UNIT-3: Python conditional and iterative statements

Python conditional and iterative statements

if statement , if…elif statement , if….. elif…..else statment , nested if

1. if Statement.

  • The if statement is the most basic decision-making statement.
  • It checks whether a given condition is True. If the condition evaluates to True, the indented block of code under the if statement executes; otherwise, the block is skipped.
  • Syntax:
if condition:
    # Code to execute if condition is True
  • The condition is a Boolean expression (i.e., it evaluates to True or False).
  • If condition is True, the code inside the if block executes.
  • If condition is False, the code is skipped.
age = 20
if age >= 18:
    print("You are eligible to vote.")  # This line executes if age is 18 or more

2. if...elif Statement.

  • The if...elif statement allows checking multiple conditions sequentially.
  • If the first condition is False, Python checks the next elif condition.
  • The first condition that evaluates to True is executed, and the rest are ignored.
  • Syntax
if condition1:
    # Code to execute if condition1 is True
elif condition2:
    # Code to execute if condition1 is False and condition2 is True
elif condition3:
    # Code to execute if both condition1 and condition2 are False, but condition3 is True
  • Only one condition is executed.
  • If condition1 is True, its block runs, and the rest are ignored.
  • If condition1 is False but condition2 is True, the elif block runs.

Example:

marks = 85

if marks >= 90:
    print("Grade: A")
elif marks >= 80:
    print("Grade: B")
elif marks >= 70:
    print("Grade: C")

3. if...elif...else Statement.

This is similar to if...elif, but it also includes an else block that executes when none of the conditions are True.

Syntax.

if condition1:
    # Code to execute if condition1 is True
elif condition2:
    # Code to execute if condition1 is False and condition2 is True
else:
    # Code to execute if all conditions are False
  • The else block always executes if no conditions are True.
  • It acts as a “default” case.
temperature = 10

if temperature > 30:
    print("It's a hot day.")
elif temperature > 15:
    print("The weather is pleasant.")
else:
    print("It's cold outside.")

4. Nested if Statement.

  • A nested if statement means writing an if statement inside another if statement.
  • This allows checking multiple conditions in a hierarchical way.

Syntax:

if condition1:
    if condition2:
        # Code executes if both condition1 and condition2 are True
  • The inner if block executes only if the outer if condition is True.

Example:

num = 10

if num > 0:  # Outer if statement
    print("Number is positive")

    if num % 2 == 0:  # Inner if statement
        print("It is even")

Iterative statements.

While Loop and Control Statements.

1. while Loop

A while loop is used when we do not know the exact number of iterations beforehand. It keeps running as long as the condition is True.

Syntax:

while condition:
    # Code to execute while condition is True
  • The loop first checks the condition.
  • If the condition is True, the block inside the loop executes.
  • After execution, it checks the condition again and repeats if True.
  • If False, the loop terminates.
  • EXAMPLE
count = 1
while count <= 5:
    print("Count:", count)
    count += 1  # Increment count

2. Nested while Loop.

A nested loop means a while loop inside another while loop. The inner loop executes fully for each iteration of the outer loop.

Syntax:

while condition1:
    while condition2:
        # Code executes when both conditions are True

EXAMPLE

i = 1
while i <= 3:  
    j = 1
    while j <= 2:
        print(f"i={i}, j={j}")
        j += 1
    i += 1

3. break Statement

The break statement stops the loop immediately, even if the condition is still True.

Example:

x = 1
while x <= 5:
    if x == 3:
        break  # Loop stops when x = 3
    print("x:", x)
    x += 1

4. continue Statement

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

Example:

x = 0
while x < 5:
    x += 1
    if x == 3:
        continue  # Skip when x = 3
    print("x:", x)

for Loop and Control Statements.

1. for Loop

A for loop is used when we know the number of iterations. It is commonly used for iterating over sequences like lists, strings, tuples, dictionaries, and ranges.

Syntax:

for variable in sequence:
    # Code executes for each item in sequence

EXAMPLE

for num in [1, 2, 3, 4, 5]:
    print("Number:", num)

2. range() Function.

The range() function generates a sequence of numbers.

Syntax:

range(start, stop, step)
  • start: (Optional) Starting value (default = 0).
  • stop: End value (not included in range).
  • step: (Optional) Difference between numbers (default = 1).
for i in range(1, 6):
    print(i)

3. break Statement in for Loop

Example:

for num in range(1, 6):
    if num == 3:
        break  # Stop when num = 3
    print(num)

4. continue Statement in for Loop

Example:

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

5. pass Statement

The pass statement is a placeholder that does nothing.

Example:

for num in range(1, 6):
    if num == 3:
        pass  # Placeholder for future code
    print(num)

6. else with for Loop

The else block executes if the loop completes normally (i.e., without break).

Example:

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

7. Nested for Loop

Example:

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

Python Lists:

  • A list in Python is a built-in data structure used to store multiple items in a single variable.
  • It is one of the most commonly used data structures because of its flexibility and easy manipulation.
  • A list in Python is a mutable, ordered collection of elements that can store heterogeneous data types.
  • It is one of the most versatile data structures, widely used in programming for data storage and manipulation.

Key Characteristics of a List:

  • Ordered: Items maintain a specific order.
  • Mutable: Can be modified after creation.
  • Allows Duplicate Values: Elements can be repeated.
  • Can Store Different Data Types: Strings, integers, floats, and even other lists.

Why Use Lists?

✅ Store Multiple Items – Instead of creating separate variables, a list can store multiple values.
✅ Dynamic Size – Lists can grow and shrink dynamically.
✅ Mutable – Lists allow modification (adding, removing, updating items).
✅ Efficient Iteration – Easily loop over elements.
✅ Supports Various Data Types – Can hold numbers, strings, booleans, and even other lists.

1. Creating a List

Lists in Python are created using square brackets [ ], with elements separated by commas.

Syntax:

list_name = [item1, item2, item3, ...]

Examples.

# List of integers
numbers = [10, 20, 30, 40, 50]

# List of strings
fruits = ["apple", "banana", "cherry"]

# Mixed data types
mixed_list = [1, "Hello", 3.14, True]

# Empty list
empty_list = []

# Nested list (list inside a list)
nested_list = [[1, 2, 3], ["a", "b", "c"]]

print(numbers)
print(fruits)
print(mixed_list)
print(empty_list)
print(nested_list)

2. Indexing in Lists

  • Each element in a list is assigned a numeric index that helps in accessing elements.
  • Indexing helps in retrieving specific elements from a list.

🔹 Positive Indexing (Starts from 0):

Element "apple" "banana" "cherry"
Index 0 1 2

🔹 Negative Indexing (Starts from -1):

Element "apple" "banana" "cherry"
Index -3 -2 -1 
fruits = ["apple", "banana", "cherry"]

print(fruits[0])    # First element
print(fruits[-1])   # Last element using negative index
print(fruits[1])    # Second element

📍 3. Accessing List Members

  • We can retrieve elements using indexing or iteration.
  • We can access individual or multiple elements from a list.

🔹 Example:

numbers = [10, 20, 30, 40, 50]

print(numbers[2])  # Accessing the third element (30)
print(numbers[-1])  # Accessing the last element (50)

📍 4. Range in Lists (Slicing)

List slicing extracts a portion of the list.

SYNTAX.

list[start:end:step]
  • start: Beginning index (default is 0).
  • end: Ending index (exclusive).
  • step: Skips elements (default is 1).

EXAMPLE.

numbers = [10, 20, 30, 40, 50, 60, 70]

print(numbers[1:4])    # Elements from index 1 to 3
print(numbers[:3])     # First 3 elements
print(numbers[3:])     # Elements from index 3 to end
print(numbers[::2])    # Every second element
print(numbers[::-1])   # Reverse the list

📍 5. List Methods.

Python provides built-in methods to modify or manipulate  lists.

1️⃣ append() – Add an Item to the End

fruits = ["apple", "banana"]
fruits.append("cherry")
print(fruits)

2️⃣ clear() – Remove All Items

fruits = ["apple", "banana"]
fruits.clear()
print(fruits)

3️⃣ copy() – Copy a List

original = [1, 2, 3]
copy_list = original.copy()
print(copy_list)

4️⃣ count() – Count Occurrences.

numbers = [1, 2, 2, 3, 4, 2]
print(numbers.count(2))

5️⃣ index() – Find the Index of an Item

fruits = ["apple", "banana", "cherry"]
print(fruits.index("banana"))

6️⃣ insert() – Insert at Specific Position

fruits = ["apple", "cherry"]
fruits.insert(1, "banana")
print(fruits)

7️⃣ pop() – Remove and Return an Item

fruits = ["apple", "banana", "cherry"]
removed_item = fruits.pop(1)
print(removed_item)
print(fruits)

8️⃣ remove() – Remove by Value

numbers = [1, 2, 3, 2, 4]
numbers.remove(2)
print(numbers)

9️⃣ reverse() – Reverse a List

numbers = [1, 2, 3]
numbers.reverse()
print(numbers)

🔟 sort() – Sort a List

numbers = [5, 2, 8, 1]
numbers.sort()
print(numbers)
numbers.sort(reverse=True)
print(numbers)

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