Python for loop

The for loop in Python is used to iterate over a sequence (like a list, tuple, string, dictionary, or range) and execute a block of code repeatedly for each element in that sequence. Here’s a detailed explanation of how it works and its key features:

Syntax of a for loop

for variable in sequence:
    # Code block to execute

Components:

1. for: The keyword that initiates the iteration.
2. variable: The placeholder where at the present iteration the value of each object in the sequence is inserted.
3. in: The keyword associating variable with the sequence.
4. sequence: An iterable object like list, range, tuple, string, dictionary or set.
5. Code block: An indented block of code that gets executed for every iteration.

How It Works

1. The loop takes one element out of the sequence and assigns that to the variable.
2. The code block is run with the value of the variable at present.
3. This process continues until all the elements in the sequence have been traversed.

Examples

1. Iterating Through a List

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

Output:

apple
banana
cherry

2. Using range( ) in a for loop

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

Output:

0
1
2
3
4

Here, range(5) will generate numbers from 0 to 4.

3. Iterating Through a String

word = "Python"
for letter in word:
   print(letter)

Output:

P
y
t
h
o
n

4. Iterating Through a Dictionary

person = {"name": "Alice", "age": 25}
for key, value in person.items():
    print(key, ":", value)

Output:

name : Alice
age : 25

Common Features

1. break Statement
   Stops the loop prematurely when a condition is met.

for i in range(10):
    if i == 5:
       break
    print(i)

Output:

0
1
2
3
4

2. continue Statement
Skips the current iteration and proceeds to the next.

for i in range(5):
    if i == 2:
       continue
    print(i)

Output:

0
1
3
4

3. else Clause
Executes after the loop completes normally (i.e., without a break).

for i in range(3):
    print(i)
else:
    print("Loop completed")

Output:

0
1
2
Loop completed

Iterating Over Complex Structures

1. Nested Loops
   You can use loops inside loops to work with multidimensional data.

matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
for row in matrix:
    for value in row:
         print(value, end=" ")

Output:

1 2 3 4 5 6 7 8 9

2. Enumerate
Returns both index and value during iteration.

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

Output:

0 apple
1 banana
2 cherry

3. Zip
Combines two or more sequences for iteration.

names = ["Alice", "Bob"]
ages = [25, 30]
for name, age in zip(names, ages):
    print(name, "is", age, "years old")

Output:

Alice is 25 years old
Bob is 30 years old

Summary:

• Efficient: Iterates directly over items without needing indices.
• Flexible: Works with any iterable (lists, tuples, strings, etc.).
• Customizable: Use break, continue, else, enumerate, or zip for advanced functionality.