Loops in Python

Loops in Python allow executing a block of code multiple times, making programs more efficient. Python provides two types of loops:

  1. for Loop – Used when iterating over sequences like lists, tuples, or strings.
  2. while Loop – Used when the number of iterations is not known beforehand, and execution depends on a condition.

Additionally, Python offers loop control statements (break, continue, pass) to modify loop behavior.

1. for Loop

A for loop iterates over a sequence (list, tuple, dictionary, string, or range).

Syntax:

for variable in sequence:
    # Code to execute

Example 1: Printing Numbers from 0 to 10

for x in range(11):
    print(x)

Output:

0
1
2
3
4
5
6
7
8
9
10

Example 2: Iterating Over a List

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

Output:

apple
banana
cherry

Example 3: Printing Odd Numbers from 0 to 20

for x in range(21):
    if x % 2 != 0:
        print(x)

Output:

1
3
5
7
9
11
13
15
17
19

Example 4: Reverse Loop (Descending Order)

for x in range(10, 0, -1):
    print(x)

Output:

10
9
8
7
6
5
4
3
2
1

2. while Loop

A while loop runs until a specified condition is False. It is useful when the number of iterations is unknown.

Syntax:

while condition:
    # Code to execute

Example 1: Printing Numbers from 1 to 10

x = 1
while x <= 10:
    print(x)
    x += 1

Output:

1
2
3
4
5
6
7
8
9
10

Example 2: Sum of First n Natural Numbers

n = int(input("Enter a number: "))
sum = 0
i = 1
while i <= n:
    sum += i
    i += 1
print("Sum of first", n, "numbers is:", sum)

If the user enters 5, the output will be:

Sum of first 5 numbers is: 15

3. Loop Control Statements

Loop control statements alter the normal flow of loops. Python provides three main control statements:

3.1 break Statement (Exits the Loop)

The break statement stops the loop immediately when a condition is met.

for i in range(10):
    if i == 7:
        print("Processing stopped at 7")
        break
    print(i)

Output:

0
1
2
3
4
5
6
Processing stopped at 7

3.2 continue Statement (Skips Current Iteration)

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

for i in range(10):
    if i % 2 == 0:
        continue
    print(i)

Output:

1
3
5
7
9

3.3 pass Statement (Does Nothing)

The pass statement acts as a placeholder where code is required but no operation is needed.

for i in range(5):
    if i == 3:
        pass  # Placeholder for future implementation
    print(i)

Output:

0
1
2
3
4

4. Nested Loops

A loop inside another loop is called a nested loop.

Example: Printing a Multiplication Table

for i in range(1, 6):
    for j in range(1, 6):
        print(i * j, end="\t")
    print()

Output:

1	2	3	4	5
2	4	6	8	10
3	6	9	12	15
4	8	12	16	20
5	10	15	20	25

5. else in Loops

Python allows an else block in loops, which executes only if the loop completes normally without break.

Example: Checking a List of Items

cart = [10, 20, 30, 40, 50]
for item in cart:
    if item >= 500:
        print("Cannot process this order")
        break
    print(item)
else:
    print("All items processed successfully!")

Output:

10
20
30
40
50
All items processed successfully!

6. Infinite Loops

An infinite loop runs indefinitely until manually stopped.

Example: Infinite Loop Using while True

i = 0
while True:
    i += 1
    print("Hello", i)
    if i == 5:
        break  # Stops after 5 iterations

Output:

Hello 1
Hello 2
Hello 3
Hello 4
Hello 5

7. Key Differences Between for and while Loops

Feature
for Loop
while Loop
Usage
Used when the number of iterations is known
Used when the number of iterations is unknown
Condition
Iterates over a sequence (list, string, range, etc.)
Runs until a condition becomes False
Example
for i in range(5):
while i < 5:

8. Summary of Loop Control Statements

Control Statement
Description
Example
break
Exits the loop immediately
if x == 5: break
continue
Skips the current iteration
if x == 5: continue
pass
Does nothing, acts as a placeholder
if x == 5: pass

Conclusion

  • Python provides two types of loops: for (when the number of iterations is known) and while (when the number of iterations is unknown).
  • Loop control statements (break, continue, pass) modify loop behavior.
  • Nested loops allow complex iterations.
  • else in loops executes only if the loop completes normally.