Control Flow

Control flow determines the order in which statements are executed in a Python program. By using conditions and loops, a program can make decisions, repeat tasks, and execute different blocks of code based on a particular situation.

What is Control Flow?

Normally, a Python program executes statements from top to bottom. However, real-world programs need to make decisions and repeat certain operations.

Control flow statements allow us to change the normal execution of a program.

For example, an application may need to check whether a student has passed an exam before displaying a certificate. This decision can be implemented using an if statement.

Types of Control Flow Statements

Python provides several ways to control the execution of a program.

Conditional Statements

Conditional statements allow a program to execute different blocks of code depending on whether a condition is true or false.

Python mainly provides three conditional keywords: if, elif, and else.

if Statement

The if statement executes a block of code only when its condition is true.

Syntax

if condition:
    statement

Example

age = 25

if age >= 18:
    print("You are eligible to vote.")

Output

You are eligible to vote.

if-else Statement

The else block is executed when the if condition is false.

age = 16

if age >= 18:
    print("You are eligible to vote.")
else:
    print("You are not eligible to vote.")

Output

You are not eligible to vote.

if-elif-else Statement

The elif keyword is used when there are multiple conditions to check.

marks = 75

if marks >= 90:
    print("Grade A+")
elif marks >= 75:
    print("Grade A")
elif marks >= 60:
    print("Grade B")
elif marks >= 40:
    print("Grade C")
else:
    print("Fail")

Output

Grade A

Nested if Statement

An if statement placed inside another if statement is called a nested conditional statement.

age = 25
has_id = True

if age >= 18:

    if has_id:
        print("Entry allowed.")
    else:
        print("ID required.")

else:
    print("Entry not allowed.")

Output:

Entry allowed.

For Loop

A for loop is used to iterate over items in a sequence such as a list, tuple, string, set, dictionary, or range.

Syntax

for variable in sequence:
    statement

Example

courses = ["Python", "C#", "Java"]

for course in courses:
    print(course)

Output

Python
C#
Java

for Loop with range()

The range() function generates a sequence of numbers that can be used with a for loop.

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

Output

1
2
3
4
5

The ending value 6 is not included.

While Loop

A while loop repeatedly executes a block of code as long as its condition remains true.

Syntax

while condition:
    statement

Example

number = 1

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

Output

1
2
3
4
5

for Loop vs while Loop

break Statement

The break statement immediately terminates the loop.

for number in range(1, 10):

    if number == 5:
        break

    print(number)

Output

1
2
3
4

continue Statement

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

for number in range(1, 6):

    if number == 3:
        continue

    print(number)

Output

1
2
4
5

pass Statement

The pass statement does nothing. It is used as a placeholder when a statement is syntactically required but the implementation is not ready yet.

for number in range(1, 4):

    if number == 2:
        pass

    print(number)

Output

1
2
3

Nested Loops

A loop inside another loop is called a nested loop.

Nested loops are useful for working with tables, matrices, patterns, and multidimensional data.

for row in range(1, 4):

    for column in range(1, 4):
        print(row, column)

Example 📩🫠

Suppose CIIT Training Institute wants to determine whether a student is eligible for a certificate based on marks and attendance.

marks = 78
attendance = 82

if marks >= 40 and attendance >= 75:
    print("Certificate Eligible")
else:
    print("Certificate Not Eligible")

Output

Certificate Eligible

Menu Example 🤓🌍

Control flow can also be used to build a simple menu-based application.

choice = 2

if choice == 1:
    print("Python Course")
elif choice == 2:
    print("C# Course")
elif choice == 3:
    print("Java Course")
else:
    print("Invalid Choice")

Output

C# Course

Common Mistakes

• Forgetting the colon : after if, elif, else, for, or while.
• Using incorrect indentation inside a block.
• Creating an infinite while loop by forgetting to update the loop variable.
• Using = when a comparison requires ==.
• Forgetting that range() excludes its ending value.

= vs ==

One common beginner mistake is confusing assignment and comparison operators.

Indentation in Control Flow

Python uses indentation to define a block of code. Unlike some languages, Python does not use curly braces to define blocks.

age = 25

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

Both print statements belong to the if block because they are indented.

marks = int(input("Enter your marks: "))

if marks >= 90:
    print("Grade A+")
elif marks >= 75:
    print("Grade A")
elif marks >= 60:
    print("Grade B")
elif marks >= 40:
    print("Grade C")
else:
    print("Fail")

Enter your marks: 85
Grade A