For Loop

Introduction

A for loop is one of the most commonly used looping statements in Python. It is used to execute a block of code repeatedly for every element present in a sequence or collection. Unlike a while loop, which depends on a condition, a for loop is mainly used when the number of iterations is already known or when every element of a collection needs to be processed.

The for loop is known as a definite loop because Python knows in advance how many iterations will occur. It automatically moves from one element to the next until every item in the sequence has been processed. After the last element is processed, the loop terminates automatically without requiring any additional condition.

In Python, almost every iterable object can be used with a for loop. These iterable objects include lists, tuples, strings, dictionaries, sets, and even sequences generated by the range() function. During each iteration, the current element is assigned to a loop variable, allowing the programmer to perform operations on that element.

The for loop is widely used in software development because it provides a simple and readable way to traverse collections of data. Whether you are displaying student names, calculating the sum of numbers, searching through records, processing files, or performing repetitive tasks, the for loop is often the most suitable choice.

Compared to many other programming languages, Python's for loop is concise and easy to understand. It eliminates the need to manually initialize and update loop variables in many situations, making programs shorter, cleaner, and less prone to logical errors.

Syntax of For Loop

The for loop in Python is used to iterate over the elements of a sequence or any iterable object. During each iteration, Python automatically assigns the current element of the sequence to the loop variable and executes the statements inside the loop body. Once all elements have been processed, the loop terminates automatically.

Unlike many other programming languages, Python does not require separate initialization, condition checking, and update expressions. The for loop automatically handles the iteration process, making programs simpler and easier to read.

General Syntax

for item in sequence:
    # statements to be executed
    statements

Understanding the Syntax

Flow of Execution

1. Python reads the first element of the sequence.
2. The first element is assigned to the loop variable.
3. The statements inside the loop are executed.
4. Python automatically moves to the next element.
5. The same process repeats until all elements have been processed.
6. After the last element, the loop terminates automatically.

Example

fruits = ["Apple", "Mango", "Orange"]

for fruit in fruits:
    print(fruit)

Output

Dry Run of the Program

Iterating Over Sequences

One of the greatest advantages of Python's for loop is its ability to iterate over different types of sequences. A sequence is a collection of elements arranged in a specific order. Examples of sequences include lists, tuples, strings, and objects generated by the range() function.

During each iteration, the for loop automatically picks the next element from the sequence and stores it in the loop variable. The statements inside the loop are then executed using that value. Once every element has been processed, the loop terminates automatically.

1. Iterating Over a List

A list is one of the most commonly used sequence types in Python. When a for loop is used with a list, Python visits every element one by one, starting from the first element and ending with the last element.

fruits = [
    "apple",
    "banana",
    "cherry"
]

for fruit in fruits:
    print("I like", fruit)

Output

Dry Run

Since the list contains three elements, the loop executes exactly three times. Each time, the variable fruit stores one element of the list.

2. Iterating Over a Tuple

Tuples are ordered collections similar to lists, but they are immutable, meaning their values cannot be modified after creation. A for loop can iterate over tuples in exactly the same way as lists.

colors = (
    "Red",
    "Green",
    "Blue"
)

for color in colors:
    print(color)

Output

Python automatically visits every tuple element from left to right until all values have been processed.

3. Iterating Over a String

A string is simply a sequence of characters. Therefore, the for loop processes one character during each iteration.

word = "Python"

for char in word:
    print(char)

Output

Dry Run

4. Iterating Over a Dictionary

A dictionary stores data in the form of key-value pairs. When a dictionary is directly used in a for loop, Python iterates over its keys by default.

student = {
    "name": "John",
    "age": 21,
    "city": "Delhi"
}

for key in student:
    print(key)

Output

Notice that only the dictionary keys are printed because a dictionary automatically iterates over its keys.

Accessing Both Keys and Values

To print both keys and values together, use the items() method.

student = {
    "name": "John",
    "age": 21,
    "city": "Delhi"
}

for key, value in student.items():
    print(key, ":", value)

5. Iterating Over a Set

A set is an unordered collection of unique elements. A for loop can iterate over every element of a set. Since sets are unordered, the output order may differ every time the program is executed.

colors = {
    "Red",
    "Green",
    "Blue"
}

for color in colors:
    print(color)

6. Using enumerate()

Sometimes we need both the index and the value while traversing a sequence. Python provides the enumerate() function for this purpose.

fruits = [
    "Apple",
    "Banana",
    "Mango"
]

for index, fruit in enumerate(fruits):
    print(index, fruit)

Output

By default, indexing begins from 0. You can also specify another starting value using enumerate(sequence, start).

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

7. Using zip()

The zip() function combines two or more sequences into a single sequence. It is useful when multiple collections need to be processed simultaneously.

names = [
    "John",
    "Alice",
    "Bob"
]

marks = [
    90,
    85,
    95
]

for name, mark in zip(names, marks):
    print(name, ":", mark)

Output

The zip() function stops when the shortest sequence ends.

Sequence Comparison

Nested For Loops

A nested for loop is a loop placed inside another for loop. The outer loop executes first, and for every iteration of the outer loop, the inner loop executes completely.

Nested loops are useful whenever a problem involves two dimensions or repeated combinations. They are commonly used for pattern printing, multiplication tables, matrix operations, coordinate generation, searching in two-dimensional data, and processing rows and columns.

General Syntax

for outer_variable in outer_sequence:

    for inner_variable in inner_sequence:

        statements

The inner loop always completes all of its iterations before the outer loop proceeds to its next iteration.

Example 1: Basic Nested Loop

for i in [1, 2]:

    for j in ['A', 'B']:

        print(i, j)

Output

Dry Run

Observe that for every value of i, the inner loop runs completely before the next value of i is processed.

Example 2: Multiplication Table

Nested loops are frequently used to generate multiplication tables.

for i in range(1, 4):

    for j in range(1, 6):

        print(i * j, end=" ")

    print()

Output

Example 3: Printing a Square Pattern

Pattern-printing problems are among the most common applications of nested loops in programming and university examinations.

for i in range(4):

    for j in range(4):

        print("*", end=" ")

    print()

Output

Example 4: Traversing a Matrix

A matrix is a two-dimensional collection. Nested loops make it easy to process every row and every column.

matrix = [

    [1, 2, 3],

    [4, 5, 6],

    [7, 8, 9]

]

for row in matrix:

    for value in row:

        print(value, end=" ")

    print()

Time Complexity

If the outer loop executes n times and the inner loop also executes n times, the total number of iterations is approximately n × n, written as O(n²).

Code Examples

The best way to understand the for loop is by solving practical problems. The following examples demonstrate how the loop can be used to process sequences, perform calculations, count elements, and solve common programming tasks frequently asked in university practical examinations.

Example 1: Calculating the Sum of Numbers in a List

This program adds every number stored inside a list using a for loop.

numbers = [4, 8, 2, 10, 5]

total = 0

for num in numbers:
    total += num

print("Total Sum =", total)

Output

Dry Run

Example 2: Finding the Largest Number

This program finds the maximum value stored inside a list without using Python's built-in max() function.

numbers = [18, 42, 9, 67, 31]

largest = numbers[0]

for num in numbers:

    if num > largest:
        largest = num

print("Largest =", largest)

Initially, the first element is assumed to be the largest. During every iteration, Python compares the current number with the current largest value and updates it whenever a larger number is found.

Example 3: Finding the Smallest Number

numbers = [18, 42, 9, 67, 31]

smallest = numbers[0]

for num in numbers:

    if num < smallest:
        smallest = num

print("Smallest =", smallest)

Example 4: Counting Vowels in a String

The following program counts the total number of vowels present in a string.

word = "Programming"

count = 0

for ch in word:

    if ch.lower() in "aeiou":
        count += 1

print("Total Vowels =", count)

Example 5: Counting Even and Odd Numbers

numbers = [4, 7, 12, 15, 18, 9]

even = 0
odd = 0

for num in numbers:

    if num % 2 == 0:
        even += 1
    else:
        odd += 1

print("Even =", even)
print("Odd =", odd)

Example 6: Counting Characters

text = "Python"

count = 0

for ch in text:

    count += 1

print("Characters =", count)

Summary

The for loop is one of Python's most important control structures. It is used for definite iteration, which means the loop executes once for every element present in a sequence. Unlike a while loop, a for loop automatically moves to the next element and terminates when the sequence ends.

Python's for loop can iterate over different sequence types such as lists, tuples, strings, dictionaries, sets, and objects created using the range() function. It is also commonly used for solving practical programming problems such as calculating sums, searching for maximum or minimum values, counting characters, traversing matrices, and printing patterns.

Nested for loops further extend the capability of iteration by allowing programmers to work with two-dimensional data structures and complex repetitive tasks.

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