Skip to main content

Python Control Flow

Python Control Flow

Control flow in Python allows you to make decisions and repeat actions using conditional statements and loops. This tutorial explores how to use if, elif, else, and loop structures to control your program’s execution.

01. What Is Control Flow?

Control flow determines the order in which your code executes, enabling dynamic behavior based on conditions or iterations. Python provides conditional statements (if) and loops (for, while) for this purpose.

Example: Basic Control Flow

# Conditional statement
age = 20
if age >= 18:
    print("You are an adult")  # Executes if condition is True

# Loop
for i in range(3):
    print(f"Iteration {i}")  # Repeats 3 times

Output:

You are an adult
Iteration 0
Iteration 1
Iteration 2

Explanation:

  • if age >= 18 - Checks a condition and executes the block if true.
  • for i in range(3) - Iterates over a sequence, executing the block for each value.

02. Conditional Statements

Conditional statements allow your program to execute different code based on conditions using if, elif, and else.

2.1 Using if, elif, else

Example: Conditional Logic

score = 85
if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
elif score >= 70:
    print("Grade: C")
else:
    print("Grade: F")

Output:

Grade: B

2.2 Nested Conditionals

Example: Nested if Statements

age = 16
has_permit = True
if age >= 16:
    if has_permit:
        print("You can drive with a permit")
    else:
        print("You need a permit to drive")
else:
    print("You are too young to drive")

Output:

You can drive with a permit

2.3 Invalid Conditional Syntax

Example: Syntax Error

if 5 > 3  # Missing colon (SyntaxError)
    print("This won't work")

Output:

SyntaxError: expected ':'

Explanation:

  • if 5 > 3 - Missing a colon (:) causes a SyntaxError.

03. Loops

Loops allow you to repeat code blocks using for or while statements.

3.1 For Loops

Example: For Loop

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(f"Fruit: {fruit}")

Output:

Fruit: apple
Fruit: banana
Fruit: cherry

3.2 While Loops

Example: While Loop

count = 1
while count <= 3:
    print(f"Count: {count}")
    count += 1

Output:

Count: 1 Count: 2 Count: 3

3.3 Infinite Loop Error

Example: Infinite Loop

while True:
    print("This runs forever!")  # Infinite loop (avoid unless intentional)

Output:

This runs forever! (Repeats indefinitely)

Explanation:

  • while True - Runs indefinitely unless interrupted (e.g., Ctrl+C).

04. Loop Control Statements

Use break, continue, and pass to modify loop behavior.

Example: Break and Continue

for i in range(5):
    if i == 2:
        continue  # Skip this iteration
    if i == 4:
        break  # Exit loop
    print(f"Number: {i}")

Output:

Number: 0
Number: 1
Number: 3

05. Effective Usage

5.1 Recommended Practices

  • Keep conditions simple and readable.

Example: Clear Conditions

# Good: Simple condition
if temperature > 30:
    print("It's hot!")

# Avoid: Overly complex
if temperature > 30 and not is_raining and wind_speed < 20:
    print("It's a nice day!")
  • Use break to exit loops early when possible.
  • Ensure loops have a clear exit condition to avoid infinite loops.

5.2 Practices to Avoid

  • Avoid deeply nested conditionals or loops (aim for 2-3 levels max).

Example: Excessive Nesting

# Avoid: Hard to read
if x > 0:
    if y > 0:
        if z > 0:
            print("All positive")
  • Don’t forget to update loop variables in while loops.

06. Common Use Cases

6.1 User Input Validation

Use conditionals to validate user input before processing.

Example: Input Validation

age = int(input("Enter your age: "))
if age < 0 or age > 120:
    print("Invalid age!")
else:
    print(f"You are {age} years old")

Output:

Enter your age: 25
You are 25 years old

6.2 Processing Lists

Loops are ideal for iterating over collections like lists.

Example: Summing a List

numbers = [10, 20, 30]
total = 0
for num in numbers:
    total += num
print(f"Sum: {total}")

Output:

Sum: 60

Conclusion

Python’s control flow tools—conditionals and loops—enable dynamic and repetitive tasks in your programs. By mastering if, for, and while, you can build flexible, efficient code. Key takeaways:

  • Use if, elif, else for decision-making.
  • Iterate with for for sequences and while for condition-based loops.
  • Control loops with break and continue for efficiency.
  • Keep conditions and loops simple to avoid errors like infinite loops.

With these skills, you’re ready to create dynamic Python programs with ease!

Comments

Post a Comment