What is a Control Structure in Python?

A control structure in Python is a way of controlling the order in which a program's instructions are executed.

Main Types of Control Structures

There are mainly 3 types:

1. Sequence

Code runs from top to bottom, one statement after another.

print("Step 1")
print("Step 2")
print("Step 3")

Output:

Step 1
Step 2
Step 3

Python executes the statements in order.

Real-life example:

This is sequence.

2. Selection / Decision

The program makes a decision based on a condition.

Python uses:

• if
• if-else
• if-elif-else

Example

age = 20

if age >= 18:
    print("You are an adult")
else:
    print("You are a minor")

Output:

You are an adult

3. Repetition / Iteration

Sometimes we want to repeat the same code multiple times.

Python uses:

• for loop
• while loop

Example using for loop

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

Output:

Hello
Hello
Hello
Hello
Hello

if Statement

An if statement is a control structure used to make a decision. It executes a particular block of instructions only when a given condition is true.

marks = 75

if marks >= 50:
    print("Pass")

Output:

Pass

Explanation: Since 75 >= 50 is True, the message "Pass" is displayed.

Syntax of if Statement

if condition:
    statement

if-else Statement

An if-else statement is a control structure used to make a decision between two alternatives. The if block executes when the condition is True, while the else block executes when the condition is False.

Example 1

marks = 40

if marks >= 50:
    print("Pass")
else:
    print("Fail")

Output:

Fail

Explanation: Since 40 >= 50 is False, the else block is executed and "Fail" is displayed.

Example 2

marks = 75

if marks >= 50:
    print("Pass")
else:
    print("Fail")

Output:

Pass

Explanation: Since 75 >= 50 is True, the if block is executed and "Pass" is displayed.

Syntax of if-else Statement

if condition:
    statement
else:
    statement

Nested Condition

Definition: A nested condition is a condition placed inside another condition. It is used when we need to check a second condition only after the first condition is true.

Nested if

An if statement inside another if statement.

Syntax

if condition1:
    if condition2:
        statement

Example

age = 20
citizen = True

if age >= 18:
    if citizen:
        print("Eligible")

Output:

Eligible

Nested if-else

An if-else statement can also be placed inside another if.

Syntax

if condition1:
    if condition2:
        statement1
    else:
        statement2
else:
    statement3

Example

age = 20
has_card = False

if age >= 18:
    if has_card:
        print("Entry allowed")
    else:
        print("Card required")
else:
    print("Under age")

Output:

Card required

Looping Construct

Definition: A looping construct is a control structure used to repeat a set of instructions again and again until a specific condition is met or for a specific number of times.

It is also called a repetitive structure or iterative structure.

While Loop

Definition: A while loop is a looping construct that repeats a set of instructions as long as a given condition is true.

Syntax

while condition:
    statement

Example

num = 1

while num <= 5:
    print(num)
    num = num + 1

Output:

1
2
3
4
5

How it works

1. num starts at 1.
2. The condition num <= 5 is checked.
3. If it is true, the statement is executed.
4. num increases by 1.
5. The condition is checked again.
6. When num becomes 6, the condition becomes false and the loop stops.

Important Programs for Exam Preparation

The following programs are important from an exam point of view. Practice these programs carefully to understand the working of the while loop.

1. Program to Print Numbers from 1 to 10

num = 1

while num <= 10:
    print(num)
    num += 1    # num += 1 means num = num + 1

Output:

1
2
3
4
5
6
7
8
9
10

2. Program to Print Even Numbers from 1 to 20

i = 2

while i <= 20:
    print(i)
    i += 2    # i += 2 means i = i + 2

Output:

2
4
6
8
10
12
14
16
18
20

3. Program to Print Odd Numbers from 1 to 20

i = 1

while i <= 20:
    print(i)
    i += 2    # i += 2 means i = i + 2

Output:

1
3
5
7
9
11
13
15
17
19

4. Program to Print Table of a Number Stored in a Variable

num = 5
i = 1

while i <= 10:
    print(num, "x", i, "=", num * i)
    i += 1    # i += 1 means i = i + 1

Output:

5 x 1 = 5
5 x 2 = 10
5 x 3 = 15
5 x 4 = 20
5 x 5 = 25
5 x 6 = 30
5 x 7 = 35
5 x 8 = 40
5 x 9 = 45
5 x 10 = 50

5. Program to Print Table of a Number Entered by the User

num = int(input("Enter a number: "))
i = 1

while i <= 10:
    print(num, "x", i, "=", num * i)
    i += 1    # i += 1 means i = i + 1

Sample Input:

Enter a number: 7

Output:

7 x 1 = 7
7 x 2 = 14
7 x 3 = 21
7 x 4 = 28
7 x 5 = 35
7 x 6 = 42
7 x 7 = 49
7 x 8 = 56
7 x 9 = 63
7 x 10 = 70

For Loop

Definition: A for loop is a looping construct that repeats a set of instructions for each item in a sequence or for a specific number of times.

Syntax

for variable in sequence:
    statement

Example

for num in range(1, 6):
    print(num)

Output:

1
2
3
4
5

How it works

1. range(1, 6) generates numbers from 1 to 5.
2. The for loop takes one number at a time.
3. The value is stored in the variable num.
4. The print() statement displays the value of num.
5. The loop repeats until all numbers in the sequence have been processed.

Important Programs for Exam Preparation

The following programs are important from an exam point of view. Practice these programs carefully to understand the working of the for loop.

1. Program to Print Numbers from 1 to 10

for num in range(1, 11):
    print(num)

Output:

1
2
3
4
5
6
7
8
9
10

2. Program to Print Even Numbers from 1 to 20

for i in range(2, 21, 2):
    print(i)

Output:

2
4
6
8
10
12
14
16
18
20

3. Program to Print Odd Numbers from 1 to 20

for i in range(1, 21, 2):
    print(i)

Output:

1
3
5
7
9
11
13
15
17
19

4. Program to Print Table of a Number Stored in a Variable

num = 5

for i in range(1, 11):
    print(num, "x", i, "=", num * i)

Output:

5 x 1 = 5
5 x 2 = 10
5 x 3 = 15
5 x 4 = 20
5 x 5 = 25
5 x 6 = 30
5 x 7 = 35
5 x 8 = 40
5 x 9 = 45
5 x 10 = 50

5. Program to Print Table of a Number Entered by the User

num = int(input("Enter a number: "))

for i in range(1, 11):
    print(num, "x", i, "=", num * i)

Sample Input:

Enter a number: 7

Output:

7 x 1 = 7
7 x 2 = 14
7 x 3 = 21
7 x 4 = 28
7 x 5 = 35
7 x 6 = 42
7 x 7 = 49
7 x 8 = 56
7 x 9 = 63
7 x 10 = 70

range() Function

Definition: The range() function is used to generate a sequence of numbers within a given range. It is commonly used with loops to repeat a task a specific number of times.

Syntax

range(start, stop, step)

• start → The number where the sequence starts.
• stop → The number where the sequence stops (this number is not included).
• step → The amount by which the number increases or decreases.

Example 1: range(5)

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

Output:

0
1
2
3
4

Notice that 5 is not included.

Example 2: range(1, 6)

for i in range(1, 6):
    print(i)

Output:

1
2
3
4
5

Example 3: Using Step

for i in range(2, 11, 2):
    print(i)

Output:

2
4
6
8
10

Here:

• 2 → Starting number
• 11 → Stopping point (not included)
• 2 → Increase by 2

end Statement

Definition: The end statement is used with the print() function to stop line breaking and keep the output on the same line.

Syntax

print(value, end="something")

Example 1: Default print()

print("Hello")
print("World")

Output:

Hello
World

Here, print() automatically moves to the next line.

Example 2: Using end=" "

print("Hello", end=" ")
print("World")

Output:

Hello World

end=" " tells print() to stay on the same line and add a space.

Example 3: Using end=""

print("Hello", end="")
print("World")

Output:

HelloWorld

Example 4: Using end="-"

print("Hello", end="-")
print("World")

Output:

Hello-World

Nested Loop

Definition: A nested loop is a loop placed inside another loop.

Nested loops are mostly used to print patterns, such as triangles, squares, rectangles, pyramids, and other shapes.

Syntax of Nested Loop

Using for Loop

for variable1 in range():
    for variable2 in range():
        statement

Using while Loop

while condition1:
    while condition2:
        statement

Examples of Nested Loop

Example 1: Square Pattern

for i in range(1, 5):
    for j in range(1, 5):
        print("*", end=" ")
    print()

Output:

* * * *
* * * *
* * * *
* * * *

Here:

• The outer loop controls the rows.
• The inner loop controls the columns.
• Together, they create the square pattern.

Example 2: Triangle Pattern

for i in range(1, 5):
    for j in range(i):
        print("*", end=" ")
    print()

Output:

*
* *
* * *
* * * *

Square Pattern Using while Loop

i = 1

while i <= 4:
    j = 1

    while j <= 4:
        print("*", end=" ")
        j += 1    # j += 1 means j = j + 1

    print()
    i += 1        # i += 1 means i = i + 1

Output:

* * * *
* * * *
* * * *
* * * *

Triangle Pattern Using while Loop

i = 1

while i <= 4:
    j = 1

    while j <= i:
        print("*", end=" ")
        j += 1    # j += 1 means j = j + 1

    print()
    i += 1        # i += 1 means i = i + 1

Output:

*
* *
* * *
* * * *

Libraries

Definition: A library is a collection of pre-written code, functions, and tools that we can use in a program instead of writing everything from the beginning.

Why do we use libraries?

Libraries save time and effort because they provide ready-made functions for different tasks.

1. random Library

The random library is used to generate random numbers or random values.

Example:

import random

num = random.randint(1, 10)

print(num)

Output:

7

The output can be any number from 1 to 10, so it may be different each time.

2. statistics Library

The statistics library is used to perform statistical calculations, such as finding the mean, median, and mode.

Example:

import statistics

marks = [60, 70, 80, 90, 100]

print(statistics.mean(marks))

Output:

80

Here, mean() calculates the average of the given numbers.

Another Example:

import statistics

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

print(statistics.median(numbers))
print(statistics.mode(numbers))

Output:

20
20

• mean() → finds the average.
• median() → finds the middle value.
• mode() → finds the most repeated value.

Python List

Definition

A list is a collection of multiple values stored in a single variable.

A list can store different types of data, such as numbers, strings, or a combination of values.

Creating a List

A list is created using square brackets [ ], and values are separated by commas.

Syntax

list_name = [value1, value2, value3, ...]

Example 1: List of Numbers

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

print(numbers)

Output:

[10, 20, 30, 40, 50]

Example 2: List of Names

names = ["Ali", "Ahmed", "Sara", "John"]

print(names)

Output:

['Ali', 'Ahmed', 'Sara', 'John']

Example 3: List with Different Data Types

student = ["Ali", 20, 85.5]

print(student)

Output:

['Ali', 20, 85.5]

List Elements and Index Numbers

List Elements

The values stored inside a list are called list elements or list items.

Example:

names = ["Ali", "Ahmed", "Sara", "John"]

Here:

• Ali is a list element.
• Ahmed is a list element.
• Sara is a list element.
• John is a list element.

Index Number

An index number is the position number of an element in a list.

In a list, the index number starts from 0.

Element	Ali	Ahmed	Sara	John
Index	0	1	2	3

Accessing List Items

Definition: Accessing list items means getting or retrieving a particular value from a list.

List items are accessed using their index number.

Important Point

The index of a list starts from 0.

For example:

names = ["Ali", "Ahmed", "Sara", "John"]

Item	Ali	Ahmed	Sara	John
Index	0	1	2	3

Example

names = ["Ali", "Ahmed", "Sara", "John"]

print(names[0])
print(names[2])

Output:

Ali
Sara

Here:

• names[0] → accesses Ali.
• names[2] → accesses Sara.

Accessing Items from the End

Python also allows negative indexing.

names = ["Ali", "Ahmed", "Sara", "John"]

print(names[-1])
print(names[-2])

Output:

John
Sara

Here:

• -1 → last item.
• -2 → second-last item.

Modifying a List

Definition: Modifying a list means changing, adding, or removing elements from an existing list.

Python provides different operations to modify a list, such as append, remove, sort, and reverse.

1. append()

The append() method is used to add a new element at the end of a list.

Example:

numbers = [10, 20, 30]

numbers.append(40)

print(numbers)

Output:

[10, 20, 30, 40]

Here, 40 is added at the end of the list.

2. remove()

The remove() method is used to remove a specific element from a list.

Example:

numbers = [10, 20, 30, 40]

numbers.remove(20)

print(numbers)

Output:

[10, 30, 40]

Here, 20 is removed from the list.

3. sort()

The sort() method is used to arrange the elements of a list in ascending order.

Example:

numbers = [40, 10, 30, 20]

numbers.sort()

print(numbers)

Output:

[10, 20, 30, 40]

4. reverse()

The reverse() method is used to reverse the order of elements in a list.

Example:

numbers = [10, 20, 30, 40]

numbers.reverse()

print(numbers)

Output:

[40, 30, 20, 10]

Quick Summary

Operation	Purpose
append()	Adds an element at the end
remove()	Removes a specific element
sort()	Arranges elements in ascending order
reverse()	Reverses the order of elements

Testing and Debugging

1. Testing

Definition: Testing is the process of checking a program to find errors or problems and to make sure it works correctly.

Types of Testing

1. Unit Testing

Testing a small individual part of a program, such as a function.

Example: Testing a function that calculates the sum of two numbers.

2. Integration Testing

Testing whether different parts of a program work correctly together.

Example: Checking whether the login system works correctly with the database.

3. Functional Testing

Testing whether the program's features work according to the requirements.

Example: Testing whether a calculator correctly performs addition, subtraction, multiplication, and division.

4. Regression Testing

Testing the program again after making changes or fixing errors to make sure the old features still work correctly.

Example: You fix the login system. After fixing it, you test the login system and other features to make sure the changes did not break anything else.

2. Debugging

Definition: Debugging is the process of finding and fixing errors in a program.

Common Debugging Techniques

1. Print Statement

We can use print() to check the values of variables and understand where the program is going wrong.

Example:

num = 10
result = num * 2

print("num =", num)
print("result =", result)

Output:

num = 10
result = 20

This helps us check whether the values are correct.

2. Debugging Tools

Debugging tools help us find and understand errors step by step.

They can allow us to:

• Run the program one line at a time.
• Check variable values.
• Find where the error occurs.
• Stop the program at a particular line using a breakpoint.

3. Error Messages

When an error occurs, the program provides an error message that tells us what went wrong and often shows the line where the error occurred.

Example:

num = 10
print(numm)

Error:

NameError: name 'numm' is not defined

Here, the error message tells us that numm has not been defined. The correct variable is num.

Quick Summary

Topic	Meaning
Testing	Checking whether the program works correctly
Unit Testing	Testing one small part
Integration Testing	Testing different parts together
Functional Testing	Testing whether features work correctly
Regression Testing	Testing again after changes
Debugging	Finding and fixing errors
Print Statement	Checking values and program flow
Debugging Tools	Finding errors step by step
Error Messages	Information about what went wrong

Multiple Choice Questions (MCQs) on Python Programming

Test Yourself: Interactive MCQs (Operating System)

Multiple Choice Questions (MCQs) on Python Programming

1. What is a control structure in Python?

a. A way to store data b. A way to control the order of program execution c. A type of library d. A type of variable

2. How many main types of control structures are discussed?

a. 2 b. 3 c. 4 d. 5

3. Which control structure executes statements from top to bottom?

a. Sequence b. Selection c. Iteration d. Nesting

4. Which control structure is used to make decisions?

a. Sequence b. Selection c. Iteration d. Library

5. Which Python statement executes a block only when a condition is true?

a. for b. while c. if d. print

6. Which keyword is used when the if condition is false?

a. else b. elif c. for d. while

7. What is the output of the following code?
marks = 75
if marks >= 50:
    print("Pass")

a. Fail b. Error c. Pass d. 75

8. Which statement provides two alternatives based on a condition?

a. if b. if-else c. for d. while

9. What is a nested condition?

a. A loop inside a list b. A condition inside another condition c. A list inside a loop d. A function inside a library

10. In a nested if statement, the second condition is checked when:

a. The first condition is true b. The first condition is false c. The program ends d. The loop starts

11. What is a looping construct used for?

a. Storing values b. Repeating a set of instructions c. Creating a library d. Defining a list

12. Which two loops are commonly used in Python?

a. if and else b. for and while c. try and except d. print and input

13. A while loop repeats statements as long as:

a. A condition is true b. A condition is false c. A list is empty d. A program is closed

14. Which keyword is used to create a while loop?

a. loop b. repeat c. while d. during

15. What is the output of this code?
num = 1
while num <= 5:
    print(num)
    num += 1

a. 1, 2, 3, 4, 5 b. 0, 1, 2, 3, 4 c. 1, 2, 3, 4 d. 2, 3, 4, 5

16. Which expression increases a variable by 1?

a. += 1 b. -= 1 c. *= 1 d. /= 1

17. Which loop is commonly used to process each item in a sequence?

a. if b. else c. for d. print

18. What does range(1, 6) generate?

a. 1, 2, 3, 4, 5 b. 1, 2, 3, 4, 5, 6 c. 0, 1, 2, 3, 4, 5 d. 2, 3, 4, 5, 6

19. In range(2, 11, 2), what does the third value represent?

a. Starting value b. Stopping value c. Step value d. Number of loops

20. Which number is NOT included in range(5)?

a. 0 b. 1 c. 4 d. 5

21. What is the purpose of the end argument in print()?

a. To stop the program b. To control what is printed after the value c. To create a loop d. To define a variable

22. What is the output of print("Hello", end=" ") followed by print("World")?

a. Hello on one line and World on another b. HelloWorld c. Hello World d. World Hello

23. What is a nested loop?

a. A loop inside another loop b. A condition inside a loop c. A list inside a loop d. A library inside a loop

24. Nested loops are mostly used in the provided examples to:

a. Create variables b. Print patterns c. Import libraries d. Test functions

25. In a pattern program, which loop usually controls the rows?

a. Inner loop b. Outer loop c. if loop d. while condition

26. Which Python library is used to generate random values?

a. statistics b. random c. math d. list

27. Which function generates a random integer between two given values?

a. random.number() b. random.value() c. random.randint() d. random.integer()

28. Which library is used for statistical calculations?

a. random b. statistics c. loops d. testing

29. Which function from the statistics library calculates the average?

a. median() b. mode() c. average() d. mean()

30. Which function finds the most repeated value in a set of data?

a. mean() b. median() c. mode() d. repeat()

31. Which brackets are used to create a Python list?

a. ( ) b. { } c. [ ] d. < >

32. What is the index number of the first element in a Python list?

a. 0 b. 1 c. -1 d. 2

33. What does names[-1] access?

a. First item b. Second item c. Last item d. Middle item

34. Which list method adds an element at the end of a list?

a. remove() b. append() c. sort() d. reverse()

35. Which list method removes a specific element?

a. append() b. remove() c. sort() d. reverse()

36. Which list method arranges elements in ascending order?

a. append() b. remove() c. sort() d. reverse()

37. What is testing?

a. Writing code without checking it b. Checking a program to find errors and ensure it works correctly c. Deleting a program d. Creating a list

38. Which type of testing checks a small individual part of a program?

a. Integration testing b. Functional testing c. Regression testing d. Unit testing

39. What is debugging?

a. Finding and fixing errors in a program b. Creating random numbers c. Printing a list d. Repeating a program

40. Which debugging technique can be used to check variable values and program flow?

a. sort() b. print() c. range() d. append()

FAQs

Python Programming FAQs

What is a control structure in Python?

A control structure is a way of controlling the order in which a program's instructions are executed.

What are the three main types of control structures in Python?

The three main types are sequence, selection or decision, and repetition or iteration.

What is a sequence control structure?

A sequence control structure executes program statements from top to bottom, one statement after another.

What is an if statement in Python?

An if statement is used to make a decision. It executes a particular block of instructions only when the given condition is true.

What is an if-else statement?

An if-else statement makes a decision between two alternatives. The if block executes when the condition is true, while the else block executes when the condition is false.

What is a nested condition in Python?

A nested condition is a condition placed inside another condition. It is useful when a second condition needs to be checked after the first condition is true.

What is a looping construct?

A looping construct is a control structure used to repeat a set of instructions again and again until a condition is met or for a specific number of times.

What is a while loop in Python?

A while loop repeats a set of instructions as long as a given condition remains true.

What is a for loop in Python?

A for loop repeats a set of instructions for each item in a sequence or for a specific number of times.

What is the range() function in Python?

The range() function generates a sequence of numbers within a specified range and is commonly used with for loops.

What is a nested loop?

A nested loop is a loop placed inside another loop. Nested loops are commonly used to create patterns such as squares, rectangles, and triangles.

What is a Python library?

A library is a collection of pre-written code, functions, and tools that can be used in a program instead of writing everything from the beginning.

What is a Python list?

A list is a collection of multiple values stored in a single variable. Lists are created using square brackets and can contain different types of data.

What is testing in programming?

Testing is the process of checking a program to find errors or problems and to make sure that it works correctly.

What is debugging in Python?

Debugging is the process of finding and fixing errors in a program. Print statements, debugging tools, and error messages can help identify problems.

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