UNIT-4: Python conditional and iterative statements

UNIT-4: Python conditional and iterative statements

UNIT-4: Python conditional and iterative statements

4.1 Conditional Statements in Python

  • Conditional statements in Python allow decision-making in programs.
  • The if, if-elif, if-elif-else, and nested if statements help execute specific blocks of code based on conditions.

4.1.1 If Statement

  • The if statement checks a condition. If the condition is True, the block of code inside if executes.

Syntax:

if condition:
    # Code block to execute if condition is True

Example:

num = 10

if num > 5:
    print("The number is greater than 5")
✅ Since num is 10 (greater than 5), the condition is True, so the print statement executes.

4.1.2 If…Elif Statement

  • The if-elif statement is used when multiple conditions need to be checked one after another.

Syntax:

if condition1:
    # Code block if condition1 is True
elif condition2:
    # Code block if condition2 is True

Example.

marks = 75

if marks >= 90:
    print("Grade: A")
elif marks >= 75:
    print("Grade: B")
✅ Since marks = 75, the first condition (marks >= 90) is False, but the second (marks >= 75) is True, so "Grade: B" prints.

4.1.3 If…Elif…Else Statement

  • The if-elif-else statement provides a default block (else) that executes if none of the conditions are True.

Syntax:

if condition1:
    # Executes if condition1 is True
elif condition2:
    # Executes if condition2 is True
else:
    # Executes if none of the conditions are True

Example:

temperature = 15

if temperature > 30:
    print("It's hot outside.")
elif temperature > 20:
    print("The weather is warm.")
else:
    print("It's cold.")

✅ Since temperature = 15, both conditions are False, so the else block executes.

4.1.4 Nested If Statement

  • A nested if is an if statement inside another if statement.

Syntax:

if condition1:
    if condition2:
        # Executes if both condition1 and condition2 are True

Example:

age = 20
has_id = True

if age >= 18:
    if has_id:
        print("You are allowed to enter.")
    else:
        print("You need an ID.")
else:
    print("You are underaged.")
✅ Since age = 20 (≥ 18) and has_id = True, both conditions are True, so "You are allowed to enter." prints.

4.2 Iterative Statements in Python.

  • Iterative statements (loops) repeat a block of code multiple times until a certain condition is met. Python provides two types of loops:
  1. while loop
  2. for loop
  • Additionally, Python has control statements like break, continue, and pass to manage loops.

4.2.1 While Loop

  • A while loop executes repeatedly as long as a given condition is True.

Syntax:

while condition:
    # Code to execute

Example: While Loop

count = 1

while count <= 5:
    print("Count:", count)
    count += 1
✅ The loop runs until count becomes 6, which makes the condition False.

Nested While Loop

  • A nested while loop is a loop inside another while loop.

Example:

i = 1
while i <= 3:
    j = 1
    while j <= 2:
        print(f"i={i}, j={j}")
        j += 1
    i += 1
✅ The inner loop (j) runs twice for each iteration of the outer loop (i).

Break and Continue Statements in While Loop

  • break → Stops the loop immediately.
  • continue → Skips the rest of the loop and moves to the next iteration.
num = 1

while num <= 5:
    if num == 3:
        break  # Exit loop when num is 3
    print("Number:", num)
    num += 1
✅ The loop stops when num reaches 3 due to break.
Example: Continue Statement:
num = 0

while num < 5:
    num += 1
    if num == 3:
        continue  # Skip iteration when num is 3
    print("Number:", num)
✅ When num = 3, continue skips the print statement.

4.2.2 For Loop

  • A for loop is used to iterate over sequences (like lists, tuples, strings, and ranges).

Syntax:

for variable in sequence:
    # Code to execute

Example: For Loop

for num in range(1, 6):
    print("Number:", num)
✅ The loop iterates over the sequence 1, 2, 3, 4, 5.

Using range() in a For Loop

  • The range() function generates a sequence of numbers.
Syntax Description Example
range(n) Generates numbers from 0 to n-1 range(5) → 0,1,2,3,4
range(start, end) Generates numbers from start to end-1 range(2,6) → 2,3,4,5
range(start, end, step) Generates numbers with a step range(1,10,2) → 1,3,5,7,9

Example:

for num in range(2, 10, 2):
    print(num)
✅ The loop prints even numbers from 2 to 8.

Break and Continue in For Loop

  • Similar to while, break and continue can be used in for loops.
Example: Break Statement
for num in range(1, 6):
    if num == 4:
        break
    print(num)
✅ The loop stops when num reaches 4.
Example: Continue Statement
for num in range(1, 6):
    if num == 3:
        continue  # Skip 3
    print(num)
✅ The number 3 is skipped.

Using pass in a Loop

  • The pass statement is a placeholder for future code.
Example:
for num in range(5):
    pass  # Placeholder
✅ The loop runs but does nothing.

Else with For Loop

  • The else block runs after the loop completes normally (without break).

Example:

for num in range(1, 4):
    print(num)
else:
    print("Loop completed successfully!")
✅ The else block executes only if the loop is not broken.
Example: Break with Else
for num in range(1, 4):
    print(num)
    if num == 2:
        break
else:
    print("Loop completed!")
✅ The else block does not execute because the loop breaks early.

Nested For Loop

  • A for loop inside another for loop is called a nested loop.

Example:

for i in range(1, 3):
    for j in range(1, 4):
        print(f"i={i}, j={j}")
✅ The inner loop (j) runs three times for each iteration of the outer loop (i).

4.3 List in Python.

  • A list in Python is a mutable, ordered collection that can store multiple items of different types (integers, floats, strings, etc.).
  • Lists allow easy modifications, such as adding, removing, and sorting elements.

4.3.1 Creating a List

A list is defined using square brackets [], with elements separated by commas.

Example:

# Creating a list with different data types
fruits = ["Apple", "Banana", "Cherry"]
numbers = [10, 20, 30, 40]
mixed = [1, "Hello", 3.5, True]

print(fruits)
print(numbers)
print(mixed)

4.3.2 Indexing in a List

Each element in a list has an index, starting from 0 for the first item.
List Item "A" "B" "C" "D"
Index 0 1 2 3
Negative Index -4 -3 -2 -1 
letters = ["A", "B", "C", "D"]

# Accessing elements using positive index
print(letters[0])  # Output: A
print(letters[2])  # Output: C

# Accessing elements using negative index
print(letters[-1])  # Output: D
print(letters[-3])  # Output: B

4.3.3 Accessing List Members

Lists can be accessed using indexing, slicing, and loops.

Using Indexing

colors = ["Red", "Green", "Blue"]
print(colors[1])  # Output: Green

Using Slicing

  • Slicing extracts a portion of a list using [start:end:step].
numbers = [10, 20, 30, 40, 50, 60]

print(numbers[1:4])  # Output: [20, 30, 40]
print(numbers[:3])   # Output: [10, 20, 30] (Start from index 0)
print(numbers[2:])   # Output: [30, 40, 50, 60] (Till last index)
print(numbers[::2])  # Output: [10, 30, 50] (Skip every 2nd element)

Using a Loop.

animals = ["Cat", "Dog", "Rabbit"]

for animal in animals:
    print(animal)

4.3.4 Range in List

  • We can use the range() function to generate a sequence of numbers for indexing and slicing.
Example:
numbers = list(range(1, 11))  # Generates numbers from 1 to 10
print(numbers)

4.3.5 List Methods

  • Python provides built-in methods to manipulate lists.

1. append() – Adds an element to the end of the list.

fruits = ["Apple", "Banana"]
fruits.append("Cherry")

print(fruits)  # Output: ['Apple', 'Banana', 'Cherry']

2. clear() – Removes all elements from the list.

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

print(numbers)  # Output: []

3. copy() – Creates a copy of the list.

original = [10, 20, 30]
copy_list = original.copy()

print(copy_list)  # Output: [10, 20, 30]

4. count() – Counts the occurrences of an element

nums = [1, 2, 3, 1, 2, 1]
print(nums.count(1))  # Output: 3

5. index() – Finds the index of the first occurrence of an element.

colors = ["Red", "Green", "Blue", "Green"]
print(colors.index("Green"))  # Output: 1

6. insert() – Inserts an element at a specific index

languages = ["Python", "Java"]
languages.insert(1, "C++")

print(languages)  # Output: ['Python', 'C++', 'Java']

7. pop() – Removes and returns the last element (or a specified index)

numbers = [10, 20, 30, 40]

print(numbers.pop())    # Output: 40 (removes last element)
print(numbers.pop(1))   # Output: 20 (removes element at index 1)
print(numbers)          # Output: [10, 30]

8. remove() – Removes the first occurrence of a specified element

fruits = ["Apple", "Banana", "Cherry"]
fruits.remove("Banana")

print(fruits)  # Output: ['Apple', 'Cherry']

9. reverse() – Reverses the order of elements

nums = [1, 2, 3, 4]
nums.reverse()

print(nums)  # Output: [4, 3, 2, 1]

10. sort() – Sorts the list in ascending (default) or descending order

numbers = [40, 10, 30, 20]

numbers.sort()  # Sort in ascending order
print(numbers)  # Output: [10, 20, 30, 40]

numbers.sort(reverse=True)  # Sort in descending order
print(numbers)  # Output: [40, 30, 20, 10]

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