Loops Cheatsheet

Complete Python Loops cheatsheet with clear examples for quick revision

WHAT IS A LOOP?

A loop is used to repeat a block of code multiple times until a condition is met. Instead of writing the same code again and again, we use loops.

Real-life examples:

• Asking attendance of 50 students one by one

• Printing numbers from 1 to 10

Why do we need loops?

• To save time

• To reduce code repetition

• To work with lists, strings, and ranges

• To automate repetitive tasks

Types of loops in Python

• while loop

• for loop

---

while Loop

Definition A while loop is used when we do not know how many times the loop will run. It runs as long as a condition is true.

Syntax

while condition:
    code to repeat

1

Example: Factorial using while loop

result = 1
i = 1
while i <= 5:
    result = result * i
    i = i + 1

print(f'The factorial of 5 is:{result}')
# OUTPUT
# The factorial of 5 is: 120

2

Example: Print numbers from 1 to 5

i = 1
while i <= 5:
    print(i)
    i += 1
# Note -> i += 1 is very important, otherwise the loop will run forever (infinite loop)

3

Example: Print each character of a string using while loop

name = "Python"
i = 0
while i < len(name):
    print(name[i])
    i += 1

4

Example: Print each word in a sentence using while loop

sentence = "Python is fun"
words = sentence.split()
i = 0
while i < len(words):
    print(words[i])
    i += 1
# OUTPUT :
# Python
# is
# fun

---

for Loop

Definition A for loop is used when we know in advance how many times the loop should run.

Syntax

for variable in sequence:
    code to repeat

1

Example: Print numbers from 1 to 5

for i in range(1, 6):
    print(i)
# OUTPUT:
# 1
# 2
# 3
# 4
# 5

Note: range(1, 6) means numbers from 1 to 5 (6 is excluded).

2

Example: Print each character of a string

for ch in "Python":
    print(ch)
# OUTPUT:
# P
# y
# t
# h
# o
# n

3

Example: Loop through a list

fruits = ["apple", "banana", "mango"]
for each_fruit in fruits:
    print(each_fruit)
# OUTPUT:
# apple
# banana
# mango

Common use of range()

• range(5) → 0 to 4

• range(1, 5) → 1 to 4

• range(1, 10, 2) → 1, 3, 5, 7, 9

Iterating using range()

Definition

range() is used to generate a sequence of numbers. It is commonly used with for loops when we want to repeat something a fixed number of times.

Syntax

range(start, stop, step)

• start → starting number (default: 0)

• stop → ending number (not included)

• step → gap between numbers (default: 1)

Examples:

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

# Print numbers from 1 to 10
for i in range(1, 11):
    print(i)

# Print even numbers
for i in range(2, 11, 2):
    print(i)

---

Iterating using enumerate()

Definition

enumerate() is used when we want both the index and the value while looping through a sequence (list, string, etc.).

Syntax

enumerate(sequence, start=0)

Examples:

# Loop through a list with index
fruits = ["apple", "banana", "mango"]
for index, fruit in enumerate(fruits):
    print(index, fruit)
# Output :
# 0 apple
# 1 banana
# 2 mango

# Start index from 1
for index, fruit in enumerate(fruits, start=1):
    print(index, fruit)

---

Infinite Loop

If the condition never becomes false, the loop runs forever.

# Example of infinite loop 
i = 1
while i <= 5:
    print(i)
# Note -> Always update the condition variable.

---

Loop Control Statements

These statements control the flow of loops.

1

break

Definition : break is used to stop the loop immediately.

for i in range(1, 10):
    if i == 5:
        break
    print(i)
    
# OUTPUT:
# 1
# 2
# 3
# 4

2

continue

Definition : continue is used to skip the current iteration and move to the next one.

for i in range(1, 6):
    if i == 3:
        continue
    print(i)
# OUTPUT:
# 1
# 2
# 4
# 5

3

pass

Definition : pass is used when we don’t want to write code yet, but need the loop syntax.

for i in range(5):
    pass
# NOTE -> No output, no error.

---

Nested Loops (Loop inside Loop)

Definition : A loop inside another loop is called a nested loop.

1

Example 1

persons = [{'name': 'John', 'sex': 'Male'}, {'name': 'Jane', 'sex': 'Female'}]
for person in persons:
    for key in person:
        print(key, ":", person[key])
    print(" ")
    
# OUTPUT:
# name : John
# sex : Male
#
# name : Jane
# sex : Female

2

Example 2

days = ['Monday', 'Tuesday', 'Wednesday']
fruits = ['apple', 'banana', 'guava']
for day in days:
    for fruit in fruits:
        print(day, fruit)
        
# OUTPUT:
# Monday apple
# Monday banana
# Monday guava
# Tuesday apple
# Tuesday banana
# Tuesday guava
# Wednesday apple
# Wednesday banana
# Wednesday guava