Python provides two primary loop structures: for loops and while loops.

• For loops iterate over sequences (like lists, tuples, or strings), executing a code block for each element. The loop ends after all elements have been processed.
• While loops repeat a code block as long as a specified condition remains true. The loop terminates when the condition becomes false.

Python also includes control keywords like break and continue:

• break exits the current loop entirely.
• continue skips the remaining code in the current iteration and proceeds to the next one.

These tools offer flexibility for implementing repetitive tasks in Python programs.

While Loop Syntax

The basic structure of a while loop is:

while condition:
    statements

Indentation and colon are esssential. Python does not have a do...while construct.

Example: Summing Numbers from 1 to 100

target = 100
total = 0
current = 1

while current <= target:
    total += current
    current += 1

print(f"Sum from 1 to {target} is: {total}")

Output:

Sum from 1 to 100 is: 5050

Infinite Loops

An infinite loop occurs when the condition never becomes false. This is useful for continuous operations, like server requests.

value = 1
while value == 1:  # Always true
    user_input = int(input("Enter a number: "))
    print(f"You entered: {user_input}")

print("Loop ended.")

Press CTRL+C to interrupt an infinite loop.

While Loop with Else Clause

An else block executes after the loop finishes, but not if the loop is terminated by break.

counter = 0
while counter < 5:
    print(f"{counter} is less than 5")
    counter += 1
else:
    print(f"{counter} is not less than 5")

Output:

0 is less than 5
1 is less than 5
2 is less than 5
3 is less than 5
4 is less than 5
5 is not less than 5

Single-Statement While Loops

If the loop body contains only one statement, it can be written on the same line:

active = True
while active: print("Looping...")
print("Done.")

For Loop Syntax

for loops iterate over iterable objects:

for element in sequence:
    statements

Example: Iterating Over a List

websites = ["Baidu", "Google", "Huawei", "Taobao"]
for site in websites:
    print(site)

Output:

Baidu
Google
Huawei
Taobao

Example: Iterating Over a String

text = "hello,world!"
for char in text:
    print(char)

For Loop with Else Clause

Similar to while, an else block runs after the loop completes, unless break is used.

for num in range(6):
    print(num)
else:
    print("Loop finished.")

Output:

0
1
2
3
4
5
Loop finished.

Example with Break

companies = ["Baidu", "Google", "Huawei", "Taobao"]
for company in companies:
    if company == "Huawei":
        print("Found Huawei!")
        break
    print(f"Processing {company}")
else:
    print("No Huawei found.")
print("Search complete.")

Output:

Processing Baidu
Processing Google
Found Huawei!
Search complete.

Using the range() Function

range() generates number sequences for iteration.

Basic Usage

for i in range(5):
    print(i)

Output:

0
1
2
3
4

Specifying Start and End

for i in range(5, 9):
    print(i)

Output:

5
6
7
8

Using a Step Value

for i in range(0, 10, 3):
    print(i)

Output:

0
3
6
9

Negative Steps

for i in range(-10, -100, -30):
    print(i)

Output:

-10
-40
-70

Iterating with Indexes

items = ['Google', 'Baidu', 'Runoob', 'Taobao', 'QQ']
for index in range(len(items)):
    print(index, items[index])

Output:

0 Google
1 Baidu
2 Runoob
3 Taobao
4 QQ

Creating Lists with range()

number_list = list(range(5))
print(number_list)

Output:

[0, 1, 2, 3, 4]

Break and Cnotinue Statements

Break in While Loop

value = 5
while value > 0:
    value -= 1
    if value == 2:
        break
    print(value)
print("Loop terminated.")

Output:

4
3
Loop terminated.

Continue in While Loop

value = 5
while value > 0:
    value -= 1
    if value == 2:
        continue
    print(value)
print("Loop ended.")

Output:

4
3
1
0
Loop ended.

Continue in For Loop

for char in "hello,world!":
    if char == "e":
        continue
    print(f"Current character: {char}")

counter = 10
while counter > 0:
    counter -= 1
    if counter == 5:
        continue
    print(f"Counter value: {counter}")
print("Execution complete.")

Break in For Loop

for char in "Hello,World":
    if char == "e":
        break
    print(f"Current character: {char}")

counter = 10
while counter > 0:
    print(f"Counter value: {counter}")
    counter -= 1
    if counter == 5:
        break
print("Process finished.")

Prime Number Example

for number in range(2, 10):
    for divisor in range(2, number):
        if number % divisor == 0:
            print(f"{number} equals {divisor} * {number // divisor}")
            break
    else:
        print(f"{number} is a prime number")

Output:

2 is a prime number
3 is a prime number
4 equals 2 * 2
5 is a prime number
6 equals 2 * 3
7 is a prime number
8 equals 2 * 4
9 equals 3 * 3

Pass Statement

pass is a null operation used as a placeholder.

Example in Loop

while True:
    pass  # Wait for keyboard interrupt (Ctrl+C)

Minimal Class Definition

class EmptyClass:
    pass

Using Pass in a Loop

for char in "Runoob":
    if char == "o":
        pass
        print("Pass block executed")
    print(f"Current character: {char}")
print("Done.")

Output:

Current character: R
Current character: u
Current character: n
Pass block executed
Current character: o
Pass block executed
Current character: o
Current character: b
Done.