of the loop when i == 3
    print("i =", i)
 
# Output:
# i = 1
# i = 2
# i = 4
# i = 5
 

When to Use for vs. while

Situation	Use This Loop
You know how many times to loop / have a sequence	for
You want to loop until a condition is met	while

Back Matter

Source

• based_on: Week 1 Module 1

References