Loops (for, while)

1. Introduction

Loops are used to execute a block of code multiple times. Python provides two main types of loops:

• for loop (used for iterating over sequences like lists, tuples, strings, etc.)
• while loop (executes until a condition becomes False)

Loops help in reducing redundancy and improving code efficiency.

2. The for Loop

The for loop is used when you want to iterate over a sequence like a list, tuple, dictionary, string, or range.

Syntax

for variable in sequence:
    # Code to execute

Example: Iterating Over a List

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

Output

apple
banana
cherry

Using range() in for Loop

The range() function generates a sequence of numbers.

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

Output

0
1
2
3
4

Using range(start, stop, step)

for i in range(1, 10, 2):  # Start from 1, stop at 10, step by 2
    print(i)

Output

1
3
5
7
9

Iterating Over a String

for char in "Python":
    print(char)

Output

P
y
t
h
o
n

3. The while Loop

The while loop runs as long as the condition is True.

Syntax

while condition:
    # Code to execute

Example: Counting from 1 to 5

count = 1
while count <= 5:
    print(count)
    count += 1  # Increment count

Output

1
2
3
4
5

4. Controlling Loops (break and continue)

Using break (Stops the Loop)

for num in range(1, 10):
    if num == 5:
        break  # Stops when num is 5
    print(num)

Output

1
2
3
4

Using continue (Skips Current Iteration)

for num in range(1, 6):
    if num == 3:
        continue  # Skips number 3
    print(num)

Output

1
2
4
5

5. Using else with Loops

Python allows else with loops, which executes if the loop completes normally (i.e., without break).

Example: Using else with for

for i in range(1, 4):
    print(i)
else:
    print("Loop completed successfully!")

Output

1
2
3
Loop completed successfully!

Example: Using else with while

x = 1
while x < 4:
    print(x)
    x += 1
else:
    print("Loop completed!")

Output

1
2
3
Loop completed!

6. Nested Loops

A loop inside another loop.

for i in range(1, 3):
    for j in range(1, 4):
        print(f"i={i}, j={j}")

Output

i=1, j=1
i=1, j=2
i=1, j=3
i=2, j=1
i=2, j=2
i=2, j=3

7. Infinite Loops (Avoid!)

An infinite loop runs forever if the stopping condition is never met.

while True:
    print("This will run forever!")  # Press Ctrl+C to stop

8. Practical Example: Sum of Numbers

total = 0
for num in range(1, 6):
    total += num
print("Sum:", total)

Output

Sum: 15

9. Summary

for loops iterate over sequences like lists, strings, and ranges.
while loops run as long as the condition is True.
break stops the loop, and continue skips an iteration.
else runs when loops finish normally.
Nested loops allow loops inside loops.